diff --git a/.github/actions/install-binaryen/action.yml b/.github/actions/install-binaryen/action.yml
new file mode 100644
index 0000000000..b3c3615521
--- /dev/null
+++ b/.github/actions/install-binaryen/action.yml
@@ -0,0 +1,90 @@
+name: Install Binaryen
+
+inputs:
+  repository:
+    description: 'Repository name with owner. For example, actions/checkout'
+    default: WebAssembly/binaryen
+  ref:
+    description: >
+      The branch, tag or SHA to checkout. When checking out the repository that
+      triggered a workflow, this defaults to the reference or SHA for that
+      event.  Otherwise, uses the default branch.
+    default: latest
+  build:
+    description: Whether we should build from source
+    default: false
+runs:
+  using: composite
+  steps:
+    - name: Restore cached binaryen
+      if: ${{ inputs.build && inputs.build != 'false' }}
+      id: cache-binaryen
+      uses: actions/cache/restore@v4
+      with:
+        path: binaryen
+        key: ${{ runner.os }}-binaryen-${{ inputs.ref }}
+
+    - name: Checkout binaryen
+      if: ${{ inputs.build  && inputs.build != 'false' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      uses: actions/checkout@v4
+      with:
+        repository: ${{ inputs.repository }}
+        path: binaryen
+        submodules: true
+        ref: ${{ inputs.ref == 'latest' && 'main' || inputs.ref }}
+
+    - name: Install ninja (Linux)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Linux' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      shell: bash
+      run: sudo apt-get install ninja-build
+
+    - name: Install ninja (MacOS)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'macOS' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      shell: bash
+      run: brew install ninja
+
+    - name: Build binaryen
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os != 'Windows' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      working-directory: ./binaryen
+      shell: bash
+      run: |
+        cmake -G Ninja .
+        ninja
+
+    - name: Install binaryen build dependencies (Windows)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Windows' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      working-directory: ./binaryen
+      shell: bash
+      run: opam install conf-cmake conf-c++
+
+    - name: Build binaryen (Windows)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Windows' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      working-directory: ./binaryen
+      shell: bash
+      run: |
+        opam exec -- cmake . -DBUILD_STATIC_LIB=ON -DBUILD_TESTS=off -DINSTALL_LIBS=off -DCMAKE_C_COMPILER=x86_64-w64-mingw32-gcc
+        make -j 4
+
+    - name: Cache binaryen
+      if: ${{ inputs.build && inputs.build != 'false' && steps.cache-binaryen.outputs.cache-hit != 'true' }}
+      uses: actions/cache/save@v4
+      with:
+        path: binaryen
+        key: ${{ runner.os }}-binaryen-${{ inputs.ref }}
+
+    - name: Set binaryen's path
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os != 'Windows' }}
+      shell: bash
+      run: echo "$GITHUB_WORKSPACE/binaryen/bin" >> $GITHUB_PATH
+
+    - name: Copy binaryen's tools (Windows)
+      if: ${{ inputs.build && inputs.build != 'false' && runner.os == 'Windows' }}
+      shell: bash
+      run: cp $GITHUB_WORKSPACE/binaryen/bin/wasm-{merge,opt}.exe _opam/bin
+
+    - name: Download Binaryen
+      if: ${{ ! inputs.build || inputs.build == 'false' }}
+      uses: Aandreba/setup-binaryen@v1.0.0
+      with:
+          token: ${{ github.token }}
+          version: ${{ inputs.ref }}
diff --git a/.github/workflows/build-wasm_of_ocaml.yml b/.github/workflows/build-wasm_of_ocaml.yml
index 70ee887c56..55250a1f3d 100644
--- a/.github/workflows/build-wasm_of_ocaml.yml
+++ b/.github/workflows/build-wasm_of_ocaml.yml
@@ -16,6 +16,8 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
+        os-name:
+          - Ubuntu
         ocaml-compiler:
           - "4.14"
           - "5.0"
@@ -27,30 +29,50 @@ jobs:
           - false
         all_jane_street_tests:
           - false
+        wasi:
+          - false
         include:
           - os: macos-latest
+            os-name: MacOS
             ocaml-compiler: "5.3"
             separate_compilation: true
             jane_street_tests: false
             all_jane_street_tests: false
+            wasi: false
           - os: windows-latest
+            os-name: Windows
             ocaml-compiler: "5.2"
             separate_compilation: true
             jane_street_tests: true
             all_jane_street_tests: true
+            wasi: false
           - os: ubuntu-latest
+            os-name: Ubuntu
             ocaml-compiler: "5.2"
             separate_compilation: true
             jane_street_tests: true
             all_jane_street_tests: true
+            wasi: false
           - os: ubuntu-latest
+            os-name: Ubuntu
             ocaml-compiler: "5.2"
             separate_compilation: false
             jane_street_tests: true
             all_jane_street_tests: false
+            wasi: false
+          - os: ubuntu-latest
+            os-name: Ubuntu
+            ocaml-compiler: "5.3"
+            separate_compilation: true
+            jane_street_tests: false
+            all_jane_street_tests: false
+            wasi: true
 
     runs-on: ${{ matrix.os }}
 
+    name:
+       ${{ matrix.wasi && 'WASI / ' || '' }}${{ (! matrix.separate_compilation) && 'Whole program / ' || ''}}${{ matrix.ocaml-compiler }} / ${{ matrix.os-name }}${{ matrix.all_jane_street_tests && ' / Jane Street tests' || ''}}
+
     steps:
       - name: Set git to use LF
         if: ${{ matrix.os == 'windows-latest' && matrix.ocaml-compiler < 5.2 }}
@@ -77,15 +99,65 @@ jobs:
         with:
           node-version: latest
 
+      - name: Set-up Rust toolchain
+        if: matrix.wasi
+        uses: actions-rust-lang/setup-rust-toolchain@v1
+
+      - name: Checkout Wasmtime
+        if: matrix.wasi
+        uses: actions/checkout@v4
+        with:
+          repository: bytecodealliance/wasmtime
+          path: wasmtime
+          submodules: true
+
+      - name: Build Wasmtime
+        if: matrix.wasi
+        working-directory: ./wasmtime
+        run: |
+          cargo build
+          echo `pwd`/target/debug >> "$GITHUB_PATH"
+
+      - name: Checkout Virgil
+        if: matrix.wasi
+        uses: actions/checkout@v4
+        with:
+          repository: titzer/virgil
+          path: virgil
+
+      - name: Build Virgil
+        if: matrix.wasi
+        working-directory: ./virgil
+        run: |
+          export PATH=$PATH:`pwd`/bin
+          echo `pwd`/bin >> "$GITHUB_PATH"
+          make
+
+      - name: Checkout Wizard engine
+        if: matrix.wasi
+        uses: actions/checkout@v4
+        with:
+          repository: titzer/wizard-engine
+          path: wizard-engine
+
+      - name: Build Wizard engine
+        if: matrix.wasi
+        working-directory: ./wizard-engine
+        run: |
+          make -j 4
+          echo `pwd`/bin >> "$GITHUB_PATH"
+
       - name: Set-up OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v3
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
 
-      - name: Set-up Binaryen
-        uses: Aandreba/setup-binaryen@v1.0.0
+      - name: Install Binaryen
+        uses: ./wasm_of_ocaml/.github/actions/install-binaryen
         with:
-          token: ${{ github.token }}
+          build: true
+          repository: vouillon/binaryen
+          ref: stack-switching-fixes
 
       - name: Pin faked binaryen-bin package
         # It's faster to use a cached version
@@ -127,7 +199,7 @@ jobs:
           opam install num cohttp-lwt-unix ppx_expect cstruct uucp
 
       - name: Run tests
-        if: ${{ matrix.separate_compilation }}
+        if: ${{ matrix.separate_compilation && ! matrix.wasi }}
         working-directory: ./wasm_of_ocaml
         run: opam exec -- dune build @runtest-wasm
 
@@ -136,11 +208,33 @@ jobs:
       # See https://github.com/libuv/libuv/issues/3622
 
       - name: Run tests with CPS effects
-        if: ${{ matrix.ocaml-compiler >= '5.' && matrix.separate_compilation }}
+        if: ${{ matrix.ocaml-compiler >= '5.' && matrix.separate_compilation && ! matrix.wasi }}
         continue-on-error: ${{ matrix.os == 'windows-latest' }}
         working-directory: ./wasm_of_ocaml
         run: opam exec -- dune build @runtest-wasm --profile with-effects
 
+      - name: Run tests (WASI runtime - node)
+        if: ${{ false }}
+        working-directory: ./wasm_of_ocaml
+        run: opam exec -- dune build @runtest-wasm --profile wasi
+
+      - name: Run tests (WASI runtime - wasmtime)
+        if: ${{ false }}
+        working-directory: ./wasm_of_ocaml
+        env:
+          WASM_ENGINE: wasmtime
+          WASI_FLAGS: --enable trap-on-exception
+          RUST_BACKTRACE: 0
+        continue-on-error: true
+        run: opam exec -- dune build @runtest-wasm --profile wasi
+
+      - name: Run tests (WASI runtime - Wizard engine)
+        if: ${{ matrix.wasi }}
+        working-directory: ./wasm_of_ocaml
+        env:
+          WASM_ENGINE: wizard-fast
+        run: opam exec -- dune build @runtest-wasm --profile wasi
+
       - name: Run Base tests
         if: matrix.all_jane_street_tests
         continue-on-error: ${{ matrix.os == 'windows-latest' }}
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 02aae3e8d5..7150cc9919 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -127,10 +127,12 @@ jobs:
       - run: opam install conf-pkg-config conf-mingw-w64-g++-i686 conf-mingw-w64-g++-x86_64
         if: runner.os == 'Windows'
 
-      - name: Set-up Binaryen
-        uses: Aandreba/setup-binaryen@v1.0.0
+      - name: Install Binaryen
+        uses: ./.github/actions/install-binaryen
         with:
-          token: ${{ github.token }}
+          build: true
+          repository: vouillon/binaryen
+          ref: stack-switching-fixes
 
       - name: Install faked binaryen-bin package
         # It's faster to use a cached version
diff --git a/CHANGES.md b/CHANGES.md
index 23a63c84bf..d1f73325eb 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,9 @@
+# dev
+
+## Features/Changes
+* Runtime: support more Unix functions (#1829)
+* Compiler: use a Wasm text files preprocessor (#1822)
+
 # 6.0.1 (2025-02-07) - Lille
 
 ## Features/Changes
diff --git a/compiler/bin-wasm_of_ocaml/compile.ml b/compiler/bin-wasm_of_ocaml/compile.ml
index 4f3587d8c7..cd967dcaea 100644
--- a/compiler/bin-wasm_of_ocaml/compile.ml
+++ b/compiler/bin-wasm_of_ocaml/compile.ml
@@ -73,6 +73,54 @@ let output_gen output_file f =
   Code.Var.set_stable (Config.Flag.stable_var ());
   Filename.gen_file output_file f
 
+let with_runtime_files ~runtime_wasm_files f =
+  let inputs =
+    List.map
+      ~f:(fun file ->
+        { Wat_preprocess.module_name = "env"
+        ; file
+        ; source =
+            (if Link.Wasm_binary.check_file ~file
+             then File
+             else Contents (Js_of_ocaml_compiler.Fs.read_file file))
+        })
+      runtime_wasm_files
+  in
+  Wat_preprocess.with_preprocessed_files ~variables:[] ~inputs f
+
+let build_runtime ~runtime_file =
+  (* Keep this variables in sync with gen/gen.ml *)
+  let variables =
+    [ "wasi", Config.Flag.wasi (); "trap-on-exception", Config.Flag.trap_on_exception () ]
+  in
+  match
+    List.find_opt Runtime_files.precompiled_runtimes ~f:(fun (flags, _) ->
+        assert (
+          List.length flags = List.length variables
+          && List.for_all2 ~f:(fun (k, _) (k', _) -> String.equal k k') flags variables);
+        Poly.equal flags variables)
+  with
+  | Some (_, contents) -> Fs.write_file ~name:runtime_file ~contents
+  | None ->
+      let inputs =
+        List.map
+          ~f:(fun (module_name, contents) ->
+            { Wat_preprocess.module_name
+            ; file = module_name ^ ".wat"
+            ; source = Contents contents
+            })
+          (if Config.Flag.wasi ()
+           then ("libc", Runtime_files.wasi_libc) :: Runtime_files.wat_files
+           else Runtime_files.wat_files)
+      in
+      Runtime.build
+        ~link_options:[ "-g" ]
+        ~opt_options:[ "-g"; "-O2" ]
+        ~variables:
+          (List.map ~f:(fun (k, v) : (_ * Wat_preprocess.value) -> k, Bool v) variables)
+        ~inputs
+        ~output_file:runtime_file
+
 let link_and_optimize
     ~profile
     ~sourcemap_root
@@ -91,7 +139,7 @@ let link_and_optimize
   let enable_source_maps = Option.is_some opt_sourcemap_file in
   Fs.with_intermediate_file (Filename.temp_file "runtime" ".wasm")
   @@ fun runtime_file ->
-  Fs.write_file ~name:runtime_file ~contents:Runtime_files.wasm_runtime;
+  build_runtime ~runtime_file;
   Fs.with_intermediate_file (Filename.temp_file "wasm-merged" ".wasm")
   @@ fun temp_file ->
   opt_with
@@ -100,11 +148,15 @@ let link_and_optimize
      then Some (Filename.temp_file "wasm-merged" ".wasm.map")
      else None)
   @@ fun opt_temp_sourcemap ->
+  (with_runtime_files ~runtime_wasm_files
+  @@ fun runtime_inputs ->
   Binaryen.link
-    ~runtime_files:(runtime_file :: runtime_wasm_files)
-    ~input_files:wat_files
+    ~inputs:
+      (({ Binaryen.module_name = "env"; file = runtime_file } :: runtime_inputs)
+      @ List.map ~f:(fun file -> { Binaryen.module_name = "OCaml"; file }) wat_files)
     ~opt_output_sourcemap:opt_temp_sourcemap
-    ~output_file:temp_file;
+    ~output_file:temp_file
+    ());
   Fs.with_intermediate_file (Filename.temp_file "wasm-dce" ".wasm")
   @@ fun temp_file' ->
   opt_with
@@ -113,7 +165,10 @@ let link_and_optimize
   @@ fun opt_temp_sourcemap' ->
   let primitives =
     Binaryen.dead_code_elimination
-      ~dependencies:Runtime_files.dependencies
+      ~dependencies:
+        (if Config.Flag.wasi ()
+         then Runtime_files.wasi_dependencies
+         else Runtime_files.dependencies)
       ~opt_input_sourcemap:opt_temp_sourcemap
       ~opt_output_sourcemap:opt_temp_sourcemap'
       ~input_file:temp_file
@@ -124,7 +179,8 @@ let link_and_optimize
     ~opt_input_sourcemap:opt_temp_sourcemap'
     ~opt_output_sourcemap:opt_sourcemap
     ~input_file:temp_file'
-    ~output_file;
+    ~output_file
+    ();
   Option.iter
     ~f:(update_sourcemap ~sourcemap_root ~sourcemap_don't_inline_content)
     opt_sourcemap_file;
@@ -133,20 +189,23 @@ let link_and_optimize
 let link_runtime ~profile runtime_wasm_files output_file =
   Fs.with_intermediate_file (Filename.temp_file "runtime" ".wasm")
   @@ fun runtime_file ->
-  Fs.write_file ~name:runtime_file ~contents:Runtime_files.wasm_runtime;
+  build_runtime ~runtime_file;
   Fs.with_intermediate_file (Filename.temp_file "wasm-merged" ".wasm")
   @@ fun temp_file ->
+  with_runtime_files ~runtime_wasm_files
+  @@ fun runtime_inputs ->
   Binaryen.link
     ~opt_output_sourcemap:None
-    ~runtime_files:(runtime_file :: runtime_wasm_files)
-    ~input_files:[]
-    ~output_file:temp_file;
+    ~inputs:({ Binaryen.module_name = "env"; file = runtime_file } :: runtime_inputs)
+    ~output_file:temp_file
+    ();
   Binaryen.optimize
     ~profile
     ~opt_input_sourcemap:None
     ~opt_output_sourcemap:None
     ~input_file:temp_file
     ~output_file
+    ()
 
 let generate_prelude ~out_file =
   Filename.gen_file out_file
@@ -186,7 +245,8 @@ let build_prelude z =
     ~input_file:prelude_file
     ~output_file:tmp_prelude_file
     ~opt_input_sourcemap:None
-    ~opt_output_sourcemap:None;
+    ~opt_output_sourcemap:None
+    ();
   Zip.add_file z ~name:"prelude.wasm" ~file:tmp_prelude_file;
   predefined_exceptions
 
@@ -216,7 +276,13 @@ let build_js_runtime ~primitives ?runtime_arguments () =
   in
   let prelude = Link.output_js always_required_js in
   let init_fun =
-    match Parse_js.parse (Parse_js.Lexer.of_string Runtime_files.js_runtime) with
+    match
+      Parse_js.parse
+        (Parse_js.Lexer.of_string
+           (if Config.Flag.wasi ()
+            then Runtime_files.js_wasi_launcher
+            else Runtime_files.js_launcher))
+    with
     | [ (Expression_statement f, _) ] -> f
     | _ -> assert false
   in
@@ -413,7 +479,8 @@ let run
            ~opt_input_sourcemap:None
            ~opt_output_sourcemap:opt_tmp_map_file
            ~input_file:wat_file
-           ~output_file:tmp_wasm_file;
+           ~output_file:tmp_wasm_file
+           ();
          { Link.unit_name; unit_info; strings; fragments }
        in
        cont unit_data unit_name tmp_wasm_file opt_tmp_map_file
@@ -455,9 +522,12 @@ let run
              tmp_wasm_file
          in
          let wasm_name =
-           Printf.sprintf
-             "code-%s"
-             (String.sub (Digest.to_hex (Digest.file tmp_wasm_file)) ~pos:0 ~len:20)
+           if Config.Flag.wasi ()
+           then "code"
+           else
+             Printf.sprintf
+               "code-%s"
+               (String.sub (Digest.to_hex (Digest.file tmp_wasm_file)) ~pos:0 ~len:20)
          in
          let tmp_wasm_file' = Filename.concat tmp_dir (wasm_name ^ ".wasm") in
          Sys.rename tmp_wasm_file tmp_wasm_file';
diff --git a/compiler/bin-wasm_of_ocaml/dune b/compiler/bin-wasm_of_ocaml/dune
index 19598ca764..024b987d36 100644
--- a/compiler/bin-wasm_of_ocaml/dune
+++ b/compiler/bin-wasm_of_ocaml/dune
@@ -1,6 +1,6 @@
-(executable
- (name wasm_of_ocaml)
- (public_name wasm_of_ocaml)
+(executables
+ (names wasm_of_ocaml wasmoo_link_wasm)
+ (public_names wasm_of_ocaml -)
  (package wasm_of_ocaml-compiler)
  (libraries
   jsoo_cmdline
@@ -25,9 +25,13 @@
  (target runtime_files.ml)
  (deps
   gen/gen.exe
-  ../../runtime/wasm/runtime.wasm
   ../../runtime/wasm/runtime.js
-  ../../runtime/wasm/deps.json)
+  ../../runtime/wasm/deps.json
+  ../../runtime/wasm/runtime-wasi.js
+  ../../runtime/wasm/deps-wasi.json
+  ../../runtime/wasm/libc.wasm
+  (glob_files ../../runtime/wasm/*.wat)
+  (glob_files ../../runtime/wasm/runtime-*.wasm))
  (action
   (with-stdout-to
    %{target}
diff --git a/compiler/bin-wasm_of_ocaml/gen/gen.ml b/compiler/bin-wasm_of_ocaml/gen/gen.ml
index b7a20c4e3e..36f17fb970 100644
--- a/compiler/bin-wasm_of_ocaml/gen/gen.ml
+++ b/compiler/bin-wasm_of_ocaml/gen/gen.ml
@@ -1,13 +1,70 @@
 let read_file ic = really_input_string ic (in_channel_length ic)
 
+(* Keep the two variables below in sync with function build_runtime in
+   ../compile.ml *)
+
+let default_flags = [ "trap-on-exception", false ]
+
+let interesting_runtimes = [ [ "wasi", false ]; [ "wasi", true ] ]
+
+let name_runtime l =
+  let flags = List.filter_map (fun (k, v) -> if v then Some k else None) l in
+  String.concat "-" ("runtime" :: (if flags = [] then [ "standard" ] else flags))
+  ^ ".wasm"
+
+let print_flags f flags =
+  Format.fprintf
+    f
+    "@[<2>[ %a ]@]"
+    (Format.pp_print_list
+       ~pp_sep:(fun f () -> Format.fprintf f ";@ ")
+       (fun f (k, v) ->
+         Format.fprintf f "@[\"%s\",@ %s@]" k (if v then "true" else "false")))
+    flags
+
 let () =
   let () = set_binary_mode_out stdout true in
   Format.printf
-    "let wasm_runtime = \"%s\"@."
+    "let js_launcher = \"%s\"@."
     (String.escaped (read_file (open_in_bin Sys.argv.(1))));
   Format.printf
-    "let js_runtime = \"%s\"@."
+    "let dependencies = \"%s\"@."
     (String.escaped (read_file (open_in_bin Sys.argv.(2))));
   Format.printf
-    "let dependencies = \"%s\"@."
-    (String.escaped (read_file (open_in_bin Sys.argv.(3))))
+    "let js_wasi_launcher = \"%s\"@."
+    (String.escaped (read_file (open_in_bin Sys.argv.(3))));
+  Format.printf
+    "let wasi_dependencies = \"%s\"@."
+    (String.escaped (read_file (open_in_bin Sys.argv.(4))));
+  Format.printf
+    "let wasi_libc = \"%s\"@."
+    (String.escaped (read_file (open_in_bin Sys.argv.(5))));
+  let wat_files, runtimes =
+    List.partition
+      (fun f -> Filename.check_suffix f ".wat")
+      (Array.to_list (Array.sub Sys.argv 6 (Array.length Sys.argv - 6)))
+  in
+  Format.printf
+    "let wat_files = [%a]@."
+    (Format.pp_print_list (fun f file ->
+         Format.fprintf
+           f
+           "\"%s\", \"%s\"; "
+           Filename.(chop_suffix (basename file) ".wat")
+           (String.escaped (read_file (open_in_bin file)))))
+    wat_files;
+  Format.printf
+    "let precompiled_runtimes = [%a]@."
+    (Format.pp_print_list (fun f flags ->
+         let flags = flags @ default_flags in
+         let name = name_runtime flags in
+         match List.find_opt (fun file -> Filename.basename file = name) runtimes with
+         | None -> failwith ("Missing runtime " ^ name)
+         | Some file ->
+             Format.fprintf
+               f
+               "%a, \"%s\"; "
+               print_flags
+               flags
+               (String.escaped (read_file (open_in_bin file)))))
+    interesting_runtimes
diff --git a/compiler/bin-wasm_of_ocaml/info.ml b/compiler/bin-wasm_of_ocaml/info.ml
index c297de5b6c..9a440d48c9 100644
--- a/compiler/bin-wasm_of_ocaml/info.ml
+++ b/compiler/bin-wasm_of_ocaml/info.ml
@@ -32,9 +32,9 @@ let make ~name ~doc ~description =
     ; `S "AUTHORS"
     ; `P "Jerome Vouillon, Hugo Heuzard."
     ; `S "LICENSE"
-    ; `P "Copyright (C) 2010-2024."
+    ; `P "Copyright (C) 2010-2025."
     ; `P
-        "js_of_ocaml is free software, you can redistribute it and/or modify it under \
+        "wasm_of_ocaml is free software, you can redistribute it and/or modify it under \
          the terms of the GNU Lesser General Public License as published by the Free \
          Software Foundation, with linking exception; either version 2.1 of the License, \
          or (at your option) any later version."
diff --git a/compiler/bin-wasm_of_ocaml/link_wasm.ml b/compiler/bin-wasm_of_ocaml/link_wasm.ml
new file mode 100644
index 0000000000..3a87a32a37
--- /dev/null
+++ b/compiler/bin-wasm_of_ocaml/link_wasm.ml
@@ -0,0 +1,110 @@
+(* Wasm_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *)
+
+open Cmdliner
+open Js_of_ocaml_compiler.Stdlib
+open Wasm_of_ocaml_compiler
+
+type binaryen_options =
+  { common : string list
+  ; opt : string list
+  ; merge : string list
+  }
+
+type options =
+  { input_modules : (string * string) list
+  ; output_file : string
+  ; variables : Preprocess.variables
+  ; binaryen_options : binaryen_options
+  }
+
+let options =
+  let input_modules =
+    let doc =
+      "Specify an input module with name $(i,NAME) in Wasm text file $(i,FILE)."
+    in
+    Arg.(
+      value
+      & pos_right 0 (pair ~sep:':' string string) []
+      & info [] ~docv:"NAME:FILE" ~doc)
+  in
+  let output_file =
+    let doc = "Specify the Wasm binary output file $(docv)." in
+    Arg.(required & pos 0 (some string) None & info [] ~docv:"WASM_FILE" ~doc)
+  in
+  let binaryen_options =
+    let doc = "Pass option $(docv) to binaryen tools" in
+    Arg.(value & opt_all string [] & info [ "binaryen" ] ~docv:"OPT" ~doc)
+  in
+  let opt_options =
+    let doc = "Pass option $(docv) to $(b,wasm-opt)" in
+    Arg.(value & opt_all string [] & info [ "binaryen-opt" ] ~docv:"OPT" ~doc)
+  in
+  let merge_options =
+    let doc = "Pass option $(docv) to $(b,wasm-merge)" in
+    Arg.(value & opt_all string [] & info [ "binaryen-merge" ] ~docv:"OPT" ~doc)
+  in
+  let build_t input_modules output_file variables common opt merge =
+    `Ok
+      { input_modules; output_file; variables; binaryen_options = { common; opt; merge } }
+  in
+  let t =
+    Term.(
+      const build_t
+      $ input_modules
+      $ output_file
+      $ Preprocess.variable_options
+      $ binaryen_options
+      $ opt_options
+      $ merge_options)
+  in
+  Term.ret t
+
+let link
+    { input_modules; output_file; variables; binaryen_options = { common; merge; opt } } =
+  let inputs =
+    List.map
+      ~f:(fun (module_name, file) ->
+        { Wat_preprocess.module_name
+        ; file
+        ; source =
+            (if Link.Wasm_binary.check_file ~file
+             then File
+             else Contents (Js_of_ocaml_compiler.Fs.read_file file))
+        })
+      input_modules
+  in
+  Runtime.build
+    ~link_options:(common @ merge)
+    ~opt_options:(common @ opt)
+    ~variables:(Preprocess.set_variables variables)
+    ~inputs
+    ~output_file
+
+let info =
+  Info.make
+    ~name:"link-wasm"
+    ~doc:"Wasm linker"
+    ~description:
+      "$(b,wasmoo_link_wasm) is a Wasm linker. It takes as input a list of Wasm text \
+       files, preprocesses them, links them together, and outputs a single Wasm binary \
+       module"
+
+let term = Cmdliner.Term.(const link $ options)
+
+let command = Cmdliner.Cmd.v info term
diff --git a/compiler/bin-wasm_of_ocaml/link_wasm.mli b/compiler/bin-wasm_of_ocaml/link_wasm.mli
new file mode 100644
index 0000000000..952975461c
--- /dev/null
+++ b/compiler/bin-wasm_of_ocaml/link_wasm.mli
@@ -0,0 +1,19 @@
+(* Wasm_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *)
+
+val command : unit Cmdliner.Cmd.t
diff --git a/compiler/bin-wasm_of_ocaml/preprocess.ml b/compiler/bin-wasm_of_ocaml/preprocess.ml
new file mode 100644
index 0000000000..277527ff64
--- /dev/null
+++ b/compiler/bin-wasm_of_ocaml/preprocess.ml
@@ -0,0 +1,152 @@
+(* Wasm_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *)
+
+open Cmdliner
+open Js_of_ocaml_compiler.Stdlib
+open Wasm_of_ocaml_compiler
+
+let () = Sys.catch_break true
+
+let read_contents ch =
+  let buf = Buffer.create 65536 in
+  let b = Bytes.create 65536 in
+  let rec read () =
+    let n = input ch b 0 (Bytes.length b) in
+    if n > 0
+    then (
+      Buffer.add_subbytes buf b 0 n;
+      read ())
+  in
+  read ();
+  Buffer.contents buf
+
+type variables =
+  { enable : string list
+  ; disable : string list
+  ; set : (string * string) list
+  }
+
+type options =
+  { input_file : string option
+  ; output_file : string option
+  ; variables : variables
+  }
+
+let variable_options =
+  let enable =
+    let doc = "Set preprocessor variable $(docv) to true." in
+    let arg =
+      Arg.(value & opt_all (list string) [] & info [ "enable" ] ~docv:"VAR" ~doc)
+    in
+    Term.(const List.flatten $ arg)
+  in
+  let disable =
+    let doc = "Set preprocessor variable $(docv) to false." in
+    let arg =
+      Arg.(value & opt_all (list string) [] & info [ "disable" ] ~docv:"VAR" ~doc)
+    in
+    Term.(const List.flatten $ arg)
+  in
+  let set =
+    let doc = "Set preprocessor variable $(i,VAR) to value $(i,VALUE)." in
+    let arg =
+      Arg.(
+        value
+        & opt_all (list (pair ~sep:'=' string string)) []
+        & info [ "set" ] ~docv:"VAR=VALUE" ~doc)
+    in
+    Term.(const List.flatten $ arg)
+  in
+  let build_t enable disable set = { enable; disable; set } in
+  Term.(const build_t $ enable $ disable $ set)
+
+let options =
+  let input_file =
+    let doc =
+      "Use the Wasm text file $(docv) as input (default to the standard input)."
+    in
+    Arg.(value & pos 0 (some string) None & info [] ~docv:"INPUT_FILE" ~doc)
+  in
+  let output_file =
+    let doc = "Specify the output file $(docv) (default to the standard output)." in
+    Arg.(value & opt (some string) None & info [ "o" ] ~docv:"OUTPUT_FILE" ~doc)
+  in
+  let build_t input_file output_file variables =
+    `Ok { input_file; output_file; variables }
+  in
+  let t = Term.(const build_t $ input_file $ output_file $ variable_options) in
+  Term.ret t
+
+let set_variables { enable; disable; set } =
+  List.map ~f:(fun nm -> nm, Wat_preprocess.Bool true) enable
+  @ List.map ~f:(fun nm -> nm, Wat_preprocess.Bool false) disable
+  @ List.map ~f:(fun (nm, v) -> nm, Wat_preprocess.String v) set
+
+let preprocess { input_file; output_file; variables } =
+  let with_input f =
+    match input_file with
+    | None -> f stdin
+    | Some file ->
+        let ch = open_in file in
+        let res = f ch in
+        close_in ch;
+        res
+  in
+  let with_output f =
+    match output_file with
+    | Some "-" | None -> f stdout
+    | Some file -> Filename.gen_file file f
+  in
+  let contents = with_input read_contents in
+  let res =
+    Wat_preprocess.f
+      ~filename:(Option.value ~default:"-" input_file)
+      ~contents
+      ~variables:(set_variables variables)
+  in
+  with_output (fun ch -> output_string ch res)
+
+let term = Cmdliner.Term.(const preprocess $ options)
+
+let info =
+  Info.make
+    ~name:"preprocess"
+    ~doc:"Wasm text file preprocessor"
+    ~description:"$(b,wasmoo_util pp) is a Wasm text file preprocessor."
+
+let command = Cmdliner.Cmd.v info term
+
+(* Adapted from
+   https://github.com/ocaml/opam/blob/fbbe93c3f67034da62d28c8666ec6b05e0a9b17c/s
+rc/client/opamArg.ml#L759 *)
+let alias_command ?orig_name cmd term name =
+  let orig =
+    match orig_name with
+    | Some s -> s
+    | None -> Cmd.name cmd
+  in
+  let doc = Printf.sprintf "An alias for $(b,%s)." orig in
+  let man =
+    [ `S "DESCRIPTION"
+    ; `P (Printf.sprintf "$(mname)$(b, %s) is an alias for $(mname)$(b, %s)." name orig)
+    ; `P (Printf.sprintf "See $(mname)$(b, %s --help) for details." orig)
+    ]
+  in
+  Cmd.v (Cmd.info name ~docs:"COMMAND ALIASES" ~doc ~man) term
+
+let command_alias = alias_command ~orig_name:"preprocess" command term "pp"
diff --git a/compiler/bin-wasm_of_ocaml/preprocess.mli b/compiler/bin-wasm_of_ocaml/preprocess.mli
new file mode 100644
index 0000000000..9ad1de2fff
--- /dev/null
+++ b/compiler/bin-wasm_of_ocaml/preprocess.mli
@@ -0,0 +1,32 @@
+(* Wasm_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *)
+
+type variables =
+  { enable : string list
+  ; disable : string list
+  ; set : (string * string) list
+  }
+
+val variable_options : variables Cmdliner.Term.t
+
+val set_variables :
+  variables -> (string * Wasm_of_ocaml_compiler.Wat_preprocess.value) list
+
+val command : unit Cmdliner.Cmd.t
+
+val command_alias : unit Cmdliner.Cmd.t
diff --git a/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml b/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml
index fdb2df384e..1ffcba57c5 100644
--- a/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml
+++ b/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml
@@ -48,7 +48,13 @@ let () =
         (Cmdliner.Cmd.group
            ~default:Compile.term
            (Compile.info "wasm_of_ocaml")
-           [ Link.command; Build_runtime.command; Compile.command ])
+           [ Link.command
+           ; Build_runtime.command
+           ; Compile.command
+           ; Preprocess.command
+           ; Preprocess.command_alias
+           ; Link_wasm.command
+           ])
     with
     | Ok (`Ok () | `Help | `Version) ->
         if !warnings > 0 && !werror
diff --git a/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml b/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml
new file mode 100644
index 0000000000..d9f7a24766
--- /dev/null
+++ b/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml
@@ -0,0 +1,43 @@
+(* Js_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ * Copyright (C) 2013 Hugo Heuzard
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *)
+
+open Js_of_ocaml_compiler.Stdlib
+
+let (_ : int) =
+  try
+    Cmdliner.Cmd.eval
+      ~catch:false
+      ~argv:(Jsoo_cmdline.normalize_argv ~warn:(warn "%s") Sys.argv)
+      Link_wasm.command
+  with
+  | (Match_failure _ | Assert_failure _ | Not_found) as exc ->
+      let backtrace = Printexc.get_backtrace () in
+      Format.eprintf
+        "%s: You found a bug. Please report it at \
+         https://github.com/ocsigen/js_of_ocaml/issues :@."
+        Sys.argv.(0);
+      Format.eprintf "Error: %s@." (Printexc.to_string exc);
+      prerr_string backtrace;
+      exit 1
+  | Failure s ->
+      Format.eprintf "%s: Error: %s@." Sys.argv.(0) s;
+      exit 1
+  | exc ->
+      Format.eprintf "%s: Error: %s@." Sys.argv.(0) (Printexc.to_string exc);
+      exit 1
diff --git a/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.mli b/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.mli
new file mode 100644
index 0000000000..cc6700682b
--- /dev/null
+++ b/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.mli
@@ -0,0 +1,17 @@
+(* Wasm_of_ocaml compiler
+ * http://www.ocsigen.org/js_of_ocaml/
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, with linking exception;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *)
diff --git a/compiler/lib-wasm/binaryen.ml b/compiler/lib-wasm/binaryen.ml
index 93b0b7b7fb..af3ef0ed13 100644
--- a/compiler/lib-wasm/binaryen.ml
+++ b/compiler/lib-wasm/binaryen.ml
@@ -36,6 +36,8 @@ let common_options () =
     ; "--enable-bulk-memory"
     ; "--enable-nontrapping-float-to-int"
     ; "--enable-strings"
+    ; "--enable-multimemory" (* To keep wasm-merge happy *)
+    ; "--enable-stack-switching"
     ]
   in
   if Config.Flag.pretty () then "-g" :: l else l
@@ -45,18 +47,20 @@ let opt_flag flag v =
   | None -> []
   | Some v -> [ flag; Filename.quote v ]
 
-let link ~runtime_files ~input_files ~opt_output_sourcemap ~output_file =
+type link_input =
+  { module_name : string
+  ; file : string
+  }
+
+let link ?options ~inputs ~opt_output_sourcemap ~output_file () =
   command
     ("wasm-merge"
     :: (common_options ()
+       @ Option.value ~default:[] options
        @ List.flatten
            (List.map
-              ~f:(fun runtime_file -> [ Filename.quote runtime_file; "env" ])
-              runtime_files)
-       @ List.flatten
-           (List.map
-              ~f:(fun input_file -> [ Filename.quote input_file; "OCaml" ])
-              input_files)
+              ~f:(fun { file; module_name } -> [ Filename.quote file; module_name ])
+              inputs)
        @ [ "-o"; Filename.quote output_file ]
        @ opt_flag "--output-source-map" opt_output_sourcemap))
 
@@ -109,13 +113,19 @@ let dead_code_elimination
   filter_unused_primitives primitives usage_file
 
 let optimization_options =
-  [| [ "-O2"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
-   ; [ "-O2"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
-   ; [ "-O3"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
+  [| [ "-O2"; "--skip-pass=inlining-optimizing" ]
+   ; [ "-O2"; "--skip-pass=inlining-optimizing" ]
+   ; [ "-O3"; "--skip-pass=inlining-optimizing" ]
   |]
 
-let optimize ~profile ~opt_input_sourcemap ~input_file ~opt_output_sourcemap ~output_file
-    =
+let optimize
+    ~profile
+    ?options
+    ~opt_input_sourcemap
+    ~input_file
+    ~opt_output_sourcemap
+    ~output_file
+    () =
   let level =
     match profile with
     | None -> 1
@@ -124,7 +134,8 @@ let optimize ~profile ~opt_input_sourcemap ~input_file ~opt_output_sourcemap ~ou
   command
     ("wasm-opt"
      :: (common_options ()
-        @ optimization_options.(level - 1)
+        @ (if Config.Flag.trap_on_exception () then [] else [ "--traps-never-happen" ])
+        @ Option.value ~default:optimization_options.(level - 1) options
         @ [ Filename.quote input_file; "-o"; Filename.quote output_file ])
     @ opt_flag "--input-source-map" opt_input_sourcemap
     @ opt_flag "--output-source-map" opt_output_sourcemap)
diff --git a/compiler/lib-wasm/binaryen.mli b/compiler/lib-wasm/binaryen.mli
index 3e07e06f88..4ce5bbd916 100644
--- a/compiler/lib-wasm/binaryen.mli
+++ b/compiler/lib-wasm/binaryen.mli
@@ -16,12 +16,18 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  *)
 
+type link_input =
+  { module_name : string
+  ; file : string
+  }
+
 val link :
-     runtime_files:string list
-  -> input_files:string list
+     ?options:string list
+  -> inputs:link_input list
   -> opt_output_sourcemap:string option
   -> output_file:string
   -> unit
+  -> unit
 
 val dead_code_elimination :
      dependencies:string
@@ -33,8 +39,10 @@ val dead_code_elimination :
 
 val optimize :
      profile:Driver.profile option
+  -> ?options:string list
   -> opt_input_sourcemap:string option
   -> input_file:string
   -> opt_output_sourcemap:string option
   -> output_file:string
   -> unit
+  -> unit
diff --git a/compiler/lib-wasm/dune b/compiler/lib-wasm/dune
index 2a54c9316f..90e6dcddee 100644
--- a/compiler/lib-wasm/dune
+++ b/compiler/lib-wasm/dune
@@ -4,4 +4,6 @@
  (synopsis "Wasm_of_ocaml compiler library")
  (libraries js_of_ocaml_compiler)
  (flags
-  (:standard -w -7-37 -safe-string -open Js_of_ocaml_compiler)))
+  (:standard -w -7-37 -safe-string -open Js_of_ocaml_compiler))
+ (preprocess
+  (pps sedlex.ppx)))
diff --git a/compiler/lib-wasm/gc_target.ml b/compiler/lib-wasm/gc_target.ml
index 7a1bd27867..cda5f4a92c 100644
--- a/compiler/lib-wasm/gc_target.ml
+++ b/compiler/lib-wasm/gc_target.ml
@@ -573,9 +573,13 @@ module Value = struct
        return ())
       (val_int (if negate then Arith.eqz n else n))
 
-  let eq x y = eq_gen ~negate:false x y
+  let eq x y =
+    if Config.Flag.wasi () then val_int (ref_eq x y) else eq_gen ~negate:false x y
 
-  let neq x y = eq_gen ~negate:true x y
+  let neq x y =
+    if Config.Flag.wasi ()
+    then val_int (Arith.eqz (ref_eq x y))
+    else eq_gen ~negate:true x y
 
   let ult = binop Arith.(ult)
 
@@ -1294,7 +1298,12 @@ module Math = struct
     { W.params = List.init ~len:n ~f:(fun _ : W.value_type -> F64); result = [ F64 ] }
 
   let unary name x =
-    let* f = register_import ~import_module:"Math" ~name (Fun (float_func_type 1)) in
+    let* f =
+      register_import
+        ~import_module:(if Config.Flag.wasi () then "env" else "Math")
+        ~name
+        (Fun (float_func_type 1))
+    in
     let* x = x in
     return (W.Call (f, [ x ]))
 
@@ -1337,7 +1346,12 @@ module Math = struct
   let log10 f = unary "log10" f
 
   let binary name x y =
-    let* f = register_import ~import_module:"Math" ~name (Fun (float_func_type 2)) in
+    let* f =
+      register_import
+        ~import_module:(if Config.Flag.wasi () then "env" else "Math")
+        ~name
+        (Fun (float_func_type 2))
+    in
     let* x = x in
     let* y = y in
     return (W.Call (f, [ x; y ]))
@@ -1676,21 +1690,34 @@ let handle_exceptions ~result_typ ~fall_through ~context body x exn_handler =
          x
          (block_expr
             { params = []; result = [ Value.value ] }
-            (let* exn =
-               block_expr
-                 { params = []; result = [ externref ] }
-                 (let* e =
-                    try_expr
-                      { params = []; result = [ externref ] }
-                      (body
-                         ~result_typ:[ externref ]
-                         ~fall_through:`Skip
-                         ~context:(`Skip :: `Skip :: `Catch :: context))
-                      [ ocaml_tag, 1, Value.value; js_tag, 0, externref ]
-                  in
-                  instr (W.Push e))
-             in
-             instr (W.CallInstr (f, [ exn ]))))
+            (if Config.Flag.wasi ()
+             then
+               let* e =
+                 try_expr
+                   { params = []; result = [ Value.value ] }
+                   (body
+                      ~result_typ:[ Value.value ]
+                      ~fall_through:`Skip
+                      ~context:(`Skip :: `Catch :: context))
+                   [ ocaml_tag, 0, Value.value ]
+               in
+               instr (W.Push e)
+             else
+               let* exn =
+                 block_expr
+                   { params = []; result = [ externref ] }
+                   (let* e =
+                      try_expr
+                        { params = []; result = [ externref ] }
+                        (body
+                           ~result_typ:[ externref ]
+                           ~fall_through:`Skip
+                           ~context:(`Skip :: `Skip :: `Catch :: context))
+                        [ ocaml_tag, 1, Value.value; js_tag, 0, externref ]
+                    in
+                    instr (W.Push e))
+               in
+               instr (W.CallInstr (f, [ exn ]))))
      in
      let* () = no_event in
      exn_handler ~result_typ ~fall_through ~context)
diff --git a/compiler/lib-wasm/generate.ml b/compiler/lib-wasm/generate.ml
index 308f8d5602..cb1eefcfea 100644
--- a/compiler/lib-wasm/generate.ml
+++ b/compiler/lib-wasm/generate.ml
@@ -1078,6 +1078,35 @@ module Generate (Target : Target_sig.S) = struct
       :: context.other_fields;
     name
 
+  let add_missing_primitives ~context l =
+    let failwith_desc = W.Fun { params = [ Value.value ]; result = [] } in
+    List.iter l ~f:(fun (exported_name, arity) ->
+        let name = Code.Var.fresh_n exported_name in
+        let locals, body =
+          function_body
+            ~context
+            ~param_names:[]
+            ~body:
+              (let* failwith =
+                 register_import ~import_module:"env" ~name:"caml_failwith" failwith_desc
+               in
+               let* msg =
+                 Constant.translate (String (exported_name ^ " not implemented"))
+               in
+               let* () = instr (CallInstr (failwith, [ msg ])) in
+               push Value.unit)
+        in
+        context.other_fields <-
+          W.Function
+            { name
+            ; exported_name = Some exported_name
+            ; typ = func_type arity
+            ; param_names = []
+            ; locals
+            ; body
+            }
+          :: context.other_fields)
+
   let entry_point context toplevel_fun entry_name =
     let typ, param_names, body = entry_point ~toplevel_fun in
     let locals, body = function_body ~context ~param_names ~body in
@@ -1238,6 +1267,10 @@ let add_init_function =
   let module G = Generate (Gc_target) in
   G.add_init_function
 
+let add_missing_primitives =
+  let module G = Generate (Gc_target) in
+  G.add_missing_primitives
+
 let output ch ~context =
   let module G = Generate (Gc_target) in
   let fields = G.output ~context in
diff --git a/compiler/lib-wasm/generate.mli b/compiler/lib-wasm/generate.mli
index 773917310b..d183c865c0 100644
--- a/compiler/lib-wasm/generate.mli
+++ b/compiler/lib-wasm/generate.mli
@@ -34,6 +34,9 @@ val add_start_function : context:Code_generation.context -> Wasm_ast.var -> unit
 
 val add_init_function : context:Code_generation.context -> to_link:string list -> unit
 
+val add_missing_primitives :
+  context:Code_generation.context -> (string * int) list -> unit
+
 val output : out_channel -> context:Code_generation.context -> unit
 
 val wasm_output : out_channel -> context:Code_generation.context -> unit
diff --git a/compiler/lib-wasm/link.ml b/compiler/lib-wasm/link.ml
index b5822d31ea..81ec5b5954 100644
--- a/compiler/lib-wasm/link.ml
+++ b/compiler/lib-wasm/link.ml
@@ -104,6 +104,19 @@ module Wasm_binary = struct
     if not (String.equal s header)
     then failwith (file ^ " is not a Wasm binary file (bad magic)")
 
+  let check ~contents = String.starts_with ~prefix:header contents
+
+  let check_file ~file =
+    let ch = open_in file in
+    let res =
+      try
+        let s = really_input_string ch 8 in
+        String.equal s header
+      with End_of_file -> false
+    in
+    close_in ch;
+    res
+
   type t =
     { ch : in_channel
     ; limit : int
@@ -160,7 +173,20 @@ module Wasm_binary = struct
 
   let reftype' i ch =
     match i with
-    | 0x6a | 0x6b | 0x6c | 0x6d | 0x6e | 0x6f | 0x70 | 0x71 | 0x72 | 0x73 -> ()
+    | 0x68
+    | 0x69
+    | 0x6a
+    | 0x6b
+    | 0x6c
+    | 0x6d
+    | 0x6e
+    | 0x6f
+    | 0x70
+    | 0x71
+    | 0x72
+    | 0x73
+    | 0x74
+    | 0x75 -> ()
     | 0x63 | 0x64 -> heaptype ch
     | _ ->
         Format.eprintf "Unknown reftype %x@." i;
@@ -168,12 +194,13 @@ module Wasm_binary = struct
 
   let reftype ch = reftype' (input_byte ch) ch
 
-  let valtype ch =
-    let i = read_uint ch in
+  let valtype' i ch =
     match i with
-    | 0x7b | 0x7c | 0x7d | 0x7e | 0x7f -> ()
+    | 0x7B | 0x7C | 0x7D | 0x7E | 0x7F -> ()
     | _ -> reftype' i ch
 
+  let valtype ch = valtype' (read_uint ch) ch
+
   let limits ch =
     match input_byte ch with
     | 0 -> ignore (read_uint ch)
@@ -188,32 +215,99 @@ module Wasm_binary = struct
     reftype ch;
     limits ch
 
+  type comptype =
+    | Func of { arity : int }
+    | Struct
+    | Array
+    | Cont
+
+  let supertype ch =
+    match input_byte ch with
+    | 0 -> ()
+    | 1 -> ignore (read_uint ch)
+    | _ -> assert false
+
+  let storagetype ch =
+    let i = read_uint ch in
+    match i with
+    | 0x78 | 0x77 -> ()
+    | _ -> valtype' i ch
+
+  let fieldtype ch =
+    storagetype ch;
+    ignore (input_byte ch)
+
+  let comptype i ch =
+    match i with
+    | 0x5D ->
+        ignore (read_sint ch);
+        Cont
+    | 0x5E ->
+        fieldtype ch;
+        Array
+    | 0x5F ->
+        ignore (vec fieldtype ch);
+        Struct
+    | 0x60 ->
+        let params = vec valtype ch in
+        let _ = vec valtype ch in
+        Func { arity = List.length params }
+    | c -> failwith (Printf.sprintf "Unknown comptype %d" c)
+
+  let subtype i ch =
+    match i with
+    | 0x50 ->
+        supertype ch;
+        comptype (input_byte ch) ch
+    | 0x4F ->
+        supertype ch;
+        comptype (input_byte ch) ch
+    | _ -> comptype i ch
+
+  let rectype ch =
+    match input_byte ch with
+    | 0x4E -> vec (fun ch -> subtype (input_byte ch) ch) ch
+    | i -> [ subtype i ch ]
+
+  type importdesc =
+    | Func of int
+    | Table
+    | Mem
+    | Global
+    | Tag
+
   type import =
     { module_ : string
     ; name : string
+    ; desc : importdesc
     }
 
   let import ch =
     let module_ = name ch in
     let name = name ch in
     let d = read_uint ch in
-    let _ =
+    let desc =
       match d with
-      | 0 -> ignore (read_uint ch)
-      | 1 -> tabletype ch
-      | 2 -> memtype ch
+      | 0 -> Func (read_uint ch)
+      | 1 ->
+          tabletype ch;
+          Table
+      | 2 ->
+          memtype ch;
+          Mem
       | 3 ->
           let _typ = valtype ch in
           let _mut = input_byte ch in
-          ()
+          Global
       | 4 ->
           assert (read_uint ch = 0);
-          ignore (read_uint ch)
+          ignore (read_uint ch);
+          Tag
       | _ ->
           Format.eprintf "Unknown import %x@." d;
           assert false
     in
-    { module_; name }
+    { module_; name; desc }
 
   let export ch =
     let name = name ch in
@@ -243,6 +337,7 @@ module Wasm_binary = struct
   type interface =
     { imports : import list
     ; exports : string list
+    ; types : comptype array
     }
 
   let read_interface ch =
@@ -250,7 +345,11 @@ module Wasm_binary = struct
       match next_section ch with
       | None -> i
       | Some s ->
-          if s.id = 2
+          if s.id = 1
+          then
+            find_sections
+              { i with types = Array.of_list (List.flatten (vec rectype ch.ch)) }
+          else if s.id = 2
           then find_sections { i with imports = vec import ch.ch }
           else if s.id = 7
           then { i with exports = vec export ch.ch }
@@ -258,7 +357,7 @@ module Wasm_binary = struct
             skip_section ch s;
             find_sections i)
     in
-    find_sections { imports = []; exports = [] }
+    find_sections { imports = []; exports = []; types = [||] }
 
   let append_source_map_section ~file ~url =
     let ch = open_out_gen [ Open_wronly; Open_append; Open_binary ] 0o666 file in
@@ -392,6 +491,13 @@ let generate_start_function ~to_link ~out_file =
   Generate.wasm_output ch ~context;
   if times () then Format.eprintf "    generate start: %a@." Timer.print t1
 
+let generate_missing_primitives ~missing_primitives ~out_file =
+  Filename.gen_file out_file
+  @@ fun ch ->
+  let context = Generate.start () in
+  Generate.add_missing_primitives ~context missing_primitives;
+  Generate.wasm_output ch ~context
+
 let output_js js =
   Code.Var.reset ();
   let b = Buffer.create 1024 in
@@ -652,17 +758,20 @@ let compute_dependencies ~files_to_link ~files =
 
 let compute_missing_primitives (runtime_intf, intfs) =
   let provided_primitives = StringSet.of_list runtime_intf.Wasm_binary.exports in
-  StringSet.elements
+  StringMap.bindings
   @@ List.fold_left
-       ~f:(fun s { Wasm_binary.imports; _ } ->
+       ~f:(fun s { Wasm_binary.imports; types; _ } ->
          List.fold_left
-           ~f:(fun s { Wasm_binary.module_; name; _ } ->
-             if String.equal module_ "env" && not (StringSet.mem name provided_primitives)
-             then StringSet.add name s
-             else s)
+           ~f:(fun s { Wasm_binary.module_; name; desc } ->
+             match module_, desc with
+             | "env", Func idx when not (StringSet.mem name provided_primitives) -> (
+                 match types.(idx) with
+                 | Func { arity } -> StringMap.add name arity s
+                 | _ -> s)
+             | _ -> s)
            ~init:s
            imports)
-       ~init:StringSet.empty
+       ~init:StringMap.empty
        intfs
 
 let load_information files =
@@ -698,6 +807,69 @@ let gen_dir dir f =
     remove_directory d_tmp;
     raise exc
 
+let link_to_module ~to_link ~files_to_link ~files ~enable_source_maps:_ ~dir =
+  let process_file ~name ~module_name file =
+    Zip.with_open_in file
+    @@ fun z ->
+    let intf =
+      let ch, pos, len, _ = Zip.get_entry z ~name in
+      Wasm_binary.read_interface (Wasm_binary.from_channel ~name ch pos len)
+    in
+    ( { Wasm_link.module_name
+      ; file
+      ; code = Some (Zip.read_entry z ~name)
+      ; opt_source_map = None
+      }
+    , intf )
+  in
+  let runtime_file = fst (List.hd files) in
+  let z = Zip.open_in runtime_file in
+  let runtime, runtime_intf =
+    process_file ~name:"runtime.wasm" ~module_name:"env" runtime_file
+  in
+  let prelude =
+    { Wasm_link.module_name = "OCaml"
+    ; file = runtime_file
+    ; code = Some (Zip.read_entry z ~name:"prelude.wasm")
+    ; opt_source_map = None
+    }
+  in
+  Zip.close_in z;
+  let lst =
+    List.tl files
+    |> List.filter_map ~f:(fun (file, _) ->
+           if StringSet.mem file files_to_link
+           then Some (process_file ~name:"code.wasm" ~module_name:"OCaml" file)
+           else None)
+  in
+  let missing_primitives =
+    if Config.Flag.genprim ()
+    then compute_missing_primitives (runtime_intf, List.map ~f:snd lst)
+    else []
+  in
+  let start_module = Filename.concat dir "start.wasm" in
+  generate_start_function ~to_link ~out_file:start_module;
+  let start =
+    { Wasm_link.module_name = "OCaml"
+    ; file = start_module
+    ; code = None
+    ; opt_source_map = None
+    }
+  in
+  generate_missing_primitives ~missing_primitives ~out_file:"stubs.wasm";
+  let missing_primitives =
+    { Wasm_link.module_name = "env"
+    ; file = "stubs.wasm"
+    ; code = None
+    ; opt_source_map = None
+    }
+  in
+  ignore
+    (Wasm_link.f
+       (runtime :: prelude :: missing_primitives :: start :: List.map ~f:fst lst)
+       ~filter_export:(fun nm -> String.equal nm "_start" || String.equal nm "memory")
+       ~output_file:(Filename.concat dir "code.wasm"))
+
 let link ~output_file ~linkall ~enable_source_maps ~files =
   if times () then Format.eprintf "linking@.";
   let t = Timer.make () in
@@ -788,30 +960,35 @@ let link ~output_file ~linkall ~enable_source_maps ~files =
   if times () then Format.eprintf "    finding what to link: %a@." Timer.print t1;
   if times () then Format.eprintf "  scan: %a@." Timer.print t;
   let t = Timer.make () in
-  let interfaces, wasm_dir, link_spec =
+  let missing_primitives, wasm_dir, link_spec =
     let dir = Filename.chop_extension output_file ^ ".assets" in
     gen_dir dir
     @@ fun tmp_dir ->
     Sys.mkdir tmp_dir 0o777;
-    let start_module =
-      "start-"
-      ^ String.sub
-          (Digest.to_hex (Digest.string (String.concat ~sep:"/" to_link)))
-          ~pos:0
-          ~len:8
-    in
-    generate_start_function
-      ~to_link
-      ~out_file:(Filename.concat tmp_dir (start_module ^ ".wasm"));
-    let module_names, interfaces =
-      link_to_directory ~files_to_link ~files ~enable_source_maps ~dir:tmp_dir
-    in
-    ( interfaces
-    , dir
-    , let to_link = compute_dependencies ~files_to_link ~files in
-      List.combine module_names (None :: None :: to_link) @ [ start_module, None ] )
+    if not (Config.Flag.wasi ())
+    then (
+      let start_module =
+        "start-"
+        ^ String.sub
+            (Digest.to_hex (Digest.string (String.concat ~sep:"/" to_link)))
+            ~pos:0
+            ~len:8
+      in
+      let module_names, interfaces =
+        link_to_directory ~files_to_link ~files ~enable_source_maps ~dir:tmp_dir
+      in
+      let missing_primitives = compute_missing_primitives interfaces in
+      generate_start_function
+        ~to_link
+        ~out_file:(Filename.concat tmp_dir (start_module ^ ".wasm"));
+      ( List.map ~f:fst missing_primitives
+      , dir
+      , let to_link = compute_dependencies ~files_to_link ~files in
+        List.combine module_names (None :: None :: to_link) @ [ start_module, None ] ))
+    else (
+      link_to_module ~to_link ~files_to_link ~files ~enable_source_maps ~dir:tmp_dir;
+      [], dir, [ "code", None ])
   in
-  let missing_primitives = compute_missing_primitives interfaces in
   if times () then Format.eprintf "    copy wasm files: %a@." Timer.print t;
   let t1 = Timer.make () in
   let js_runtime =
diff --git a/compiler/lib-wasm/link.mli b/compiler/lib-wasm/link.mli
index 9ad39a4244..55424663f1 100644
--- a/compiler/lib-wasm/link.mli
+++ b/compiler/lib-wasm/link.mli
@@ -19,11 +19,23 @@
 open Stdlib
 
 module Wasm_binary : sig
+  type importdesc =
+    | Func of int
+    | Table
+    | Mem
+    | Global
+    | Tag
+
   type import =
     { module_ : string
     ; name : string
+    ; desc : importdesc
     }
 
+  val check : contents:string -> bool
+
+  val check_file : file:string -> bool
+
   val read_imports : file:string -> import list
 
   val append_source_map_section : file:string -> url:string -> unit
diff --git a/compiler/lib-wasm/runtime.ml b/compiler/lib-wasm/runtime.ml
new file mode 100644
index 0000000000..7402dd69c7
--- /dev/null
+++ b/compiler/lib-wasm/runtime.ml
@@ -0,0 +1,21 @@
+open Stdlib
+
+let build ~link_options ~opt_options ~variables ~inputs ~output_file =
+  Fs.with_intermediate_file (Filename.temp_file "runtime-merged" ".wasm")
+  @@ fun merge_file ->
+  (Wat_preprocess.with_preprocessed_files ~variables ~inputs
+  @@ fun inputs ->
+  Binaryen.link
+    ~options:link_options
+    ~opt_output_sourcemap:None
+    ~inputs
+    ~output_file:merge_file
+    ());
+  Binaryen.optimize
+    ~profile:None
+    ~options:opt_options
+    ~opt_input_sourcemap:None
+    ~input_file:merge_file
+    ~opt_output_sourcemap:None
+    ~output_file
+    ()
diff --git a/compiler/lib-wasm/runtime.mli b/compiler/lib-wasm/runtime.mli
new file mode 100644
index 0000000000..2ba99e5e59
--- /dev/null
+++ b/compiler/lib-wasm/runtime.mli
@@ -0,0 +1,7 @@
+val build :
+     link_options:string list
+  -> opt_options:string list
+  -> variables:(string * Wat_preprocess.value) list
+  -> inputs:Wat_preprocess.input list
+  -> output_file:string
+  -> unit
diff --git a/compiler/lib-wasm/wasm_link.ml b/compiler/lib-wasm/wasm_link.ml
index a53fc34a4d..13a593c5b6 100644
--- a/compiler/lib-wasm/wasm_link.ml
+++ b/compiler/lib-wasm/wasm_link.ml
@@ -23,6 +23,10 @@ type heaptype =
   | Nofunc
   | Extern
   | Noextern
+  | Exn
+  | Noexn
+  | Cont
+  | Nocont
   | Any
   | Eq
   | I31
@@ -66,6 +70,7 @@ type comptype =
       }
   | Struct of fieldtype array
   | Array of fieldtype
+  | Cont of int
 
 type subtype =
   { final : bool
@@ -147,6 +152,8 @@ module Write = struct
 
   let heaptype st ch typ =
     match (typ : heaptype) with
+    | Nocont -> byte ch 0x75
+    | Noexn -> byte ch 0x74
     | Nofunc -> byte ch 0x73
     | Noextern -> byte ch 0x72
     | None_ -> byte ch 0x71
@@ -157,6 +164,8 @@ module Write = struct
     | I31 -> byte ch 0x6C
     | Struct -> byte ch 0x6B
     | Array -> byte ch 0x6A
+    | Exn -> byte ch 0x69
+    | Cont -> byte ch 0x68
     | Type idx -> sint ch (typeidx st idx)
 
   let reftype st ch { nullable; typ } =
@@ -202,6 +211,9 @@ module Write = struct
         byte ch 1;
         uint ch (typeidx st supertype));
     match typ with
+    | Cont idx ->
+        byte ch 0x5D;
+        sint ch (typeidx st idx)
     | Array field_type ->
         byte ch 0x5E;
         fieldtype st ch field_type
@@ -569,7 +581,9 @@ module Read = struct
   let heaptype st ch =
     let i = sint ch in
     match i + 128 with
-    | 0X73 -> Nofunc
+    | 0x75 -> Nocont
+    | 0x74 -> Noexn
+    | 0x73 -> Nofunc
     | 0x72 -> Noextern
     | 0x71 -> None_
     | 0x70 -> Func
@@ -579,6 +593,8 @@ module Read = struct
     | 0x6C -> I31
     | 0x6B -> Struct
     | 0x6A -> Array
+    | 0x69 -> Exn
+    | 0x68 -> Cont
     | _ ->
         if i < 0 then failwith (Printf.sprintf "Unknown heaptype %x@." i);
         let i =
@@ -596,7 +612,9 @@ module Read = struct
 
   let reftype' st i ch =
     match i with
-    | 0X73 -> nullable Nofunc
+    | 0x75 -> nullable Nocont
+    | 0x74 -> nullable Noexn
+    | 0x73 -> nullable Nofunc
     | 0x72 -> nullable Noextern
     | 0x71 -> nullable None_
     | 0x70 -> nullable Func
@@ -606,6 +624,8 @@ module Read = struct
     | 0x6C -> nullable I31
     | 0x6B -> nullable Struct
     | 0x6A -> nullable Array
+    | 0x69 -> nullable Exn
+    | 0x68 -> nullable Cont
     | 0x63 -> nullable (heaptype st ch)
     | 0x64 -> { nullable = false; typ = heaptype st ch }
     | _ -> failwith (Printf.sprintf "Unknown reftype %x@." i)
@@ -652,6 +672,14 @@ module Read = struct
 
   let comptype st i ch =
     match i with
+    | 0x5D ->
+        let i = sint ch in
+        let i =
+          if i >= st.type_index_count
+          then lnot (i - st.type_index_count)
+          else st.type_mapping.(i)
+        in
+        Cont i
     | 0x5E -> Array (fieldtype st ch)
     | 0x5F -> Struct (vec (fieldtype st) ch)
     | 0x60 ->
@@ -1252,6 +1280,13 @@ module Scan = struct
       | 0xD1 (* ref.is_null *) | 0xD3 (* ref.eq *) | 0xD4 (* ref.as_non_null *) ->
           pos + 1 |> instructions
       | 0xD2 (* ref.func *) -> pos + 1 |> funcidx |> instructions
+      | 0xE0 (* cont.new *) -> pos + 1 |> typeidx |> instructions
+      | 0xE1 (* cont.bind *) -> pos + 1 |> typeidx |> typeidx |> instructions
+      | 0xE2 (* suspend *) -> pos + 1 |> tagidx |> instructions
+      | 0xE3 (* resume *) -> pos + 1 |> typeidx |> vector on_clause |> instructions
+      | 0xE4 (* resume_throw *) ->
+          pos + 1 |> typeidx |> tagidx |> vector on_clause |> instructions
+      | 0xE5 (* switch *) -> pos + 1 |> typeidx |> tagidx |> instructions
       | 0xFB -> pos + 1 |> gc_instruction
       | 0xFC -> (
           if debug then Format.eprintf "  %d@." (get (pos + 1));
@@ -1386,6 +1421,11 @@ module Scan = struct
       | 0 (* catch *) | 1 (* catch_ref *) -> pos + 1 |> tagidx |> labelidx
       | 2 (* catch_all *) | 3 (* catch_all_ref *) -> pos + 1 |> labelidx
       | c -> failwith (Printf.sprintf "bad catch 0x02%d@." c)
+    and on_clause pos =
+      match get pos with
+      | 0 (* on *) -> pos + 1 |> tagidx |> labelidx
+      | 1 (* on .. switch *) -> pos + 1 |> tagidx
+      | c -> failwith (Printf.sprintf "bad on clause 0x02%d@." c)
     and block_end pos =
       if debug then Format.eprintf "0x%02X (@%d) block end@." (get pos) pos;
       match get pos with
@@ -1538,30 +1578,43 @@ let rec subtype subtyping_info (i : int) i' =
   | None -> false
   | Some s -> subtype subtyping_info s i'
 
-let heap_subtype (subtyping_info : subtype array) (ty : heaptype) (ty' : heaptype) =
+let rec heap_subtype (subtyping_info : subtype array) (ty : heaptype) (ty' : heaptype) =
   match ty, ty' with
-  | (Func | Nofunc), Func
-  | Nofunc, Nofunc
-  | (Extern | Noextern), Extern
+  | Func, Func
+  | Extern, Extern
+  | Noextern, Noextern
+  | Exn, Exn
+  | Noexn, Noexn
+  | Cont, Cont
+  | Nocont, Nocont
   | (Any | Eq | I31 | Struct | Array | None_ | Type _), Any
   | (Eq | I31 | Struct | Array | None_ | Type _), Eq
-  | (I31 | None_), I31
-  | (Struct | None_), Struct
-  | (Array | None_), Array
+  | I31, I31
+  | Struct, Struct
+  | Array, Array
   | None_, None_ -> true
   | Type i, Struct -> (
       match subtyping_info.(i).typ with
       | Struct _ -> true
-      | Array _ | Func _ -> false)
+      | Array _ | Func _ | Cont _ -> false)
   | Type i, Array -> (
       match subtyping_info.(i).typ with
       | Array _ -> true
-      | Struct _ | Func _ -> false)
+      | Struct _ | Func _ | Cont _ -> false)
   | Type i, Func -> (
       match subtyping_info.(i).typ with
       | Func _ -> true
-      | Struct _ | Array _ -> false)
+      | Struct _ | Array _ | Cont _ -> false)
+  | Type i, Cont -> (
+      match subtyping_info.(i).typ with
+      | Cont _ -> true
+      | Struct _ | Array _ | Func _ -> false)
   | Type i, Type i' -> subtype subtyping_info i i'
+  | Nofunc, _ -> heap_subtype subtyping_info ty' Func
+  | Noextern, _ -> heap_subtype subtyping_info ty' Extern
+  | Noexn, _ -> heap_subtype subtyping_info ty' Exn
+  | Nocont, _ -> heap_subtype subtyping_info ty' Cont
+  | None_, _ -> heap_subtype subtyping_info ty' Any
   | _ -> false
 
 let ref_subtype subtyping_info { nullable; typ } { nullable = nullable'; typ = typ' } =
@@ -1878,7 +1931,7 @@ type input =
   ; opt_source_map : Source_map.Standard.t option
   }
 
-let f files ~output_file =
+let f ?(filter_export = fun _ -> true) files ~output_file =
   let files =
     Array.map
       ~f:(fun { module_name; file; code; opt_source_map } ->
@@ -2132,20 +2185,28 @@ let f files ~output_file =
   Array.iter ~f:Scan.clear_position_data positions;
 
   (* 7: export *)
+  let exports =
+    Array.map
+      ~f:(fun intf ->
+        map_exportable_info
+          (fun _ exports -> List.filter ~f:(fun (nm, _) -> filter_export nm) exports)
+          intf.Read.exports)
+      intfs
+  in
   let export_count =
     Array.fold_left
-      ~f:(fun count intf ->
+      ~f:(fun count exports ->
         fold_exportable_info
           (fun _ exports count -> List.length exports + count)
           count
-          intf.Read.exports)
+          exports)
       ~init:0
-      intfs
+      exports
   in
   Write.uint buf export_count;
-  let exports = Hashtbl.create 128 in
+  let export_tbl = Hashtbl.create 128 in
   Array.iteri
-    ~f:(fun i intf ->
+    ~f:(fun i exports ->
       iter_exportable_info
         (fun kind lst ->
           let map =
@@ -2158,7 +2219,7 @@ let f files ~output_file =
           in
           List.iter
             ~f:(fun (name, idx) ->
-              match Hashtbl.find exports name with
+              match Hashtbl.find export_tbl name with
               | i' ->
                   failwith
                     (Printf.sprintf
@@ -2167,11 +2228,11 @@ let f files ~output_file =
                        files.(i').file
                        files.(i).file)
               | exception Not_found ->
-                  Hashtbl.add exports name i;
+                  Hashtbl.add export_tbl name i;
                   Write.export buf kind name map.(idx))
             lst)
-        intf.Read.exports)
-    intfs;
+        exports)
+    exports;
   add_section out_ch ~id:7 buf;
 
   (* 8: start *)
@@ -2441,7 +2502,6 @@ let f files ~output_file =
 (*
 LATER
 - testsuite : import/export matching, source maps, multiple start functions, ...
-- missing instructions ==> typed continuations (?)
 - check features?
 
 MAYBE
diff --git a/compiler/lib-wasm/wasm_link.mli b/compiler/lib-wasm/wasm_link.mli
index 0c0ed0a582..4cbd769668 100644
--- a/compiler/lib-wasm/wasm_link.mli
+++ b/compiler/lib-wasm/wasm_link.mli
@@ -23,4 +23,5 @@ type input =
   ; opt_source_map : Source_map.Standard.t option
   }
 
-val f : input list -> output_file:string -> Source_map.t
+val f :
+  ?filter_export:(string -> bool) -> input list -> output_file:string -> Source_map.t
diff --git a/compiler/lib-wasm/wat_output.ml b/compiler/lib-wasm/wat_output.ml
index 27c2307801..c5fe887b26 100644
--- a/compiler/lib-wasm/wat_output.ml
+++ b/compiler/lib-wasm/wat_output.ml
@@ -444,19 +444,23 @@ let expression_or_instructions ctx st in_function =
             @ [ List (Atom "else" :: expression iff) ])
         ]
     | Try (ty, body, catches) ->
-        [ List
-            (Atom "try"
-            :: (block_type st ty
-               @ List (Atom "do" :: instructions body)
-                 :: List.map
-                      ~f:(fun (tag, i, ty) ->
-                        List
-                          (Atom "catch"
-                          :: index st.tag_names tag
-                          :: (instruction (Wasm_ast.Event Code_generation.hidden_location)
-                             @ instruction (Wasm_ast.Br (i + 1, Some (Pop ty))))))
-                      catches))
-        ]
+        if Config.Flag.trap_on_exception ()
+        then [ List (Atom "block" :: (block_type st ty @ instructions body)) ]
+        else
+          [ List
+              (Atom "try"
+              :: (block_type st ty
+                 @ List (Atom "do" :: instructions body)
+                   :: List.map
+                        ~f:(fun (tag, i, ty) ->
+                          List
+                            (Atom "catch"
+                            :: index st.tag_names tag
+                            :: (instruction
+                                  (Wasm_ast.Event Code_generation.hidden_location)
+                               @ instruction (Wasm_ast.Br (i + 1, Some (Pop ty))))))
+                        catches))
+          ]
   and instruction i =
     match i with
     | Drop e -> [ List (Atom "drop" :: expression e) ]
@@ -499,8 +503,14 @@ let expression_or_instructions ctx st in_function =
             | None -> []
             | Some e -> expression e))
         ]
-    | Throw (tag, e) -> [ List (Atom "throw" :: index st.tag_names tag :: expression e) ]
-    | Rethrow i -> [ List [ Atom "rethrow"; Atom (string_of_int i) ] ]
+    | Throw (tag, e) ->
+        if Config.Flag.trap_on_exception ()
+        then [ List [ Atom "unreachable" ] ]
+        else [ List (Atom "throw" :: index st.tag_names tag :: expression e) ]
+    | Rethrow i ->
+        if Config.Flag.trap_on_exception ()
+        then [ List [ Atom "unreachable" ] ]
+        else [ List [ Atom "rethrow"; Atom (string_of_int i) ] ]
     | CallInstr (f, l) ->
         [ List
             (Atom "call"
diff --git a/compiler/lib-wasm/wat_preprocess.ml b/compiler/lib-wasm/wat_preprocess.ml
new file mode 100644
index 0000000000..63d6ce675d
--- /dev/null
+++ b/compiler/lib-wasm/wat_preprocess.ml
@@ -0,0 +1,675 @@
+open Stdlib
+
+exception Error of (Lexing.position * Lexing.position) * string
+
+let report_error loc msg =
+  let location = MenhirLib.LexerUtil.range loc in
+  Format.eprintf "%s%s%!" location msg;
+  exit 1
+
+(****)
+
+(*
+See the WebAssembly Text Format Specification:
+https://webassembly.github.io/spec/core/text/index.html
+
+We use custom annotations to extend the syntax
+(https://github.com/WebAssembly/annotations).
+*)
+
+let digit = [%sedlex.regexp? '0' .. '9']
+
+let hexdigit = [%sedlex.regexp? '0' .. '9' | 'a' .. 'f' | 'A' .. 'F']
+
+let num = [%sedlex.regexp? digit, Star (Opt '_', digit)]
+
+let hexnum = [%sedlex.regexp? hexdigit, Star (Opt '_', hexdigit)]
+
+let uN = [%sedlex.regexp? num | "0x", hexnum]
+
+let idchar =
+  [%sedlex.regexp?
+    ( '0' .. '9'
+    | 'A' .. 'Z'
+    | 'a' .. 'z'
+    | '!'
+    | '#'
+    | '$'
+    | '%'
+    | '&'
+    | '\''
+    | '*'
+    | '+'
+    | '-'
+    | '.'
+    | '/'
+    | ':'
+    | '<'
+    | '='
+    | '>'
+    | '?'
+    | '@'
+    | '\\'
+    | '^'
+    | '_'
+    | '`'
+    | '|'
+    | '~' )]
+
+let id = [%sedlex.regexp? '$', Plus idchar]
+
+let linechar = [%sedlex.regexp? Sub (any, (10 | 13))]
+
+let newline = [%sedlex.regexp? 10 | 13 | 13, 10]
+
+let linecomment = [%sedlex.regexp? ";;", Star linechar, (newline | eof)]
+
+let keyword = [%sedlex.regexp? Plus idchar]
+
+let rec comment start_pos lexbuf =
+  match%sedlex lexbuf with
+  | ";)" -> ()
+  | "(;" ->
+      comment (Sedlexing.lexing_positions lexbuf) lexbuf;
+      comment start_pos lexbuf
+  | ';' | '(' | Plus (Sub (any, (';' | '('))) -> comment start_pos lexbuf
+  | _ -> raise (Error (start_pos, Printf.sprintf "Unclosed comment.\n"))
+
+let string_buffer = Buffer.create 256
+
+let rec string lexbuf =
+  match%sedlex lexbuf with
+  | '"' ->
+      Buffer.add_string string_buffer (Sedlexing.Utf8.lexeme lexbuf);
+      let s = Buffer.contents string_buffer in
+      Buffer.clear string_buffer;
+      s
+  | Plus
+      ( Sub (any, (0 .. 31 | 0x7f | '"' | '\\'))
+      | "\\t" | "\\n" | "\\r" | "\\'" | "\\\"" | "\\\\"
+      | '\\', hexdigit, hexdigit
+      | "\\u{", hexnum, "}" ) ->
+      Buffer.add_string string_buffer (Sedlexing.Utf8.lexeme lexbuf);
+      string lexbuf
+  | _ ->
+      raise
+        (Error (Sedlexing.lexing_positions lexbuf, Printf.sprintf "Malformed string.\n"))
+
+type pos =
+  { loc : Lexing.position
+  ; byte_loc : int
+  }
+
+type token =
+  | LPAREN
+  | RPAREN
+  | ATOM of string
+  | EOF
+
+let locs lexbuf =
+  let loc, loc' = Sedlexing.lexing_positions lexbuf in
+  let byte_loc, byte_loc' = Sedlexing.bytes_loc lexbuf in
+  { loc; byte_loc }, { loc = loc'; byte_loc = byte_loc' }
+
+let position_of_loc (pos, pos') = pos.loc, pos'.loc
+
+let rec token lexbuf =
+  match%sedlex lexbuf with
+  | '(' -> LPAREN, locs lexbuf
+  | ')' -> RPAREN, locs lexbuf
+  | keyword -> ATOM (Sedlexing.Utf8.lexeme lexbuf), locs lexbuf
+  | '"' ->
+      let string_start =
+        { loc = Sedlexing.lexing_position_start lexbuf
+        ; byte_loc = Sedlexing.lexeme_bytes_start lexbuf
+        }
+      in
+      Buffer.add_char string_buffer '"';
+      let str = string lexbuf in
+      ( ATOM str
+      , ( string_start
+        , { loc = Sedlexing.lexing_position_curr lexbuf
+          ; byte_loc = Sedlexing.lexeme_bytes_end lexbuf
+          } ) )
+  | Plus (' ' | '\t' | newline | linecomment) -> token lexbuf
+  | "(;" ->
+      comment (Sedlexing.lexing_positions lexbuf) lexbuf;
+      token lexbuf
+  | ";)" ->
+      raise
+        (Error
+           ( Sedlexing.lexing_positions lexbuf
+           , Printf.sprintf "Unmatched closing comment.\n" ))
+  | eof -> EOF, locs lexbuf
+  | _ ->
+      raise (Error (Sedlexing.lexing_positions lexbuf, Printf.sprintf "Syntax error.\n"))
+
+type t =
+  { loc : pos * pos
+  ; desc : desc
+  }
+
+and desc =
+  | Atom of string
+  | List of t list
+
+let rec parse_list lexbuf toplevel start_loc acc =
+  let tok, loc = token lexbuf in
+  match tok with
+  | LPAREN ->
+      let lst, loc = parse_list lexbuf false loc [] in
+      parse_list lexbuf toplevel start_loc ({ desc = List lst; loc } :: acc)
+  | RPAREN ->
+      if toplevel
+      then
+        raise
+          (Error
+             ( Sedlexing.lexing_positions lexbuf
+             , Printf.sprintf "Unexpected closing parenthesis.\n" ));
+      List.rev acc, (fst start_loc, snd loc)
+  | EOF ->
+      if not toplevel
+      then
+        raise
+          (Error (position_of_loc start_loc, Printf.sprintf "Unclosed parenthesis.\n"));
+      List.rev acc, (fst start_loc, snd loc)
+  | ATOM s -> parse_list lexbuf toplevel start_loc ({ loc; desc = Atom s } :: acc)
+
+let parse lexbuf =
+  let pos =
+    { loc = Sedlexing.lexing_position_start lexbuf
+    ; byte_loc = Sedlexing.lexeme_bytes_end lexbuf
+    }
+  in
+  parse_list lexbuf true (pos, pos) []
+
+let is_unsigned_integer s =
+  let lexbuf = Sedlexing.Utf8.from_string s in
+  match%sedlex lexbuf with
+  | uN, eof -> true
+  | _ -> false
+
+let hexdigit c =
+  match c with
+  | '0' .. '9' -> Char.code c - Char.code '0'
+  | 'a' .. 'f' -> Char.code c - (Char.code 'a' - 10)
+  | 'A' .. 'F' -> Char.code c - (Char.code 'A' - 10)
+  | _ -> assert false
+
+let rec parse_string_contents loc lexbuf =
+  match%sedlex lexbuf with
+  | eof ->
+      let s = Buffer.contents string_buffer in
+      Buffer.clear string_buffer;
+      s
+  | Plus (Sub (any, (0 .. 31 | 0x7f | '"' | '\\'))) ->
+      Buffer.add_string string_buffer (Sedlexing.Utf8.lexeme lexbuf);
+      parse_string_contents loc lexbuf
+  | "\\t" ->
+      Buffer.add_char string_buffer '\t';
+      parse_string_contents loc lexbuf
+  | "\\n" ->
+      Buffer.add_char string_buffer '\n';
+      parse_string_contents loc lexbuf
+  | "\\r" ->
+      Buffer.add_char string_buffer '\r';
+      parse_string_contents loc lexbuf
+  | "\\'" ->
+      Buffer.add_char string_buffer '\'';
+      parse_string_contents loc lexbuf
+  | "\\\"" ->
+      Buffer.add_char string_buffer '"';
+      parse_string_contents loc lexbuf
+  | "\\\\" ->
+      Buffer.add_char string_buffer '\\';
+      parse_string_contents loc lexbuf
+  | '\\', hexdigit, hexdigit ->
+      let s = Sedlexing.Utf8.lexeme lexbuf in
+      assert (String.length s = 3);
+      Buffer.add_char string_buffer (Char.chr ((hexdigit s.[1] * 16) + hexdigit s.[2]));
+      parse_string_contents loc lexbuf
+  | "\\u{", hexnum, "}" -> (
+      match
+        let s = Sedlexing.Utf8.lexeme lexbuf in
+        int_of_string ("0x" ^ String.sub s ~pos:3 ~len:(String.length s - 4))
+      with
+      | c when Uchar.is_valid c ->
+          Buffer.add_utf_8_uchar string_buffer (Uchar.of_int c);
+          parse_string_contents loc lexbuf
+      | _ | (exception Failure _) ->
+          Buffer.clear string_buffer;
+          raise
+            (Error
+               (position_of_loc loc, Printf.sprintf "Invalid Unicode escape sequences.\n"))
+      )
+  | _ -> assert false
+
+let parse_string loc s =
+  parse_string_contents
+    loc
+    (Sedlexing.Utf8.from_string (String.sub s ~pos:1 ~len:(String.length s - 2)))
+
+let is_string s = String.length s > 0 && Char.equal s.[0] '"'
+
+let is_keyword s =
+  let lexbuf = Sedlexing.Utf8.from_string s in
+  match%sedlex lexbuf with
+  | keyword, eof -> true
+  | _ -> false
+
+let is_id s =
+  let lexbuf = Sedlexing.Utf8.from_string s in
+  match%sedlex lexbuf with
+  | id, eof -> true
+  | _ -> false
+
+(****)
+
+module StringMap = Map.Make (String)
+
+type typ =
+  | Bool
+  | String
+  | Version
+
+type value =
+  | Bool of bool
+  | String of string
+  | Version of int * int * int
+
+type st =
+  { text : string
+  ; mutable pos : pos
+  ; variables : value StringMap.t
+  ; buf : Buffer.t
+  }
+
+let value_type v : typ =
+  match v with
+  | Bool _ -> Bool
+  | String _ -> String
+  | Version _ -> Version
+
+let type_name (t : typ) =
+  match t with
+  | Bool -> "boolean"
+  | String -> "string"
+  | Version -> "version"
+
+let variable_is_set st nm =
+  match StringMap.find_opt nm st.variables with
+  | Some (Bool true) -> true
+  | _ -> false
+
+let check_type ?typ expr actual_typ =
+  match typ with
+  | None -> ()
+  | Some typ ->
+      if Poly.(actual_typ <> typ)
+      then
+        raise
+          (Error
+             ( position_of_loc expr.loc
+             , Printf.sprintf
+                 "Expected a %s but this is a %s.\n"
+                 (type_name typ)
+                 (type_name actual_typ) ))
+
+let rec eval ?typ st expr =
+  match expr with
+  | { desc = Atom s; loc } when is_string s ->
+      check_type ?typ expr String;
+      String (parse_string loc s)
+  | { desc = Atom s; loc } when is_keyword s ->
+      if not (StringMap.mem s st.variables)
+      then
+        raise (Error (position_of_loc loc, Printf.sprintf "Unknown variable '%s'.\n" s));
+      let res = StringMap.find s st.variables in
+      check_type ?typ expr (value_type res);
+      res
+  | { desc =
+        List
+          [ { desc = Atom major; _ }
+          ; { desc = Atom minor; _ }
+          ; { desc = Atom patchlevel; _ }
+          ]
+    ; _
+    }
+    when is_unsigned_integer major
+         && is_unsigned_integer minor
+         && is_unsigned_integer patchlevel ->
+      check_type ?typ expr Version;
+      Version (int_of_string major, int_of_string minor, int_of_string patchlevel)
+  | { desc = List ({ desc = Atom "and"; _ } :: lst); _ } ->
+      check_type ?typ expr Bool;
+      Bool (List.for_all ~f:(fun expr' -> eval_bool st expr') lst)
+  | { desc = List ({ desc = Atom "or"; _ } :: lst); _ } ->
+      check_type ?typ expr Bool;
+      Bool (List.exists ~f:(fun expr' -> eval_bool st expr') lst)
+  | { desc = List [ { desc = Atom "not"; _ }; expr' ]; _ } ->
+      check_type ?typ expr Bool;
+      Bool (not (eval_bool st expr'))
+  | { desc =
+        List ({ desc = Atom (("=" | "<" | ">" | "<=" | ">=" | "<>") as op); _ } :: args)
+    ; loc
+    } -> bin_op st ?typ loc op args
+  | { loc; _ } -> raise (Error (position_of_loc loc, Printf.sprintf "Syntax error.\n"))
+
+and eval_bool st expr =
+  match eval ~typ:Bool st expr with
+  | Bool b -> b
+  | _ -> assert false
+
+and bin_op st ?typ loc op args =
+  match args with
+  | [ expr; expr' ] ->
+      check_type ?typ expr Bool;
+      let v = eval st expr in
+      let v' = eval ~typ:(value_type v) st expr' in
+      Bool
+        Poly.(
+          match op with
+          | "=" -> v = v'
+          | "<" -> v < v'
+          | ">" -> v > v'
+          | "<=" -> v <= v'
+          | ">=" -> v >= v'
+          | "<>" -> v <> v'
+          | _ -> assert false)
+  | _ -> raise (Error (position_of_loc loc, Printf.sprintf "Syntax error.\n"))
+
+(****)
+
+let write st pos' =
+  Buffer.add_substring st.buf st.text st.pos.byte_loc (pos'.byte_loc - st.pos.byte_loc);
+  st.pos <- pos'
+
+let skip st (pos' : pos) =
+  let lines = pos'.loc.pos_lnum - st.pos.loc.pos_lnum in
+  let cols =
+    pos'.loc.pos_cnum
+    - pos'.loc.pos_bol
+    - if lines > 0 then 0 else st.pos.loc.pos_cnum - st.pos.loc.pos_bol
+  in
+  Buffer.add_string st.buf (String.make (max 0 lines) '\n');
+  Buffer.add_string st.buf (String.make (max 0 cols) ' ');
+  st.pos <- pos'
+
+let insert st s =
+  Buffer.add_string st.buf s;
+  let n = String.length s in
+  st.pos <-
+    { loc = { st.pos.loc with pos_cnum = st.pos.loc.pos_cnum + n }
+    ; byte_loc = st.pos.byte_loc - 1
+    }
+
+let pred_position { loc; byte_loc } =
+  { loc = { loc with pos_cnum = loc.pos_cnum - 1 }; byte_loc = byte_loc - 1 }
+
+let rec rewrite_list st l = List.iter ~f:(rewrite st) l
+
+and rewrite st elt =
+  match elt with
+  | { desc =
+        List
+          ({ desc = Atom "try"; _ }
+          :: { desc = List ({ desc = Atom "result"; _ } :: _)
+             ; loc = pos_before_result, pos_after_result
+             }
+          :: { desc = List ({ desc = Atom "do"; loc = _, pos_after_do } :: body)
+             ; loc = _, pos_after_body
+             }
+          :: _)
+    ; loc = pos, pos'
+    }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      Buffer.add_string st.buf "(block";
+      skip st pos_before_result;
+      write st pos_after_result;
+      skip st pos_after_do;
+      rewrite_list st body;
+      write st pos_after_body;
+      skip st pos'
+  | { desc =
+        List
+          ({ desc = Atom "try"; _ }
+          :: { desc = List ({ desc = Atom "do"; loc = _, pos_after_do } :: body)
+             ; loc = _, pos_after_body
+             }
+          :: _)
+    ; loc = pos, pos'
+    }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      Buffer.add_string st.buf "(block";
+      skip st pos_after_do;
+      rewrite_list st body;
+      write st pos_after_body;
+      skip st pos'
+  | { desc = List ({ desc = Atom "throw"; _ } :: _); loc = pos, pos' }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      Buffer.add_string st.buf "(unreachable)";
+      skip st pos'
+  | { desc = List ({ desc = Atom "tag"; _ } :: _); loc = pos, pos' }
+  | { desc =
+        List
+          ({ desc = Atom "import"; _ }
+          :: _
+          :: _
+          :: { desc = List ({ desc = Atom "tag"; _ } :: _); _ }
+          :: _)
+    ; loc = pos, pos'
+    }
+  | { desc =
+        List
+          ({ desc = Atom "export"; _ }
+          :: _
+          :: { desc = List ({ desc = Atom "tag"; _ } :: _); _ }
+          :: _)
+    ; loc = pos, pos'
+    }
+    when variable_is_set st "trap-on-exception" ->
+      write st pos;
+      skip st pos'
+  | { desc =
+        List
+          [ { desc = Atom "@if"; _ }
+          ; expr
+          ; { desc = List ({ desc = Atom "@then"; loc = _, pos_after_then } :: then_body)
+            ; loc = _, pos_after_then_body
+            }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      if eval_bool st expr
+      then (
+        skip st pos_after_then;
+        rewrite_list st then_body;
+        write st (pred_position pos_after_then_body);
+        skip st pos')
+      else skip st pos'
+  | { desc =
+        List
+          [ { desc = Atom "@if"; _ }
+          ; expr
+          ; { desc = List ({ desc = Atom "@then"; loc = _, pos_after_then } :: then_body)
+            ; loc = _, pos_after_then_body
+            }
+          ; { desc = List ({ desc = Atom "@else"; loc = _, pos_after_else } :: else_body)
+            ; loc = _, pos_after_else_body
+            }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      if eval_bool st expr
+      then (
+        skip st pos_after_then;
+        rewrite_list st then_body;
+        write st (pred_position pos_after_then_body);
+        skip st pos')
+      else (
+        skip st pos_after_else;
+        rewrite_list st else_body;
+        write st (pred_position pos_after_else_body);
+        skip st pos')
+  | { desc =
+        List
+          ({ desc = Atom "@if"; _ }
+          :: _
+          :: { desc = List ({ desc = Atom "@then"; _ } :: _); _ }
+          :: { desc = List ({ desc = Atom "@else"; _ } :: _); _ }
+          :: { loc; _ }
+          :: _)
+    ; _
+    } ->
+      raise
+        (Error (position_of_loc loc, Printf.sprintf "Expecting closing parenthesis.\n"))
+  | { desc =
+        List
+          ({ desc = Atom "@if"; _ }
+          :: _
+          :: { desc = List ({ desc = Atom "@then"; _ } :: _); _ }
+          :: { loc; _ }
+          :: _)
+    ; _
+    } ->
+      raise
+        (Error
+           ( position_of_loc loc
+           , Printf.sprintf "Expecting @else clause or closing parenthesis.\n" ))
+  | { desc = List ({ desc = Atom "@if"; _ } :: _ :: { loc = pos, pos'; _ } :: _); _ }
+  | { desc = List [ { desc = Atom "@if"; _ }; { loc = _, pos; _ } ]; loc = _, pos' } ->
+      raise (Error ((pos.loc, pos'.loc), Printf.sprintf "Expecting @then clause.\n"))
+  | { desc = List [ { desc = Atom "@if"; loc = _, pos } ]; loc = _, pos' } ->
+      raise (Error ((pos.loc, pos'.loc), Printf.sprintf "Expecting condition.\n"))
+  | { desc = List ({ desc = Atom (("@then" | "@else") as nm); loc } :: _); _ } ->
+      raise
+        (Error
+           ( position_of_loc loc
+           , Printf.sprintf "Unexpected %s clause. Maybe you forgot a parenthesis.\n" nm
+           ))
+  | { desc =
+        List
+          [ { desc = Atom "@string"; _ }
+          ; { desc = Atom name; loc = loc_name }
+          ; { desc = Atom value; loc = loc_value }
+          ]
+    ; loc = pos, pos'
+    } ->
+      if not (is_id name) then raise (Error (position_of_loc loc_name, "Expecting an id"));
+      if not (is_string value)
+      then raise (Error (position_of_loc loc_value, "Expecting a string"));
+      let s = parse_string loc_value value in
+      write st pos;
+      insert
+        st
+        (Format.asprintf
+           "(global %s (ref eq) (array.new_fixed $bytes %d%a))"
+           name
+           (String.length s)
+           (fun f s ->
+             String.iter ~f:(fun c -> Format.fprintf f " (i32.const %d)" (Char.code c)) s)
+           s);
+      skip st pos'
+  | { desc = List [ { desc = Atom "@string"; _ }; { desc = Atom value; loc = loc_value } ]
+    ; loc = pos, pos'
+    } ->
+      if not (is_string value)
+      then raise (Error (position_of_loc loc_value, "Expecting a string"));
+      let s = parse_string loc_value value in
+      write st pos;
+      insert
+        st
+        (Format.asprintf
+           "(array.new_fixed $bytes %d%a)"
+           (String.length s)
+           (fun f s ->
+             String.iter ~f:(fun c -> Format.fprintf f " (i32.const %d)" (Char.code c)) s)
+           s);
+      skip st pos'
+  | { desc = List [ { desc = Atom "@string"; loc = _, pos } ]; loc = _, pos' } ->
+      raise (Error ((pos.loc, pos'.loc), Printf.sprintf "Expecting an id or a string.\n"))
+  | { desc = List ({ desc = Atom "@string"; _ } :: _ :: _ :: { loc; _ } :: _); _ } ->
+      raise
+        (Error (position_of_loc loc, Printf.sprintf "Expecting a closing parenthesis.\n"))
+  | { desc =
+        List
+          ({ desc = Atom "func"; loc = _, pos }
+          :: { desc =
+                 List
+                   [ { desc = Atom "export"; _ }
+                   ; { desc = Atom export_name; loc = export_loc }
+                   ]
+             ; loc = pos', _
+             }
+          :: l)
+    ; _
+    }
+    when variable_is_set st "name-wasm-functions"
+         && is_id ("$" ^ parse_string export_loc export_name) ->
+      write st pos;
+      insert st (Printf.sprintf " $%s " (parse_string export_loc export_name));
+      skip st pos';
+      rewrite_list st l
+  | { desc = List l; _ } -> rewrite_list st l
+  | _ -> ()
+
+(****)
+
+let ocaml_version =
+  Scanf.sscanf Sys.ocaml_version "%d.%d.%d" (fun major minor patchlevel ->
+      Version (major, minor, patchlevel))
+
+let default_settings = [ "name-wasm-functions", Bool true ]
+
+let f ~variables ~filename ~contents:text =
+  let variables =
+    List.fold_left
+      ~f:(fun m (k, v) -> StringMap.add k v m)
+      ~init:StringMap.empty
+      (default_settings @ variables)
+  in
+  let variables = StringMap.add "ocaml_version" ocaml_version variables in
+  let lexbuf = Sedlexing.Utf8.from_string text in
+  Sedlexing.set_filename lexbuf filename;
+  try
+    let t, (pos, end_pos) = parse lexbuf in
+    let st = { text; pos; variables; buf = Buffer.create (String.length text) } in
+    rewrite_list st t;
+    write st end_pos;
+    Buffer.contents st.buf
+  with Error (loc, msg) -> report_error loc msg
+
+type source =
+  | File
+  | Contents of string
+
+type input =
+  { module_name : string
+  ; file : string
+  ; source : source
+  }
+
+let with_preprocessed_files ~variables ~inputs action =
+  List.fold_left
+    ~f:(fun cont { module_name; file; source } inputs ->
+      match source with
+      | File -> cont ({ Binaryen.module_name; file } :: inputs)
+      | Contents contents ->
+          let source_file = file in
+          Fs.with_intermediate_file (Filename.temp_file module_name ".wasm")
+          @@ fun file ->
+          Fs.write_file
+            ~name:file
+            ~contents:
+              (if Link.Wasm_binary.check ~contents
+               then contents
+               else f ~variables ~filename:source_file ~contents);
+          cont ({ Binaryen.module_name; file } :: inputs))
+    ~init:action
+    inputs
+    []
diff --git a/compiler/lib-wasm/wat_preprocess.mli b/compiler/lib-wasm/wat_preprocess.mli
new file mode 100644
index 0000000000..c0b386f09d
--- /dev/null
+++ b/compiler/lib-wasm/wat_preprocess.mli
@@ -0,0 +1,22 @@
+type value =
+  | Bool of bool
+  | String of string
+  | Version of int * int * int
+
+val f : variables:(string * value) list -> filename:string -> contents:string -> string
+
+type source =
+  | File (* Binary file (skipped) *)
+  | Contents of string (* Contents to preprocess *)
+
+type input =
+  { module_name : string
+  ; file : string
+  ; source : source
+  }
+
+val with_preprocessed_files :
+     variables:(string * value) list
+  -> inputs:input list
+  -> (Binaryen.link_input list -> 'a)
+  -> 'a
diff --git a/compiler/lib/build_info.ml b/compiler/lib/build_info.ml
index 4de9956edf..b5da01bbb4 100644
--- a/compiler/lib/build_info.ml
+++ b/compiler/lib/build_info.ml
@@ -68,6 +68,7 @@ let create kind =
     | v -> Printf.sprintf "%s+%s" Compiler_version.s v
   in
   [ "use-js-string", string_of_bool (Config.Flag.use_js_string ())
+  ; "wasi", string_of_bool (Config.Flag.wasi ())
   ; "effects", string_of_effects_backend (Config.effects ())
   ; "version", version
   ; "kind", string_of_kind kind
@@ -139,9 +140,9 @@ let merge fname1 info1 fname2 info2 =
         match k, v1, v2 with
         | "kind", v1, v2 ->
             if Option.equal String.equal v1 v2 then v1 else Some (string_of_kind `Unknown)
-        | ("effects" | "use-js-string" | "version"), Some v1, Some v2
+        | ("effects" | "use-js-string" | "wasi" | "version"), Some v1, Some v2
           when String.equal v1 v2 -> Some v1
-        | (("effects" | "use-js-string" | "version") as key), v1, v2 ->
+        | (("effects" | "use-js-string" | "wasi" | "version") as key), v1, v2 ->
             raise
               (Incompatible_build_info { key; first = fname1, v1; second = fname2, v2 })
         | _, Some v1, Some v2 when String.equal v1 v2 -> Some v1
@@ -156,7 +157,7 @@ let configure t =
   StringMap.iter
     (fun k v ->
       match k with
-      | "use-js-string" -> Config.Flag.set k (bool_of_string v)
+      | "use-js-string" | "wasi" -> Config.Flag.set k (bool_of_string v)
       | "effects" -> Config.set_effects_backend (effects_backend_of_string v)
       | _ -> ())
     t
diff --git a/compiler/lib/config.ml b/compiler/lib/config.ml
index 3e662dd517..109b477a4d 100644
--- a/compiler/lib/config.ml
+++ b/compiler/lib/config.ml
@@ -101,6 +101,10 @@ module Flag = struct
   let auto_link = o ~name:"auto-link" ~default:true
 
   let es6 = o ~name:"es6" ~default:false
+
+  let wasi = o ~name:"wasi" ~default:false
+
+  let trap_on_exception = o ~name:"trap-on-exception" ~default:false
 end
 
 module Param = struct
diff --git a/compiler/lib/config.mli b/compiler/lib/config.mli
index 71642430bf..a05274a2bd 100644
--- a/compiler/lib/config.mli
+++ b/compiler/lib/config.mli
@@ -76,6 +76,10 @@ module Flag : sig
 
   val es6 : unit -> bool
 
+  val wasi : unit -> bool
+
+  val trap_on_exception : unit -> bool
+
   val enable : string -> unit
 
   val disable : string -> unit
diff --git a/compiler/lib/inline.ml b/compiler/lib/inline.ml
index 2637480062..ec5b70b6f8 100644
--- a/compiler/lib/inline.ml
+++ b/compiler/lib/inline.ml
@@ -328,7 +328,7 @@ let inline ~first_class_primitives live_vars closures name pc (outer, p) =
 
 let times = Debug.find "times"
 
-let f p live_vars =
+let f p (live_vars : Deadcode.variable_uses) =
   let first_class_primitives =
     match Config.target (), Config.effects () with
     | `JavaScript, `Disabled -> true
diff --git a/compiler/tests-check-prim/main.4.14.output b/compiler/tests-check-prim/main.4.14.output
index 837ad8207f..94bb240abc 100644
--- a/compiler/tests-check-prim/main.4.14.output
+++ b/compiler/tests-check-prim/main.4.14.output
@@ -139,18 +139,27 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_strerror
+caml_unix_access
+caml_unix_chdir
+caml_unix_chmod
 caml_unix_cleanup
 caml_unix_close
 caml_unix_closedir
+caml_unix_fchmod
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_fstat
 caml_unix_fstat_64
+caml_unix_fsync
 caml_unix_ftruncate
 caml_unix_ftruncate_64
-caml_unix_getpwuid
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_gettimeofday
 caml_unix_getuid
 caml_unix_gmtime
@@ -158,6 +167,7 @@ caml_unix_has_symlink
 caml_unix_inchannel_of_filedescr
 caml_unix_inet_addr_of_string
 caml_unix_isatty
+caml_unix_link
 caml_unix_localtime
 caml_unix_lookup_file
 caml_unix_lseek
@@ -175,14 +185,17 @@ caml_unix_readlink
 caml_unix_rename
 caml_unix_rewinddir
 caml_unix_rmdir
+caml_unix_single_write
 caml_unix_startup
 caml_unix_stat
 caml_unix_stat_64
 caml_unix_symlink
 caml_unix_time
+caml_unix_times
 caml_unix_truncate
 caml_unix_truncate_64
 caml_unix_unlink
 caml_unix_utimes
 caml_unix_write
+unix_error_message
 
diff --git a/compiler/tests-check-prim/main.5.2.output b/compiler/tests-check-prim/main.5.2.output
index bd6a3fa76b..973ddcb677 100644
--- a/compiler/tests-check-prim/main.5.2.output
+++ b/compiler/tests-check-prim/main.5.2.output
@@ -135,18 +135,27 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_strerror
+caml_unix_access
+caml_unix_chdir
+caml_unix_chmod
 caml_unix_cleanup
 caml_unix_close
 caml_unix_closedir
+caml_unix_fchmod
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_fstat
 caml_unix_fstat_64
+caml_unix_fsync
 caml_unix_ftruncate
 caml_unix_ftruncate_64
-caml_unix_getpwuid
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_gettimeofday
 caml_unix_getuid
 caml_unix_gmtime
@@ -154,6 +163,7 @@ caml_unix_has_symlink
 caml_unix_inchannel_of_filedescr
 caml_unix_inet_addr_of_string
 caml_unix_isatty
+caml_unix_link
 caml_unix_localtime
 caml_unix_lookup_file
 caml_unix_lseek
@@ -172,15 +182,18 @@ caml_unix_readlink
 caml_unix_rename
 caml_unix_rewinddir
 caml_unix_rmdir
+caml_unix_single_write
 caml_unix_startup
 caml_unix_stat
 caml_unix_stat_64
 caml_unix_symlink
 caml_unix_time
+caml_unix_times
 caml_unix_truncate
 caml_unix_truncate_64
 caml_unix_unlink
 caml_unix_utimes
 caml_unix_write
 caml_unix_write_bigarray
+unix_error_message
 
diff --git a/compiler/tests-check-prim/main.5.3.output b/compiler/tests-check-prim/main.5.3.output
index d1f580ea3e..af47aa226e 100644
--- a/compiler/tests-check-prim/main.5.3.output
+++ b/compiler/tests-check-prim/main.5.3.output
@@ -133,18 +133,27 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_strerror
+caml_unix_access
+caml_unix_chdir
+caml_unix_chmod
 caml_unix_cleanup
 caml_unix_close
 caml_unix_closedir
+caml_unix_fchmod
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_fstat
 caml_unix_fstat_64
+caml_unix_fsync
 caml_unix_ftruncate
 caml_unix_ftruncate_64
-caml_unix_getpwuid
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_gettimeofday
 caml_unix_getuid
 caml_unix_gmtime
@@ -152,6 +161,7 @@ caml_unix_has_symlink
 caml_unix_inchannel_of_filedescr
 caml_unix_inet_addr_of_string
 caml_unix_isatty
+caml_unix_link
 caml_unix_localtime
 caml_unix_lookup_file
 caml_unix_lseek
@@ -170,15 +180,18 @@ caml_unix_readlink
 caml_unix_rename
 caml_unix_rewinddir
 caml_unix_rmdir
+caml_unix_single_write
 caml_unix_startup
 caml_unix_stat
 caml_unix_stat_64
 caml_unix_symlink
 caml_unix_time
+caml_unix_times
 caml_unix_truncate
 caml_unix_truncate_64
 caml_unix_unlink
 caml_unix_utimes
 caml_unix_write
 caml_unix_write_bigarray
+unix_error_message
 
diff --git a/compiler/tests-check-prim/unix-Unix.4.14.output b/compiler/tests-check-prim/unix-Unix.4.14.output
index 7922e02de5..d6634cabe7 100644
--- a/compiler/tests-check-prim/unix-Unix.4.14.output
+++ b/compiler/tests-check-prim/unix-Unix.4.14.output
@@ -18,11 +18,8 @@ caml_reset_afl_instrumentation
 caml_unix_map_file_bytecode
 debugger
 unix_accept
-unix_access
 unix_alarm
 unix_bind
-unix_chdir
-unix_chmod
 unix_chown
 unix_chroot
 unix_clear_close_on_exec
@@ -32,22 +29,13 @@ unix_dup
 unix_dup2
 unix_environment
 unix_environment_unsafe
-unix_error_message
 unix_execv
 unix_execve
 unix_execvp
 unix_execvpe
-unix_fchmod
 unix_fchown
 unix_fork
-unix_fsync
 unix_getaddrinfo
-unix_getcwd
-unix_getegid
-unix_geteuid
-unix_getgid
-unix_getgrgid
-unix_getgrnam
 unix_getgroups
 unix_gethostbyaddr
 unix_gethostbyname
@@ -60,14 +48,12 @@ unix_getpid
 unix_getppid
 unix_getprotobyname
 unix_getprotobynumber
-unix_getpwnam
 unix_getservbyname
 unix_getservbyport
 unix_getsockname
 unix_getsockopt
 unix_initgroups
 unix_kill
-unix_link
 unix_listen
 unix_lockf
 unix_mkfifo
@@ -92,7 +78,6 @@ unix_shutdown
 unix_sigpending
 unix_sigprocmask
 unix_sigsuspend
-unix_single_write
 unix_sleep
 unix_socket
 unix_socketpair
@@ -104,7 +89,6 @@ unix_tcflush
 unix_tcgetattr
 unix_tcsendbreak
 unix_tcsetattr
-unix_times
 unix_umask
 unix_wait
 unix_waitpid
@@ -231,18 +215,26 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_unix_access
+caml_unix_chdir
+caml_unix_chmod
 caml_unix_cleanup
 caml_unix_close
 caml_unix_closedir
+caml_unix_fchmod
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_fstat
 caml_unix_fstat_64
+caml_unix_fsync
 caml_unix_ftruncate
 caml_unix_ftruncate_64
-caml_unix_getpwuid
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_gettimeofday
 caml_unix_getuid
 caml_unix_gmtime
@@ -250,6 +242,7 @@ caml_unix_has_symlink
 caml_unix_inchannel_of_filedescr
 caml_unix_inet_addr_of_string
 caml_unix_isatty
+caml_unix_link
 caml_unix_localtime
 caml_unix_lookup_file
 caml_unix_lseek
@@ -267,11 +260,13 @@ caml_unix_readlink
 caml_unix_rename
 caml_unix_rewinddir
 caml_unix_rmdir
+caml_unix_single_write
 caml_unix_startup
 caml_unix_stat
 caml_unix_stat_64
 caml_unix_symlink
 caml_unix_time
+caml_unix_times
 caml_unix_truncate
 caml_unix_truncate_64
 caml_unix_unlink
diff --git a/compiler/tests-check-prim/unix-Unix.5.2.output b/compiler/tests-check-prim/unix-Unix.5.2.output
index 25663a23da..02ee9d3515 100644
--- a/compiler/tests-check-prim/unix-Unix.5.2.output
+++ b/compiler/tests-check-prim/unix-Unix.5.2.output
@@ -9,11 +9,8 @@ caml_drop_continuation
 caml_int_as_pointer
 caml_reset_afl_instrumentation
 caml_unix_accept
-caml_unix_access
 caml_unix_alarm
 caml_unix_bind
-caml_unix_chdir
-caml_unix_chmod
 caml_unix_chown
 caml_unix_chroot
 caml_unix_clear_close_on_exec
@@ -23,22 +20,13 @@ caml_unix_dup
 caml_unix_dup2
 caml_unix_environment
 caml_unix_environment_unsafe
-caml_unix_error_message
 caml_unix_execv
 caml_unix_execve
 caml_unix_execvp
 caml_unix_execvpe
-caml_unix_fchmod
 caml_unix_fchown
 caml_unix_fork
-caml_unix_fsync
 caml_unix_getaddrinfo
-caml_unix_getcwd
-caml_unix_getegid
-caml_unix_geteuid
-caml_unix_getgid
-caml_unix_getgrgid
-caml_unix_getgrnam
 caml_unix_getgroups
 caml_unix_gethostbyaddr
 caml_unix_gethostbyname
@@ -51,14 +39,12 @@ caml_unix_getpid
 caml_unix_getppid
 caml_unix_getprotobyname
 caml_unix_getprotobynumber
-caml_unix_getpwnam
 caml_unix_getservbyname
 caml_unix_getservbyport
 caml_unix_getsockname
 caml_unix_getsockopt
 caml_unix_initgroups
 caml_unix_kill
-caml_unix_link
 caml_unix_listen
 caml_unix_lockf
 caml_unix_map_file_bytecode
@@ -84,7 +70,6 @@ caml_unix_shutdown
 caml_unix_sigpending
 caml_unix_sigprocmask
 caml_unix_sigsuspend
-caml_unix_single_write
 caml_unix_sleep
 caml_unix_socket
 caml_unix_socketpair
@@ -96,7 +81,6 @@ caml_unix_tcflush
 caml_unix_tcgetattr
 caml_unix_tcsendbreak
 caml_unix_tcsetattr
-caml_unix_times
 caml_unix_umask
 caml_unix_wait
 caml_unix_waitpid
@@ -227,10 +211,12 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_strerror
 caml_unix_cleanup
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_startup
+unix_error_message
 
diff --git a/compiler/tests-check-prim/unix-Unix.5.3.output b/compiler/tests-check-prim/unix-Unix.5.3.output
index 4184d40762..1c8c4155fd 100644
--- a/compiler/tests-check-prim/unix-Unix.5.3.output
+++ b/compiler/tests-check-prim/unix-Unix.5.3.output
@@ -8,11 +8,8 @@ caml_continuation_use
 caml_int_as_pointer
 caml_reset_afl_instrumentation
 caml_unix_accept
-caml_unix_access
 caml_unix_alarm
 caml_unix_bind
-caml_unix_chdir
-caml_unix_chmod
 caml_unix_chown
 caml_unix_chroot
 caml_unix_clear_close_on_exec
@@ -22,22 +19,13 @@ caml_unix_dup
 caml_unix_dup2
 caml_unix_environment
 caml_unix_environment_unsafe
-caml_unix_error_message
 caml_unix_execv
 caml_unix_execve
 caml_unix_execvp
 caml_unix_execvpe
-caml_unix_fchmod
 caml_unix_fchown
 caml_unix_fork
-caml_unix_fsync
 caml_unix_getaddrinfo
-caml_unix_getcwd
-caml_unix_getegid
-caml_unix_geteuid
-caml_unix_getgid
-caml_unix_getgrgid
-caml_unix_getgrnam
 caml_unix_getgroups
 caml_unix_gethostbyaddr
 caml_unix_gethostbyname
@@ -50,14 +38,12 @@ caml_unix_getpid
 caml_unix_getppid
 caml_unix_getprotobyname
 caml_unix_getprotobynumber
-caml_unix_getpwnam
 caml_unix_getservbyname
 caml_unix_getservbyport
 caml_unix_getsockname
 caml_unix_getsockopt
 caml_unix_initgroups
 caml_unix_kill
-caml_unix_link
 caml_unix_listen
 caml_unix_lockf
 caml_unix_map_file_bytecode
@@ -83,7 +69,6 @@ caml_unix_shutdown
 caml_unix_sigpending
 caml_unix_sigprocmask
 caml_unix_sigsuspend
-caml_unix_single_write
 caml_unix_sleep
 caml_unix_socket
 caml_unix_socketpair
@@ -95,7 +80,6 @@ caml_unix_tcflush
 caml_unix_tcgetattr
 caml_unix_tcsendbreak
 caml_unix_tcsetattr
-caml_unix_times
 caml_unix_umask
 caml_unix_wait
 caml_unix_waitpid
@@ -225,10 +209,12 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_strerror
 caml_unix_cleanup
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_startup
+unix_error_message
 
diff --git a/compiler/tests-check-prim/unix-Win32.4.14.output b/compiler/tests-check-prim/unix-Win32.4.14.output
index ce3b1b2851..80dfdc0f2b 100644
--- a/compiler/tests-check-prim/unix-Win32.4.14.output
+++ b/compiler/tests-check-prim/unix-Win32.4.14.output
@@ -18,23 +18,17 @@ caml_reset_afl_instrumentation
 caml_unix_map_file_bytecode
 debugger
 unix_accept
-unix_access
 unix_bind
-unix_chdir
-unix_chmod
 unix_clear_nonblock
 unix_connect
 unix_dup
 unix_dup2
 unix_environment
-unix_error_message
 unix_execv
 unix_execve
 unix_execvp
 unix_execvpe
-unix_fsync
 unix_getaddrinfo
-unix_getcwd
 unix_gethostbyaddr
 unix_gethostbyname
 unix_gethostname
@@ -47,7 +41,6 @@ unix_getservbyname
 unix_getservbyport
 unix_getsockname
 unix_getsockopt
-unix_link
 unix_listen
 unix_lockf
 unix_pipe
@@ -61,12 +54,10 @@ unix_sendto
 unix_set_nonblock
 unix_setsockopt
 unix_shutdown
-unix_single_write
 unix_sleep
 unix_socket
 unix_socketpair
 unix_string_of_inet_addr
-unix_times
 win_clear_close_on_exec
 win_create_process
 win_set_close_on_exec
@@ -196,18 +187,26 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
+caml_unix_access
+caml_unix_chdir
+caml_unix_chmod
 caml_unix_cleanup
 caml_unix_close
 caml_unix_closedir
+caml_unix_fchmod
 caml_unix_filedescr_of_fd
 caml_unix_findclose
 caml_unix_findfirst
 caml_unix_findnext
 caml_unix_fstat
 caml_unix_fstat_64
+caml_unix_fsync
 caml_unix_ftruncate
 caml_unix_ftruncate_64
-caml_unix_getpwuid
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_gettimeofday
 caml_unix_getuid
 caml_unix_gmtime
@@ -215,6 +214,7 @@ caml_unix_has_symlink
 caml_unix_inchannel_of_filedescr
 caml_unix_inet_addr_of_string
 caml_unix_isatty
+caml_unix_link
 caml_unix_localtime
 caml_unix_lookup_file
 caml_unix_lseek
@@ -232,11 +232,13 @@ caml_unix_readlink
 caml_unix_rename
 caml_unix_rewinddir
 caml_unix_rmdir
+caml_unix_single_write
 caml_unix_startup
 caml_unix_stat
 caml_unix_stat_64
 caml_unix_symlink
 caml_unix_time
+caml_unix_times
 caml_unix_truncate
 caml_unix_truncate_64
 caml_unix_unlink
diff --git a/compiler/tests-check-prim/unix-Win32.5.2.output b/compiler/tests-check-prim/unix-Win32.5.2.output
index e556d8cbee..4abee14701 100644
--- a/compiler/tests-check-prim/unix-Win32.5.2.output
+++ b/compiler/tests-check-prim/unix-Win32.5.2.output
@@ -9,10 +9,7 @@ caml_drop_continuation
 caml_int_as_pointer
 caml_reset_afl_instrumentation
 caml_unix_accept
-caml_unix_access
 caml_unix_bind
-caml_unix_chdir
-caml_unix_chmod
 caml_unix_clear_close_on_exec
 caml_unix_clear_nonblock
 caml_unix_connect
@@ -20,15 +17,12 @@ caml_unix_create_process
 caml_unix_dup
 caml_unix_dup2
 caml_unix_environment
-caml_unix_error_message
 caml_unix_execv
 caml_unix_execve
 caml_unix_execvp
 caml_unix_execvpe
 caml_unix_filedescr_of_channel
-caml_unix_fsync
 caml_unix_getaddrinfo
-caml_unix_getcwd
 caml_unix_gethostbyaddr
 caml_unix_gethostbyname
 caml_unix_gethostname
@@ -41,7 +35,6 @@ caml_unix_getservbyname
 caml_unix_getservbyport
 caml_unix_getsockname
 caml_unix_getsockopt
-caml_unix_link
 caml_unix_listen
 caml_unix_lockf
 caml_unix_map_file_bytecode
@@ -57,14 +50,12 @@ caml_unix_set_close_on_exec
 caml_unix_set_nonblock
 caml_unix_setsockopt
 caml_unix_shutdown
-caml_unix_single_write
 caml_unix_sleep
 caml_unix_socket
 caml_unix_socketpair
 caml_unix_string_of_inet_addr
 caml_unix_system
 caml_unix_terminate_process
-caml_unix_times
 caml_unix_waitpid
 debugger
 
@@ -193,7 +184,13 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
-caml_unix_getpwuid
+caml_strerror
+caml_unix_fchmod
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_getuid
 caml_unix_rewinddir
+unix_error_message
 
diff --git a/compiler/tests-check-prim/unix-Win32.5.3.output b/compiler/tests-check-prim/unix-Win32.5.3.output
index 3259dfbe8b..fdd7c21ad6 100644
--- a/compiler/tests-check-prim/unix-Win32.5.3.output
+++ b/compiler/tests-check-prim/unix-Win32.5.3.output
@@ -8,10 +8,7 @@ caml_continuation_use
 caml_int_as_pointer
 caml_reset_afl_instrumentation
 caml_unix_accept
-caml_unix_access
 caml_unix_bind
-caml_unix_chdir
-caml_unix_chmod
 caml_unix_clear_close_on_exec
 caml_unix_clear_nonblock
 caml_unix_connect
@@ -19,15 +16,12 @@ caml_unix_create_process
 caml_unix_dup
 caml_unix_dup2
 caml_unix_environment
-caml_unix_error_message
 caml_unix_execv
 caml_unix_execve
 caml_unix_execvp
 caml_unix_execvpe
 caml_unix_filedescr_of_channel
-caml_unix_fsync
 caml_unix_getaddrinfo
-caml_unix_getcwd
 caml_unix_gethostbyaddr
 caml_unix_gethostbyname
 caml_unix_gethostname
@@ -40,7 +34,6 @@ caml_unix_getservbyname
 caml_unix_getservbyport
 caml_unix_getsockname
 caml_unix_getsockopt
-caml_unix_link
 caml_unix_listen
 caml_unix_lockf
 caml_unix_map_file_bytecode
@@ -56,14 +49,12 @@ caml_unix_set_close_on_exec
 caml_unix_set_nonblock
 caml_unix_setsockopt
 caml_unix_shutdown
-caml_unix_single_write
 caml_unix_sleep
 caml_unix_socket
 caml_unix_socketpair
 caml_unix_string_of_inet_addr
 caml_unix_system
 caml_unix_terminate_process
-caml_unix_times
 caml_unix_waitpid
 debugger
 
@@ -191,7 +182,13 @@ jsoo_toplevel_init_compile
 jsoo_toplevel_init_reloc
 
 From +unix.js:
-caml_unix_getpwuid
+caml_strerror
+caml_unix_fchmod
+caml_unix_getegid
+caml_unix_geteuid
+caml_unix_getgid
+caml_unix_getpwnam
 caml_unix_getuid
 caml_unix_rewinddir
+unix_error_message
 
diff --git a/compiler/tests-io/non_ascii_filenames.ml b/compiler/tests-io/non_ascii_filenames.ml
index 437ae89d4b..f32136e9fe 100644
--- a/compiler/tests-io/non_ascii_filenames.ml
+++ b/compiler/tests-io/non_ascii_filenames.ml
@@ -13,14 +13,7 @@ let () =
   Printf.printf "reading directories\n";
   let check_file d =
     let a = Sys.readdir d in
-    if
-      not
-        (Array.exists
-           (fun x ->
-             prerr_endline x;
-             x = "accentué")
-           a)
-    then raise Not_found
+    if not (Array.exists (fun x -> x = "accentué") a) then raise Not_found
   in
   test check_file ".";
   test check_file "/static";
diff --git a/compiler/tests-jsoo/dune b/compiler/tests-jsoo/dune
index faf7fd7f64..2a89e85128 100644
--- a/compiler/tests-jsoo/dune
+++ b/compiler/tests-jsoo/dune
@@ -1,3 +1,9 @@
+(env
+ (_
+  ;; for testing
+  (env-vars
+   (FOO bar))))
+
 (library
  (name jsoo_testsuite_latest)
  (modules test_io test_floats)
@@ -5,6 +11,8 @@
  (enabled_if
   (>= %{ocaml_version} 4.14))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
@@ -16,6 +24,8 @@
  (enabled_if
   (>= %{ocaml_version} 5.1.1))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
@@ -27,6 +37,22 @@
  (enabled_if
   (>= %{ocaml_version} 5.1.1))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
+  (modes js wasm best))
+ (preprocess
+  (pps ppx_expect)))
+
+(library
+ (name jsoo_testsuite_perms)
+ (modules test_unix_perms)
+ (libraries unix)
+ ;; WASI has no notion of file permissions (it uses capabilities instead)
+ (enabled_if
+  (<> %{profile} wasi))
+ (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
@@ -41,6 +67,7 @@
    test_float16
    test_marshal_compressed
    test_parsing
+   test_unix_perms
    calc_parser
    calc_lexer))
  (libraries unix compiler-libs.common js_of_ocaml-compiler)
@@ -48,6 +75,8 @@
   (language c)
   (names bigarray_stubs))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (preprocess
   (pps ppx_expect)))
diff --git a/compiler/tests-jsoo/lib-effects/assume_no_perform.ml b/compiler/tests-jsoo/lib-effects/assume_no_perform.ml
index e655c33c4c..aca8728f15 100644
--- a/compiler/tests-jsoo/lib-effects/assume_no_perform.ml
+++ b/compiler/tests-jsoo/lib-effects/assume_no_perform.ml
@@ -125,7 +125,7 @@ let () =
 
       (* The code below should be called in direct style despite the installed
          effect handler *)
-      Js_of_ocaml.Effect_js.assume_no_perform (fun () ->
+      Jsoo_runtime.Effect.assume_no_perform (fun () ->
           let m, sd = benchmark iter_fun 5 in
           let () = printf "Iter: mean = %f, sd = %f\n%!" m sd in
 
diff --git a/compiler/tests-jsoo/lib-effects/assume_no_perform_nested_handler.ml b/compiler/tests-jsoo/lib-effects/assume_no_perform_nested_handler.ml
index c164b5462e..fc771da918 100644
--- a/compiler/tests-jsoo/lib-effects/assume_no_perform_nested_handler.ml
+++ b/compiler/tests-jsoo/lib-effects/assume_no_perform_nested_handler.ml
@@ -6,7 +6,7 @@ type _ Effect.t += Dummy : unit t
 let () =
   try_with
     (fun () ->
-      Js_of_ocaml.Effect_js.assume_no_perform (fun () ->
+      Jsoo_runtime.Effect.assume_no_perform (fun () ->
           try_with (fun () -> ()) () { effc = (fun (type a) (_ : a Effect.t) -> None) });
       perform Dummy)
     ()
diff --git a/compiler/tests-jsoo/lib-effects/assume_no_perform_unhandled.ml b/compiler/tests-jsoo/lib-effects/assume_no_perform_unhandled.ml
index ad055ac3a9..6d0516436a 100644
--- a/compiler/tests-jsoo/lib-effects/assume_no_perform_unhandled.ml
+++ b/compiler/tests-jsoo/lib-effects/assume_no_perform_unhandled.ml
@@ -6,7 +6,7 @@ type _ Effect.t += Dummy : unit t
 let must_raise () =
   try_with
     (fun () ->
-      Js_of_ocaml.Effect_js.assume_no_perform (fun () ->
+      Jsoo_runtime.Effect.assume_no_perform (fun () ->
           (* Should raise [Effect.Unhandled] despite the installed handler *)
           perform Dummy))
     ()
diff --git a/compiler/tests-jsoo/lib-effects/dune b/compiler/tests-jsoo/lib-effects/dune
index d3ca13c41f..68d1d8568a 100644
--- a/compiler/tests-jsoo/lib-effects/dune
+++ b/compiler/tests-jsoo/lib-effects/dune
@@ -1,6 +1,10 @@
 (env
  (with-effects-double-translation)
  (with-effects)
+ ; (wasi
+ ;  (wasm_of_ocaml
+ ;   (flags
+ ;    (:standard --enable effects))))
  (_
   (js_of_ocaml
    (flags
@@ -11,6 +15,8 @@
  (enabled_if
   (>= %{ocaml_version} 5))
  (inline_tests
+  (deps
+   (sandbox preserve_file_kind))
   (modes js wasm best))
  (modules
   (:standard
@@ -41,7 +47,7 @@
   assume_no_perform
   assume_no_perform_unhandled
   assume_no_perform_nested_handler)
- (libraries js_of_ocaml)
+ (libraries jsoo_runtime)
  (action
   (ignore-outputs
    (with-accepted-exit-codes
diff --git a/compiler/tests-jsoo/test_unix.ml b/compiler/tests-jsoo/test_unix.ml
new file mode 100644
index 0000000000..3546260463
--- /dev/null
+++ b/compiler/tests-jsoo/test_unix.ml
@@ -0,0 +1,77 @@
+let%expect_test "Unix.error_message" =
+  Printf.printf "%s\n" (String.lowercase_ascii (Unix.error_message ENOENT));
+  [%expect {| no such file or directory |}]
+
+let%expect_test "Unix.times" =
+  let t = Unix.times () in
+  let t' = Unix.times () in
+  let cmp v v' = v' >= v && v' <= v +. 0.1 in
+  if
+    cmp t.tms_utime t'.tms_utime
+    && cmp t.tms_stime t'.tms_stime
+    && cmp t.tms_cutime t'.tms_cutime
+    && cmp t.tms_cstime t'.tms_cstime
+  then Printf.printf "OK\n";
+  [%expect {| OK |}]
+
+let%expect_test "Unix.link" =
+  let tmp = Filename.temp_file "a" "txt" in
+  let ch = open_out tmp in
+  output_string ch "test\n";
+  close_out ch;
+  let tmp' = Filename.temp_file "a" "txt" in
+  Unix.unlink tmp';
+  Unix.link tmp tmp';
+  let ch = open_in tmp' in
+  Format.printf "%s\n" (input_line ch);
+  close_in ch;
+  let ch = open_out tmp' in
+  output_string ch "abcd\n";
+  close_out ch;
+  let ch = open_in tmp in
+  Format.printf "%s\n" (input_line ch);
+  close_in ch;
+  Unix.unlink tmp;
+  Unix.unlink tmp';
+  [%expect {|
+    test
+    abcd
+  |}]
+
+let%expect_test "Unix.readlink" =
+  let tmp' = Filename.temp_file "a" "txt" in
+  Unix.unlink tmp';
+  Unix.symlink "abcdefgh" tmp';
+  Format.printf "%s\n" (Unix.readlink tmp');
+  [%expect {| abcdefgh |}]
+
+let%expect_test "Unix.single_write" =
+  let s = "abcd efgh ijkl mnop qrst uvwx" in
+  let b = Bytes.of_string s in
+  let tmp = Filename.temp_file "a" "txt" in
+  let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] 0o666 in
+  let n = Unix.single_write fd b 0 (Bytes.length b) in
+  Unix.close fd;
+  let ch = open_in tmp in
+  let s' = really_input_string ch n in
+  Printf.printf "%b %b\n" (n > 0) (s' = String.sub s 0 n);
+  [%expect {| true true |}]
+
+let%expect_test "Unix.read" =
+  let tmp = Filename.temp_file "a" "txt" in
+  let fd = Unix.openfile tmp [ O_RDONLY ] 0o666 in
+  (try Printf.printf "write: %d\n" (Unix.write fd (Bytes.create 8) 0 8)
+   with Unix.Unix_error (_, _, _) -> Printf.printf "write failed\n");
+  Unix.close fd;
+  Unix.unlink tmp;
+  (try Printf.printf "read: %d\n" (Unix.read fd (Bytes.create 8) 0 8)
+   with Unix.Unix_error (err, _, _) ->
+     Printf.printf "%s\n" (String.lowercase_ascii (Unix.error_message err)));
+  [%expect {|
+    write failed
+    bad file descriptor
+ |}]
+
+let%expect_test "Unix.getenv" =
+  Printf.printf "%s\n" (Sys.getenv "FOO");
+  [%expect {| bar |}]
diff --git a/compiler/tests-jsoo/test_unix_perms.ml b/compiler/tests-jsoo/test_unix_perms.ml
new file mode 100644
index 0000000000..8f07952db9
--- /dev/null
+++ b/compiler/tests-jsoo/test_unix_perms.ml
@@ -0,0 +1,78 @@
+let on_windows = Sys.os_type = "Win32"
+
+let%expect_test "Unix.chmod / Unix.fchmod / Unix.access" =
+  let tmp = Filename.temp_file "a" "txt" in
+  let test ?(ok_on_windows = false) flags =
+    try
+      Unix.access tmp flags;
+      if on_windows && ok_on_windows
+      then Printf.printf "denied (success on Windows)\n"
+      else Printf.printf "success\n"
+    with
+    | Unix.Unix_error ((EPERM | EACCES), _, _) ->
+        if (not on_windows) && ok_on_windows
+        then Printf.printf "denied (success on Windows)\n"
+        else Printf.printf "denied\n"
+    | Unix.Unix_error (ENOENT, _, _) -> Printf.printf "absent\n"
+  in
+  let touch perms =
+    Unix.chmod tmp 0o600;
+    Unix.unlink tmp;
+    let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] perms in
+    Unix.close fd
+  in
+  let test_perms set =
+    set 0o200;
+    test ~ok_on_windows:true [ R_OK ];
+    test [ W_OK ];
+    test ~ok_on_windows:true [ R_OK; W_OK ];
+    [%expect
+      {|
+      denied (success on Windows)
+      success
+      denied (success on Windows)
+      |}];
+    set 0o400;
+    test [ R_OK ];
+    test [ W_OK ];
+    test [ R_OK; W_OK ];
+    [%expect {|
+    success
+    denied
+    denied |}];
+    set 0o600;
+    test [ R_OK ];
+    test [ W_OK ];
+    test [ R_OK; W_OK ];
+    [%expect {|
+    success
+    success
+    success |}];
+    set 0o000;
+    test ~ok_on_windows:true [ R_OK ];
+    test [ W_OK ];
+    test [ R_OK; W_OK ];
+    [%expect {|
+      denied (success on Windows)
+      denied
+      denied
+      |}]
+  in
+  test [ F_OK ];
+  [%expect {|
+    success |}];
+  Unix.chmod tmp 0o600;
+  Unix.unlink tmp;
+  test [ F_OK ];
+  [%expect {|
+    absent |}];
+  let fd = Unix.openfile tmp [ O_WRONLY; O_CREAT; O_TRUNC ] 0o666 in
+  test [ F_OK ];
+  [%expect {|
+    success |}];
+  if not on_windows then test_perms (Unix.fchmod fd);
+  Unix.close fd;
+  test_perms (Unix.chmod tmp);
+  test_perms touch;
+  Unix.chmod tmp 0o600;
+  Unix.unlink tmp
diff --git a/compiler/tests-ocaml/basic-io-2/dune b/compiler/tests-ocaml/basic-io-2/dune
index 121f745198..e666404c1f 100644
--- a/compiler/tests-ocaml/basic-io-2/dune
+++ b/compiler/tests-ocaml/basic-io-2/dune
@@ -1,5 +1,8 @@
 (tests
  (names io)
  (modes js wasm)
+ ;; Sys.command not available
+ (enabled_if
+  (<> %{profile} wasi))
  (action
   (run node %{test} %{dep:test-file-short-lines})))
diff --git a/compiler/tests-ocaml/effect-syntax/dune b/compiler/tests-ocaml/effect-syntax/dune
index 22b9fdf7e4..d7e7e2fa3b 100644
--- a/compiler/tests-ocaml/effect-syntax/dune
+++ b/compiler/tests-ocaml/effect-syntax/dune
@@ -1,6 +1,10 @@
 (env
  (with-effects-double-translation)
  (with-effects)
+ ; (wasi
+ ;  (wasm_of_ocaml
+ ;   (flags
+ ;    (:standard --enable=effects))))
  (_
   (js_of_ocaml
    (flags
diff --git a/compiler/tests-ocaml/effects/dune b/compiler/tests-ocaml/effects/dune
index 019935b596..bb7cc86051 100644
--- a/compiler/tests-ocaml/effects/dune
+++ b/compiler/tests-ocaml/effects/dune
@@ -1,6 +1,10 @@
 (env
  (with-effects-double-translation)
  (with-effects)
+ ; (wasi
+ ;  (wasm_of_ocaml
+ ;   (flags
+ ;    (:standard --enable=effects))))
  (_
   (js_of_ocaml
    (flags
diff --git a/compiler/tests-ocaml/lib-channels/close_in.ml b/compiler/tests-ocaml/lib-channels/close_in.ml
index 9b3717362a..8697d78c6a 100644
--- a/compiler/tests-ocaml/lib-channels/close_in.ml
+++ b/compiler/tests-ocaml/lib-channels/close_in.ml
@@ -6,8 +6,14 @@
    between 1 and IO_BUFFER_SIZE *)
 let nb_bytes = 3
 
+let temp_file =
+  let name, ch = Filename.open_temp_file "data" ".txt" in
+  output_string ch (String.make 1024 'a');
+  close_out ch;
+  name
+
 let () =
-  let ic = open_in_bin (Filename.basename Sys.argv.(0)) in
+  let ic = open_in_bin temp_file in
   seek_in ic nb_bytes;
   close_in ic;
   assert (
@@ -21,7 +27,7 @@ let () =
 
 (* A variant of #11878, which #11965 failed to fix. *)
 let () =
-  let ic = open_in_bin (Filename.basename Sys.argv.(0)) in
+  let ic = open_in_bin temp_file in
   close_in ic;
   begin try
     seek_in ic (-1);
diff --git a/compiler/tests-ocaml/lib-marshal/compressed.ml b/compiler/tests-ocaml/lib-marshal/compressed.ml
index 841cb94640..9d3b0440e2 100644
--- a/compiler/tests-ocaml/lib-marshal/compressed.ml
+++ b/compiler/tests-ocaml/lib-marshal/compressed.ml
@@ -161,8 +161,8 @@ let test_supported filename =
   if false then test 100 (actually_supported = compression_supported)
 
 let main () =
-  test_out "intext.data"; test_in "intext.data";
-  test_supported "intext.data";
-  Sys.remove "intext.data"
+  test_out "intext_compressed.data"; test_in "intext_compressed.data";
+  test_supported "intext_compressed.data";
+  Sys.remove "intext_compressed.data"
 
 let _ = main ()
diff --git a/compiler/tests-ocaml/lib-marshal/intext.ml b/compiler/tests-ocaml/lib-marshal/intext.ml
index 3e0477dffd..5340806495 100644
--- a/compiler/tests-ocaml/lib-marshal/intext.ml
+++ b/compiler/tests-ocaml/lib-marshal/intext.ml
@@ -4,7 +4,8 @@
 
 (* Test for output_value / input_value *)
 
-let max_data_depth = 500000
+let max_data_depth = 10000
+(* Reduced since we use a quadratic algorithm for sharing in the WASI runtime *)
 
 type t = A | B of int | C of float | D of string | E of char
        | F of t | G of t * t | H of int * t | I of t * float | J
diff --git a/compiler/tests-ocaml/lib-marshal/intext_par.ml b/compiler/tests-ocaml/lib-marshal/intext_par.ml
index f93c55c685..fbf0a8dec9 100644
--- a/compiler/tests-ocaml/lib-marshal/intext_par.ml
+++ b/compiler/tests-ocaml/lib-marshal/intext_par.ml
@@ -17,7 +17,8 @@ let test_size =
 
 let num_domains = 1 lsl test_size
 
-let max_data_depth = 500000
+let max_data_depth = 10000
+(* Reduced since we use a quadratic algorithm for sharing in the WASI runtime *)
 
 type t = A | B of int | C of float | D of string | E of char
        | F of t | G of t * t | H of int * t | I of t * float | J
diff --git a/compiler/tests-ocaml/lib-unix/isatty/dune b/compiler/tests-ocaml/lib-unix/isatty/dune
index 6740efe55b..852dd49d6a 100644
--- a/compiler/tests-ocaml/lib-unix/isatty/dune
+++ b/compiler/tests-ocaml/lib-unix/isatty/dune
@@ -6,7 +6,10 @@
 (tests
  (names isatty_tty)
  (enabled_if
-  (not %{env:CI=false}))
+  (and
+   (<> %{profile} wasi)
+   (not %{env:CI=false})))
+ ; WASI has no notion of tty
  ; isatty_tty does not work on the CI since we are not running in a tty there
  (libraries ocaml_testing unix)
  (modes js wasm))
diff --git a/compiler/tests-ocaml/win-unicode/dune b/compiler/tests-ocaml/win-unicode/dune
new file mode 100644
index 0000000000..08585c87e9
--- /dev/null
+++ b/compiler/tests-ocaml/win-unicode/dune
@@ -0,0 +1,4 @@
+(tests
+ (names mltest)
+ (libraries ocaml_testing unix)
+ (modes js wasm))
diff --git a/compiler/tests-ocaml/win-unicode/mltest.ml b/compiler/tests-ocaml/win-unicode/mltest.ml
new file mode 100644
index 0000000000..5c750eb231
--- /dev/null
+++ b/compiler/tests-ocaml/win-unicode/mltest.ml
@@ -0,0 +1,293 @@
+(* TEST
+ include unix;
+ hasunix;
+ flags += "-strict-sequence -w +A -warn-error +A";
+ windows-unicode;
+ toplevel;
+*)
+
+let foreign_names =
+  List.sort compare
+    [
+      "simple";
+      "\xE4\xBD\xA0\xE5\xA5\xBD"; (* "你好" *)
+      "\x73\xC5\x93\x75\x72"; (* "sœur" *)
+      "e\204\129te\204\129"; (* "été" *)
+    ]
+;;
+
+let test_files =
+  List.map (fun s -> s ^ ".txt") foreign_names
+;;
+
+let to_create_and_delete_files =
+  [
+    (* "верблюды" *)
+    "\xD0\xB2\xD0\xB5\xD1\x80\xD0\xB1\xD0\xBB\xD1\x8E\xD0\xB4\xD1\x8B";
+    "\xE9\xAA\x86\xE9\xA9\xBC"; (* "骆驼" *)
+    "\215\167\215\162\215\158\215\156"; (* "קעמל" *)
+    "\216\167\217\136\217\134\217\185"; (* "اونٹ" *)
+    "L\225\186\161c \196\145\195\160"; (* "Lạc đà" *)
+    "\224\176\146\224\176\130\224\176\159\224\177\134"; (* "ఒంటె" *)
+    "\224\174\146\224\174\159\224\175\141\224\174\159\224\174\149\224\
+     \174\174\224\175\141"; (* "ஒட்டகம்" *)
+    "\217\136\216\180\216\170\216\177"; (* "وشتر" *)
+    "\224\164\137\224\164\183\224\165\141\224\164\159\224\165\141\224\
+     \164\176\224\164\131"; (* "उष्ट्रः" *)
+    "\216\167\217\186"; (* "اٺ" *)
+  ]
+;;
+
+let foreign_names2 =
+  let rec take n l =
+    if n = 0 then []
+    else List.hd l :: take (n-1) (List.tl l)
+  in
+  take (List.length foreign_names) to_create_and_delete_files
+;;
+
+(* let env0 =
+     List.sort compare
+     (List.mapi (fun i v -> Printf.sprintf "OCAML_UTF8_VAR%d=%s" i v)
+                foreign_names2) *)
+
+(* let read_all ic = *)
+(*   set_binary_mode_in ic false; *)
+(*   let rec loop acc = *)
+(*     match input_line ic with *)
+(*     | exception End_of_file -> *)
+(*         List.rev acc *)
+(*     | s -> *)
+(*         loop (s :: acc) *)
+(*   in *)
+(*   loop [] *)
+
+(** WRAPPERS *)
+
+let getenvironmentenv s =
+  let env = Unix.environment () in
+  let rec loop i =
+    if i >= Array.length env then
+      ""
+    else begin
+      let e = env.(i) in
+      let pos = String.index e '=' in
+      if String.sub e 0 pos = s then
+        String.sub e (pos+1) (String.length e - pos - 1)
+      else
+        loop (i+1)
+    end
+  in
+  loop 0
+;;
+
+let unix_getcwd () =
+  Filename.basename (Unix.getcwd ())
+;;
+
+let sys_getcwd () =
+  Filename.basename (Sys.getcwd ())
+;;
+
+let unix_readdir s =
+  let h = Unix.opendir s in
+  let rec loop acc =
+    match Unix.readdir h with
+    | s ->
+        loop (s :: acc)
+    | exception End_of_file ->
+        Unix.closedir h;
+        acc
+  in
+  List.sort compare (loop [])
+;;
+
+let sys_readdir s =
+  List.sort compare (Array.to_list (Sys.readdir s))
+;;
+
+(* let open_process_in cmdline = *)
+(*   let f cmdline = *)
+(*     let ic as proc = Unix.open_process_in cmdline in *)
+(*     let l = List.tl (read_all ic) in *)
+(*     ignore (Unix.close_process_in proc); *)
+(*     l *)
+(*   in *)
+(*   wrap "Unix.open_process_in" f ell cmdline (list quote) *)
+
+(* let open_process_full filter cmdline env =
+     let f cmdline env =
+       let (ic, _, _) as proc =
+         Unix.open_process_full cmdline (Array.of_list env)
+       in
+       let l = read_all ic in
+       ignore (Unix.close_process_full proc);
+       List.sort compare (List.filter filter l)
+     in
+     wrap2 "Unix.open_process_full" f ell (list quote) cmdline env (list quote)
+*)
+
+let test_readdir readdir =
+  let filter s = List.mem s test_files && Filename.check_suffix s ".txt" in
+  List.filter filter (readdir Filename.current_dir_name)
+;;
+
+let test_open_in () =
+  let check s =
+    let ic = open_in s in
+    let l = input_line ic in
+    close_in ic;
+    l
+  in
+  let filter s = List.mem s test_files in
+  let files = List.filter filter (sys_readdir Filename.current_dir_name) in
+  List.map check files
+;;
+
+(* putenv not implemented
+let test_getenv () =
+  let equiv l r =
+    assert (l = r);
+    l, r
+  in
+  let doit key s =
+    Unix.putenv key s;
+    let l = equiv (Sys.getenv key) (getenvironmentenv key) in
+    let r =
+      Unix.putenv key (s ^ s);
+      equiv (Sys.getenv key) (getenvironmentenv key)
+    in
+      l, r
+  in
+  List.map2 doit foreign_names foreign_names2
+;;
+*)
+
+let test_mkdir () =
+  let doit s =
+    Unix.mkdir s 0o755;
+    Sys.file_exists s, Sys.is_directory s
+  in
+  List.map doit foreign_names
+;;
+
+let test_chdir chdir getcwd =
+  let curr = Sys.getcwd () in
+  let doit s =
+    chdir s;
+    let d = getcwd () in
+    chdir curr;
+    d
+  in
+  List.map doit foreign_names
+;;
+
+let test_rmdir () =
+  let doit s =
+    Unix.rmdir s;
+    Sys.file_exists s
+  in
+  List.map doit foreign_names
+;;
+
+let test_stat () =
+  let doit s =
+    (Unix.stat s).Unix.st_kind,
+    (Unix.lstat s).Unix.st_kind,
+    (Unix.LargeFile.stat s).Unix.LargeFile.st_kind,
+    (Unix.LargeFile.lstat s).Unix.LargeFile.st_kind
+  in
+  List.map doit to_create_and_delete_files
+;;
+
+let test_access () =
+  List.iter (fun s -> Unix.access s [Unix.F_OK]) to_create_and_delete_files
+
+let test_rename rename =
+  let doit s =
+    let s' = s ^ "-1" in
+    rename s s';
+    let x = Sys.file_exists s, Sys.file_exists s' in
+    rename s' s;
+    let y = Sys.file_exists s, Sys.file_exists s' in
+    x, y
+  in
+  List.map doit to_create_and_delete_files
+;;
+
+let test_open_out () =
+  let doit s =
+    let oc = open_out s in
+    Printf.fprintf oc "Hello, %s\n" s;
+    close_out oc;
+    let ic = open_in s in
+    let l = input_line ic in
+    close_in ic;
+    l
+  in
+  List.map doit to_create_and_delete_files
+;;
+
+let test_file_exists () =
+  List.map Sys.file_exists to_create_and_delete_files
+;;
+
+let test_remove () =
+  let doit s =
+    Sys.remove s;
+    Sys.file_exists s
+  in
+  List.map doit to_create_and_delete_files
+;;
+
+let create_file s =
+  let oc = open_out_bin s in
+  output_string oc s;
+  close_out oc
+;;
+
+let test_symlink () =
+  let foodir = "UNIQU\xE4\xBD\xA0\xE5\xA5\xBD" (* "UNIQU你好" *) in
+  let foofile = "UNIQU\xE4\xBD\xA0\xE5\xA5\xBD/\xE4\xBD\xA0\xE5\xA5\xBD.txt"
+                                                          (* "UNIQU你好/你好.txt" *)
+  in
+  let fileln = "\xE4\xBD\xA0\xE5\xA5\xBD-file-ln-s" (* "你好-file-ln-s" *) in
+  let dirln = "\xE4\xBD\xA0\xE5\xA5\xBD-dir-ln-s" (* "你好-dir-ln-s" *) in
+  Unix.mkdir foodir 0o777;
+  create_file foofile;
+  Unix.symlink ~to_dir:true foodir dirln;
+  Unix.symlink ~to_dir:false foofile fileln;
+  let res =
+    (Unix.stat fileln).Unix.st_kind = Unix.S_REG &&
+    (Unix.lstat fileln).Unix.st_kind = Unix.S_LNK
+(*  node cannot stat a link to a directory under Windows (EPERM)
+&& (Unix.stat dirln).Unix.st_kind = Unix.S_DIR
+&& (Unix.lstat dirln).Unix.st_kind = Unix.S_LNK *)
+  in
+  Sys.remove foofile;
+  Sys.remove fileln;
+  Sys.remove dirln;
+  Unix.rmdir foodir;
+  res
+;;
+
+List.iter create_file test_files;;
+
+let check_length ?(n = 4) v = assert (List.length v = n);;
+
+let t_unix_readdir = check_length @@ test_readdir unix_readdir;;
+let t_sys_readdir = check_length @@ test_readdir sys_readdir;;
+let t_open_in = check_length @@ test_open_in ();;
+let t_open_out = check_length ~n:10 @@ test_open_out ();;
+let t_file_exists = assert (List.for_all Fun.id (test_file_exists ()));;
+let t_stat = assert (List.for_all (fun x -> match x with Unix.S_REG,Unix.S_REG,Unix.S_REG,Unix.S_REG -> true | _ -> false) (test_stat ()));;
+test_access ();;
+let t_unix_rename = test_rename Unix.rename;;
+let t_sys_rename = test_rename Sys.rename;;
+assert (not (List.exists Fun.id (test_remove ())));;
+assert (List.for_all (fun (p, q) -> p && q) (test_mkdir ()));;
+let t_sys_chdir = assert (foreign_names = test_chdir Sys.chdir sys_getcwd);;
+let t_unix_chdir = assert (foreign_names = test_chdir Unix.chdir unix_getcwd);;
+assert (not (List.exists Fun.id (test_rmdir ())));;
+(*let t_getenv = test_getenv ();;*)
+assert (if Unix.has_symlink () then test_symlink () else true);;
diff --git a/compiler/tests-wasm_of_ocaml/preprocess/cram.t b/compiler/tests-wasm_of_ocaml/preprocess/cram.t
new file mode 100644
index 0000000000..d21009864c
--- /dev/null
+++ b/compiler/tests-wasm_of_ocaml/preprocess/cram.t
@@ -0,0 +1,189 @@
+Too many parentheses
+
+  $ echo '())' | wasm_of_ocaml pp
+  File "-", line 1, characters 2-3:
+  Unexpected closing parenthesis.
+  [1]
+
+  $ echo '();)' | wasm_of_ocaml pp
+  File "-", line 1, characters 2-4:
+  Unmatched closing comment.
+  [1]
+
+Missing parenthesis
+
+  $ echo '(()' | wasm_of_ocaml pp
+  File "-", line 1, characters 0-1:
+  Unclosed parenthesis.
+  [1]
+
+  $ echo '(; ()' | wasm_of_ocaml pp
+  File "-", line 1, characters 0-2:
+  Unclosed comment.
+  [1]
+
+  $ echo '(; (; ()' | wasm_of_ocaml pp
+  File "-", line 1, characters 3-5:
+  Unclosed comment.
+  [1]
+
+Unterminated string (we point at the newline)
+
+  $ echo '"abcd' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-5:
+  Malformed string.
+  [1]
+
+Bad conditional
+
+  $ echo '(@if)' | wasm_of_ocaml pp
+  File "-", line 1, characters 4-5:
+  Expecting condition.
+  [1]
+
+  $ echo '(@if a)' | wasm_of_ocaml pp
+  File "-", line 1, characters 6-7:
+  Expecting @then clause.
+  [1]
+
+  $ echo '(@if a xxx)' | wasm_of_ocaml pp
+  File "-", line 1, characters 7-10:
+  Expecting @then clause.
+  [1]
+
+  $ echo '(@if a (@then) xx)' | wasm_of_ocaml pp
+  File "-", line 1, characters 15-17:
+  Expecting @else clause or closing parenthesis.
+  [1]
+
+  $ echo '(@if a (@then) (@else) xx)' | wasm_of_ocaml pp
+  File "-", line 1, characters 23-25:
+  Expecting closing parenthesis.
+  [1]
+
+Syntax error in condition
+
+  $ echo '(@if () (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-7:
+  Syntax error.
+  [1]
+
+  $ echo '(@if (not) (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-10:
+  Syntax error.
+  [1]
+
+  $ echo '(@if (not (and) (or)) (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-21:
+  Syntax error.
+  [1]
+
+  $ echo '(@if (= "a") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-12:
+  Syntax error.
+  [1]
+
+  $ echo '(@if (= "a" "b" "c") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-20:
+  Syntax error.
+  [1]
+
+Unicode escape sequences
+
+  $ echo '(@if (= "\u{1F600}" "b") (@then))' | wasm_of_ocaml pp
+                                   
+
+  $ echo '(@if (= "\u{D800}" "b") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 8-18:
+  Invalid Unicode escape sequences.
+  [1]
+
+  $ echo '(@if (= "\u{110000}" "b") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 8-20:
+  Invalid Unicode escape sequences.
+  [1]
+
+Lonely @then or @else
+
+  $ echo '(@then)' | wasm_of_ocaml pp
+  File "-", line 1, characters 1-6:
+  Unexpected @then clause. Maybe you forgot a parenthesis.
+  [1]
+
+  $ echo '(@else)' | wasm_of_ocaml pp
+  File "-", line 1, characters 1-6:
+  Unexpected @else clause. Maybe you forgot a parenthesis.
+  [1]
+
+  $ echo '(@if (and) (@then (@else)))' | wasm_of_ocaml pp
+  File "-", line 1, characters 19-24:
+  Unexpected @else clause. Maybe you forgot a parenthesis.
+  [1]
+
+Undefined variable
+
+  $ echo '(@if a (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-6:
+  Unknown variable 'a'.
+  [1]
+
+Wrong type
+  $ echo '(@if "" (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 5-7:
+  Expected a boolean but this is a string.
+  [1]
+
+  $ echo '(@if (not "") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 10-12:
+  Expected a boolean but this is a string.
+  [1]
+
+  $ echo '(@if (and "") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 10-12:
+  Expected a boolean but this is a string.
+  [1]
+
+  $ echo '(@if (or "") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 9-11:
+  Expected a boolean but this is a string.
+  [1]
+
+  $ echo '(@if (= (and) "") (@then))' | wasm_of_ocaml pp
+  File "-", line 1, characters 14-16:
+  Expected a boolean but this is a string.
+  [1]
+
+Bad strings
+
+  $ echo '(@string)' | wasm_of_ocaml pp
+  File "-", line 1, characters 8-9:
+  Expecting an id or a string.
+  [1]
+
+  $ echo '(@string a "b")' | wasm_of_ocaml pp
+  File "-", line 1, characters 9-10:
+  Expecting an id
+  [1]
+
+  $ echo '(@string $a b)' | wasm_of_ocaml pp
+  File "-", line 1, characters 12-13:
+  Expecting a string
+  [1]
+
+  $ echo '(@string $good "\u{1F600}")' | wasm_of_ocaml pp
+  (global $good (ref eq) (array.new_fixed $bytes 4 (i32.const 240) (i32.const 159) (i32.const 152) (i32.const 128)))
+
+  $ echo '(@string $bad "\u{D800}")' | wasm_of_ocaml pp
+  File "-", line 1, characters 14-24:
+  Invalid Unicode escape sequences.
+  [1]
+
+  $ echo '(@string a)' | wasm_of_ocaml pp
+  File "-", line 1, characters 9-10:
+  Expecting a string
+  [1]
+
+  $ echo '(@string a b c)' | wasm_of_ocaml pp
+  File "-", line 1, characters 13-14:
+  Expecting a closing parenthesis.
+  [1]
diff --git a/compiler/tests-wasm_of_ocaml/preprocess/dune b/compiler/tests-wasm_of_ocaml/preprocess/dune
new file mode 100644
index 0000000000..bb37a89e75
--- /dev/null
+++ b/compiler/tests-wasm_of_ocaml/preprocess/dune
@@ -0,0 +1,21 @@
+(rule
+ (with-stdout-to
+  tests.output
+  (run
+   %{bin:wasm_of_ocaml}
+   pp
+   --enable
+   a
+   --disable
+   b
+   --set
+   c=1
+   %{dep:tests.txt})))
+
+(rule
+ (alias runtest)
+ (action
+  (diff tests.expected tests.output)))
+
+(cram
+ (deps %{bin:wasm_of_ocaml}))
diff --git a/compiler/tests-wasm_of_ocaml/preprocess/tests.expected b/compiler/tests-wasm_of_ocaml/preprocess/tests.expected
new file mode 100644
index 0000000000..9e5aa1ae61
--- /dev/null
+++ b/compiler/tests-wasm_of_ocaml/preprocess/tests.expected
@@ -0,0 +1,74 @@
+;; conditional
+              a is true                     
+                                b is false  
+              a is true  
+                         
+
+;; nested conditionals
+                                                     a is true and b is false  
+                                                                            
+
+;; not
+                                
+                    b is false  
+
+;; and
+                  true  
+                    a is true  
+                               
+                                        
+                            a is true and b is false  
+                                                      
+                                                     
+
+;; or
+                        
+                   a is true  
+                              
+                     a or b is true  
+                           a is true or b is false  
+                                                    
+                                 a or b is false  
+
+;; strings
+                           newline  
+                         quote  
+
+;; string comparisons
+                      c is 1  
+                              
+                                   
+                       c is not 2  
+
+;; version comparisons
+                                                   
+                                (4 1 1) = (4 1 1)  
+                                                   
+                                 (4 1 1) <> (4 1 0)  
+                                                     
+                                 (4 1 1) <> (4 1 2)  
+                                                     
+                                 (4 1 1) <= (4 1 1)  
+                                 (4 1 1) <= (4 1 2)  
+                                 (4 1 1) >= (4 1 0)  
+                                 (4 1 1) >= (4 1 1)  
+                                                     
+                                (4 1 1) > (4 1 0)  
+                                                   
+                                                   
+
+;; version comparisons: lexicographic order
+                                                   
+                                                   
+                                (4 1 1) < (4 1 2)  
+                                                   
+                                (4 1 1) < (4 2 0)  
+                                (4 1 1) < (5 0 1)  
+                                                   
+
+;; strings
+(global $s (ref eq) (array.new_fixed $bytes 4 (i32.const 97) (i32.const 98) (i32.const 99) (i32.const 100)))
+(array.new_fixed $bytes 4 (i32.const 97) (i32.const 98) (i32.const 99) (i32.const 100))
+(array.new_fixed $bytes 4 (i32.const 92) (i32.const 39) (i32.const 40) (i32.const 10))
+                  (array.new_fixed $bytes 4 (i32.const 97) (i32.const 98) (i32.const 99) (i32.const 100))  
+                         (array.new_fixed $bytes 4 (i32.const 97) (i32.const 98) (i32.const 99) (i32.const 100))  
diff --git a/compiler/tests-wasm_of_ocaml/preprocess/tests.txt b/compiler/tests-wasm_of_ocaml/preprocess/tests.txt
new file mode 100644
index 0000000000..98cd0bd3c8
--- /dev/null
+++ b/compiler/tests-wasm_of_ocaml/preprocess/tests.txt
@@ -0,0 +1,74 @@
+;; conditional
+(@if a (@then a is true) (@else a is false))
+(@if b (@then b is true) (@else b is false))
+(@if a (@then a is true))
+(@if b (@then b is true))
+
+;; nested conditionals
+(@if a (@then (@if b (@then a and b are true) (@else a is true and b is false)))
+ (@else (@if b (@then a is false and b is true) (@else a and b are false))))
+
+;; not
+(@if (not a) (@then a is false))
+(@if (not b) (@then b is false))
+
+;; and
+(@if (and) (@then true))
+(@if (and a) (@then a is true))
+(@if (and b) (@then b is true))
+(@if (and a b) (@then a and b are true))
+(@if (and a (not b)) (@then a is true and b is false))
+(@if (and (not a) b) (@then a is false and b is true))
+(@if (and (not a) (not b)) (@then a and b are false))
+
+;; or
+(@if (or) (@then false))
+(@if (or a) (@then a is true))
+(@if (or b) (@then b is true))
+(@if (or a b) (@then a or b is true))
+(@if (or a (not b)) (@then a is true or b is false))
+(@if (or (not a) b) (@then a is false or b is true))
+(@if (or (not a) (not b)) (@then a or b is false))
+
+;; strings
+(@if (= "\n" "\0a") (@then newline))
+(@if (= "\'" "'") (@then quote))
+
+;; string comparisons
+(@if (= c "1") (@then c is 1))
+(@if (= c "2") (@then c is 2))
+(@if (<> c "1") (@then c is not 1))
+(@if (<> c "2") (@then c is not 2))
+
+;; version comparisons
+(@if (= (4 1 1) (4 1 0)) (@then (4 1 1) = (4 1 0)))
+(@if (= (4 1 1) (4 1 1)) (@then (4 1 1) = (4 1 1)))
+(@if (= (4 1 1) (4 1 2)) (@then (4 1 1) = (4 1 2)))
+(@if (<> (4 1 1) (4 1 0)) (@then (4 1 1) <> (4 1 0)))
+(@if (<> (4 1 1) (4 1 1)) (@then (4 1 1) <> (4 1 1)))
+(@if (<> (4 1 1) (4 1 2)) (@then (4 1 1) <> (4 1 2)))
+(@if (<= (4 1 1) (4 1 0)) (@then (4 1 1) <= (4 1 0)))
+(@if (<= (4 1 1) (4 1 1)) (@then (4 1 1) <= (4 1 1)))
+(@if (<= (4 1 1) (4 1 2)) (@then (4 1 1) <= (4 1 2)))
+(@if (>= (4 1 1) (4 1 0)) (@then (4 1 1) >= (4 1 0)))
+(@if (>= (4 1 1) (4 1 1)) (@then (4 1 1) >= (4 1 1)))
+(@if (>= (4 1 1) (4 1 2)) (@then (4 1 1) >= (4 1 2)))
+(@if (> (4 1 1) (4 1 0)) (@then (4 1 1) > (4 1 0)))
+(@if (> (4 1 1) (4 1 1)) (@then (4 1 1) > (4 1 1)))
+(@if (> (4 1 1) (4 1 2)) (@then (4 1 1) > (4 1 2)))
+
+;; version comparisons: lexicographic order
+(@if (< (4 1 1) (4 1 0)) (@then (4 1 1) < (4 1 0)))
+(@if (< (4 1 1) (4 1 1)) (@then (4 1 1) < (4 1 1)))
+(@if (< (4 1 1) (4 1 2)) (@then (4 1 1) < (4 1 2)))
+(@if (< (4 1 1) (4 0 2)) (@then (4 1 1) < (4 0 2)))
+(@if (< (4 1 1) (4 2 0)) (@then (4 1 1) < (4 2 0)))
+(@if (< (4 1 1) (5 0 1)) (@then (4 1 1) < (5 0 1)))
+(@if (< (4 1 1) (3 2 1)) (@then (4 1 1) < (3 2 1)))
+
+;; strings
+(@string $s "abcd")
+(@string "abcd")
+(@string "\\\'\28\n")
+(@if (and) (@then (@string "abcd")))
+(@if (or) (@then) (@else (@string "abcd")))
diff --git a/dune b/dune
index a4064b14a9..21183b243c 100644
--- a/dune
+++ b/dune
@@ -30,6 +30,23 @@
   (binaries
    (tools/node_wrapper.exe as node)
    (tools/node_wrapper.exe as node.exe)))
+ (wasi
+  (wasm_of_ocaml
+   (build_runtime_flags
+    (:standard
+     --enable
+     wasi
+     (:include wasi_extra_flags)))
+   (flags
+    (:standard
+     --pretty
+     --enable
+     wasi
+     (:include wasi_extra_flags)))
+   (compilation_mode separate))
+  (binaries
+   (tools/node_wrapper.exe as node)
+   (tools/node_wrapper.exe as node.exe)))
  (bench_no_debug
   (flags
    (:standard \ -g))
@@ -55,6 +72,13 @@
     %{dep:VERSION}
     %{dep:tools/version/GIT-VERSION}))))
 
+(rule
+ (targets wasi_extra_flags)
+ (action
+  (with-stdout-to
+   %{targets}
+   (echo "(%{env:WASI_FLAGS=})"))))
+
 (data_only_dirs _wikidoc doc-dev janestreet)
 
 (vendored_dirs)
diff --git a/lib/deriving_json/tests/dune b/lib/deriving_json/tests/dune
index c1e0147b3d..b7772e347e 100644
--- a/lib/deriving_json/tests/dune
+++ b/lib/deriving_json/tests/dune
@@ -2,6 +2,8 @@
  (name deriving_expect_tests)
  (libraries unix js_of_ocaml js_of_ocaml.deriving)
  (inline_tests
+  (enabled_if
+   (<> %{profile} wasi))
   (modes js wasm))
  (preprocess
   (pps ppx_expect ppx_deriving_json)))
diff --git a/lib/runtime/js_of_ocaml_runtime_stubs.c b/lib/runtime/js_of_ocaml_runtime_stubs.c
index 4e9ad9bd29..44ea7ecb8a 100644
--- a/lib/runtime/js_of_ocaml_runtime_stubs.c
+++ b/lib/runtime/js_of_ocaml_runtime_stubs.c
@@ -272,6 +272,10 @@ void caml_string_of_uint8_array () {
   caml_fatal_error("Unimplemented Javascript primitive caml_string_of_uint8_array!");
 }
 
+void caml_throw_js_exception () {
+  caml_fatal_error("Unimplemented Javascript primitive caml_throw_js_exception!");
+}
+
 void caml_unmount () {
   caml_fatal_error("Unimplemented Javascript primitive caml_unmount!");
 }
diff --git a/lib/runtime/jsoo_runtime.ml b/lib/runtime/jsoo_runtime.ml
index c69b5b4937..4dee5f64a9 100644
--- a/lib/runtime/jsoo_runtime.ml
+++ b/lib/runtime/jsoo_runtime.ml
@@ -177,7 +177,7 @@ end = struct
 
   let _ = Callback.register_exception "jsError" (Exn (Obj.magic [||]))
 
-  let raise_ : t -> 'a = Js.js_expr "(function (exn) { throw exn })"
+  external raise_ : t -> 'a = "caml_throw_js_exception"
 
   external of_exn : exn -> t option = "caml_js_error_option_of_exception"
 
diff --git a/lib/tests/dune.inc b/lib/tests/dune.inc
index 85b9beb929..723489479a 100644
--- a/lib/tests/dune.inc
+++ b/lib/tests/dune.inc
@@ -2,7 +2,7 @@
 (library
  ;; lib/tests/test_css_angle.ml
  (name test_css_angle_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_css_angle)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -12,7 +12,7 @@
 (library
  ;; lib/tests/test_css_color.ml
  (name test_css_color_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_css_color)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -22,7 +22,7 @@
 (library
  ;; lib/tests/test_css_length.ml
  (name test_css_length_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_css_length)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -42,7 +42,7 @@
 (library
  ;; lib/tests/test_fun_call_2.ml
  (name test_fun_call_2_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_fun_call_2)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -62,7 +62,7 @@
 (library
  ;; lib/tests/test_nodejs_filesystem_errors.ml
  (name test_nodejs_filesystem_errors_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_nodejs_filesystem_errors)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -82,7 +82,7 @@
 (library
  ;; lib/tests/test_poly_equal.ml
  (name test_poly_equal_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_poly_equal)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -92,7 +92,7 @@
 (library
  ;; lib/tests/test_regexp.ml
  (name test_regexp_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_regexp)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -112,7 +112,7 @@
 (library
  ;; lib/tests/test_typed_array.ml
  (name test_typed_array_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_typed_array)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -122,7 +122,7 @@
 (library
  ;; lib/tests/test_unsafe_set_get.ml
  (name test_unsafe_set_get_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_unsafe_set_get)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
@@ -132,7 +132,7 @@
 (library
  ;; lib/tests/test_url.ml
  (name test_url_75)
- (enabled_if true)
+ (enabled_if (<> %{profile} wasi))
  (modules test_url)
  (libraries js_of_ocaml unix)
  (inline_tests (modes js wasm))
diff --git a/lib/tests/gen-rules/gen.ml b/lib/tests/gen-rules/gen.ml
index 980e2f0cc8..6c6dab4671 100644
--- a/lib/tests/gen-rules/gen.ml
+++ b/lib/tests/gen-rules/gen.ml
@@ -81,7 +81,8 @@ let () =
            basename
            (Hashtbl.hash prefix mod 100)
            (match enabled_if basename with
-           | Any | Not_wasm -> "true"
+           | Any -> "(<> %{profile} wasi)"
+           | Not_wasm -> "true"
            | GE5 -> "(>= %{ocaml_version} 5)"
            | No_effects_not_wasm -> "(<> %{profile} with-effects)")
            basename
diff --git a/manual/wasm_runtime.wiki b/manual/wasm_runtime.wiki
index 6547c55338..4dfd37812d 100644
--- a/manual/wasm_runtime.wiki
+++ b/manual/wasm_runtime.wiki
@@ -46,7 +46,25 @@ You can import the following functions to access or allocate integers of type in
    (func $caml_copy_int64 (param i64) (result (ref eq))))
 }}}
 
-== Implementing primitives
+== Preprocessor ==
+
+The Wasm text files are passed through a preprocessor. You can run the processor manually: {{{wasm_of_ocaml pp test.wasm}}}.
+
+This preprocessing step allows optional compilations of pieces of code depending on the version of the compiler.
+{{{
+(@if (>= ocaml_version  (5 2 0))
+   (@then ...)
+   (@else ...))
+}}}
+To form conditional expressions, the following operators are available:
+- comparisons: {{{=}}}, {{{>}}}, {{{>=}}}, {{{<}}}, {{{<=}}}, {{{<>}}};
+- boolean operators: {{{and}}}, {{{or}}}, {{{not}}}
+
+It also provides some syntactic sugar to write strings. The expression{{{(@string "ab")}}} is expanded into {{{(array.new_fixed $bytes 2 (i32.const 97) (i32.const 98))}}}. The statement {{{(@string $s "ab")}}} is an abbreviation for {{{(global $s (ref eq) (@string "ab"))}}}.
+
+To provide a better debugging experience, the function export name is used to name functions with no explicit id: {{{(func (export "foo") ...)}}}} is expanded into {{{(func $foo (export "foo") ...)}}}}.
+
+== Implementing primitives ==
 
 You define a primitive by exporting a Wasm function with parameters and return value of type {{{(ref eq)}}}.
 {{{
diff --git a/runtime/js/fs.js b/runtime/js/fs.js
index dc8cfcb0da..db8002262b 100644
--- a/runtime/js/fs.js
+++ b/runtime/js/fs.js
@@ -198,13 +198,15 @@ function caml_unmount(name) {
 
 //Provides: caml_sys_getcwd
 //Requires: caml_current_dir, caml_string_of_jsstring
+//Alias: caml_unix_getcwd
+//Alias: unix_getcwd
 function caml_sys_getcwd() {
   return caml_string_of_jsstring(caml_current_dir);
 }
 
 //Provides: caml_sys_chdir
-//Requires: caml_current_dir, caml_raise_no_such_file, resolve_fs_device, caml_trailing_slash, caml_jsstring_of_string, caml_raise_sys_error
-function caml_sys_chdir(dir) {
+//Requires: caml_current_dir, caml_raise_no_such_file, resolve_fs_device, caml_trailing_slash, caml_jsstring_of_string, caml_raise_system_error
+function caml_sys_chdir(dir, raise_unix) {
   var root = resolve_fs_device(dir);
   if (root.device.is_dir(root.rest)) {
     if (root.rest)
@@ -212,11 +214,15 @@ function caml_sys_chdir(dir) {
     else caml_current_dir = root.path;
     return 0;
   } else if (root.device.exists(root.rest)) {
-    caml_raise_sys_error(
-      "ENOTDIR: not a directory, chdir '" + caml_jsstring_of_string(dir) + "'",
+    caml_raise_system_error(
+      raise_unix,
+      "ENOTDIR",
+      "chdir",
+      "not a directory",
+      caml_jsstring_of_string(dir),
     );
   } else {
-    caml_raise_no_such_file(caml_jsstring_of_string(dir));
+    caml_raise_no_such_file(caml_jsstring_of_string(dir), raise_unix);
   }
 }
 
diff --git a/runtime/js/fs_fake.js b/runtime/js/fs_fake.js
index d5af726d7a..0273b9a8bb 100644
--- a/runtime/js/fs_fake.js
+++ b/runtime/js/fs_fake.js
@@ -203,6 +203,23 @@ MlFakeDevice.prototype.unlink = function (name, raise_unix) {
   delete this.content[name];
   return 0;
 };
+MlFakeDevice.prototype.access = function (name, f, raise_unix) {
+  var file;
+  this.lookup(name);
+  if (this.content[name]) {
+    if (this.is_dir(name))
+      caml_raise_system_error(
+        raise_unix,
+        "EACCESS",
+        "access",
+        "permission denied,",
+        this.nm(name),
+      );
+  } else {
+    caml_raise_no_such_file(this.nm(name), raise_unix);
+  }
+  return 0;
+};
 MlFakeDevice.prototype.open = function (name, f, _perms, raise_unix) {
   var file;
   this.lookup(name);
diff --git a/runtime/js/fs_node.js b/runtime/js/fs_node.js
index 5085c87d11..38be7f8cfb 100644
--- a/runtime/js/fs_node.js
+++ b/runtime/js/fs_node.js
@@ -113,6 +113,33 @@ MlNodeDevice.prototype.truncate = function (name, len, raise_unix) {
     caml_raise_nodejs_error(err, raise_unix);
   }
 };
+MlNodeDevice.prototype.access = function (name, f, raise_unix) {
+  var consts = require("node:constants");
+  var res = 0;
+  for (var key in f) {
+    switch (key) {
+      case "r":
+        res |= consts.R_OK;
+        break;
+      case "w":
+        res |= consts.W_OK;
+        break;
+      case "x":
+        res |=
+          globalThis.process?.platform === "win32" ? consts.R_OK : consts.X_OK;
+        break;
+      case "f":
+        res |= consts.F_OK;
+        break;
+    }
+  }
+  try {
+    this.fs.accessSync(this.nm(name), res);
+    return 0;
+  } catch (err) {
+    caml_raise_nodejs_error(err, raise_unix);
+  }
+};
 MlNodeDevice.prototype.open = function (name, f, perms, raise_unix) {
   var consts = require("node:constants");
   var res = 0;
@@ -226,6 +253,22 @@ MlNodeDevice.prototype.lstat = function (name, large, raise_unix) {
     caml_raise_nodejs_error(err, raise_unix);
   }
 };
+MlNodeDevice.prototype.chmod = function (name, perms, raise_unix) {
+  try {
+    this.fs.chmodSync(this.nm(name), perms);
+    return 0;
+  } catch (err) {
+    caml_raise_nodejs_error(err, raise_unix);
+  }
+};
+MlNodeDevice.prototype.link = function (target, path, raise_unix) {
+  try {
+    this.fs.linkSync(this.nm(target), this.nm(path));
+    return 0;
+  } catch (err) {
+    caml_raise_nodejs_error(err, raise_unix);
+  }
+};
 MlNodeDevice.prototype.symlink = function (to_dir, target, path, raise_unix) {
   try {
     this.fs.symlinkSync(
@@ -409,6 +452,22 @@ MlNodeFd.prototype.stat = function (large) {
     caml_raise_nodejs_error(err, /* raise Unix_error */ 1);
   }
 };
+MlNodeFd.prototype.chmod = function (perms) {
+  try {
+    this.fs.fchmodSync(this.fd, perms);
+    return 0;
+  } catch (err) {
+    caml_raise_nodejs_error(err, /* raise Unix_error */ 1);
+  }
+};
+MlNodeFd.prototype.sync = function () {
+  try {
+    this.fs.fsyncSync(this.fd);
+    return 0;
+  } catch (err) {
+    caml_raise_nodejs_error(err, /* raise Unix_error */ 1);
+  }
+};
 MlNodeFd.prototype.close = function (raise_unix) {
   try {
     this.fs.closeSync(this.fd);
diff --git a/runtime/js/jslib.js b/runtime/js/jslib.js
index 8b974912d4..6f538410f7 100644
--- a/runtime/js/jslib.js
+++ b/runtime/js/jslib.js
@@ -203,6 +203,11 @@ function caml_js_error_option_of_exception(exn) {
   return 0;
 }
 
+//Provides: caml_throw_js_exception
+function caml_throw_js_exception(exn) {
+  throw exn;
+}
+
 //Provides: caml_js_from_bool const (const)
 function caml_js_from_bool(x) {
   return !!x;
diff --git a/runtime/js/unix.js b/runtime/js/unix.js
index cb576e490e..46c29d8f19 100644
--- a/runtime/js/unix.js
+++ b/runtime/js/unix.js
@@ -11,6 +11,20 @@ function caml_unix_time() {
   return Math.floor(caml_unix_gettimeofday());
 }
 
+//Provides: caml_unix_times
+//Requires: caml_failwith
+//Alias: unix_times
+function caml_unix_times() {
+  if (globalThis.process && globalThis.process.cpuUsage) {
+    var t = globalThis.process.cpuUsage();
+    return BLOCK(0, t.user / 1e6, t.system / 1e6, 0, 0);
+  } else if (globalThis.performance && globalThis.performance.now) {
+    return BLOCK(0, globalThis.performance.now() / 1000, 0, 0, 0);
+  } else {
+    caml_failwith("caml_unix_times: not implemented");
+  }
+}
+
 //Provides: caml_unix_gmtime
 //Alias: unix_gmtime
 function caml_unix_gmtime(t) {
@@ -199,6 +213,36 @@ function make_unix_err_args(code, syscall, path, errno) {
   return args;
 }
 
+//Provides: caml_strerror
+//Requires: unix_error
+function caml_strerror(errno) {
+  const util = require("node:util");
+  if (errno >= 0) {
+    const code = unix_error[errno];
+    return util
+      .getSystemErrorMap()
+      .entries()
+      .find((x) => x[1][0] === code)[1][1];
+  } else {
+    return util.getSystemErrorMessage(errno);
+  }
+}
+
+//Provides: unix_error_message
+//Alias: caml_unix_error_message
+//Requires: caml_strerror, caml_string_of_jsstring
+function unix_error_message(err) {
+  const errno = typeof err === "number" ? err : -err[1];
+  return caml_string_of_jsstring(caml_strerror(errno));
+}
+
+//Provides: caml_unix_chdir
+//Requires: caml_sys_chdir
+//Alias: unix_chdir
+function caml_unix_chdir(dir) {
+  return caml_sys_chdir(dir, /* raise Unix_error */ true);
+}
+
 //Provides: caml_unix_stat
 //Requires: resolve_fs_device, caml_failwith
 //Alias: unix_stat
@@ -259,6 +303,17 @@ function caml_unix_lstat_64(name) {
   );
 }
 
+//Provides: caml_unix_chmod
+//Requires: resolve_fs_device, caml_failwith
+//Alias: unix_chmod
+function caml_unix_chmod(name, perms) {
+  var root = resolve_fs_device(name);
+  if (!root.device.chmod) {
+    caml_failwith("caml_unix_chmod: not implemented");
+  }
+  return root.device.chmod(root.rest, perms);
+}
+
 //Provides: caml_unix_rename
 //Requires: caml_failwith, resolve_fs_device
 //Requires: caml_raise_system_error
@@ -294,6 +349,26 @@ function caml_unix_rmdir(name) {
   return root.device.rmdir(root.rest, /* raise Unix_error */ true);
 }
 
+//Provides: caml_unix_link
+//Requires: resolve_fs_device, caml_failwith, caml_raise_system_error
+//Alias: unix_link
+function caml_unix_link(follow, src, dst) {
+  var src_root = resolve_fs_device(src);
+  var dst_root = resolve_fs_device(dst);
+  if (!src_root.device.link) {
+    caml_failwith("caml_unix_link: not implemented");
+  }
+  if (typeof follow !== "number")
+    caml_raise_system_error(/* raise Unix_error */ 1, "ENOSYS", "link");
+  if (src_root.device !== dst_root.device)
+    caml_raise_system_error(/* raise Unix_error */ 1, "EXDEV", "link");
+  return src_root.device.link(
+    src_root.rest,
+    dst_root.rest,
+    /* raise Unix_error */ true,
+  );
+}
+
 //Provides: caml_unix_symlink
 //Requires: resolve_fs_device, caml_failwith, caml_jsstring_of_string
 //Alias: unix_symlink
@@ -373,6 +448,36 @@ function caml_unix_truncate_64(name, len) {
   return 0;
 }
 
+//Provides: caml_unix_access
+//Requires: resolve_fs_device, caml_failwith
+//Alias: unix_access
+function caml_unix_access(name, flags) {
+  var f = {};
+  while (flags) {
+    switch (flags[1]) {
+      case 0:
+        f.r = 1;
+        break;
+      case 1:
+        f.w = 1;
+        break;
+      case 2:
+        f.x = 1;
+        break;
+      case 3:
+        f.f = 1;
+        break;
+    }
+    flags = flags[2];
+  }
+  var root = resolve_fs_device(name);
+  if (!root.device.access) {
+    caml_failwith("caml_unix_access: not implemented");
+  }
+  root.device.access(root.rest, f, /* raise Unix_error */ true);
+  return 0;
+}
+
 //Provides: caml_unix_open
 //Requires: resolve_fs_device, caml_sys_fds, MlChanid
 //Alias: unix_open
@@ -455,6 +560,28 @@ function caml_unix_fstat_64(fd) {
   return file.stat(/* large */ true);
 }
 
+//Provides: caml_unix_fchmod
+//Alias: unix_fchmod
+//Requires: caml_unix_lookup_file, caml_failwith
+function caml_unix_fchmod(fd, perms) {
+  var file = caml_unix_lookup_file(fd, "fchmod");
+  if (!file.chmod) {
+    caml_failwith("caml_unix_fchmod: not implemented");
+  }
+  return file.chmod(perms);
+}
+
+//Provides: caml_unix_fsync
+//Alias: unix_fsync
+//Requires: caml_unix_lookup_file, caml_failwith
+function caml_unix_fsync(fd) {
+  var file = caml_unix_lookup_file(fd, "fsync");
+  if (!file.sync) {
+    caml_failwith("caml_unix_fsync: not implemented");
+  }
+  return file.sync();
+}
+
 //Provides: caml_unix_write
 //Alias: unix_write
 //Requires: caml_unix_lookup_file, caml_uint8_array_of_bytes
@@ -471,6 +598,20 @@ function caml_unix_write(fd, buf, pos, len) {
   return written;
 }
 
+//Provides: caml_unix_single_write
+//Alias: unix_single_write
+//Requires: caml_unix_lookup_file, caml_uint8_array_of_bytes
+function caml_unix_single_write(fd, buf, pos, len) {
+  var file = caml_unix_lookup_file(fd, "write");
+  if (len === 0) return 0;
+  return file.write(
+    caml_uint8_array_of_bytes(buf),
+    pos,
+    len,
+    /* raise unix_error */ 1,
+  );
+}
+
 //Provides: caml_unix_write_bigarray
 //Alias: caml_unix_lookup_file
 //Requires: caml_ba_to_typed_array, caml_unix_lookup_file
@@ -589,10 +730,43 @@ function caml_unix_getuid(unit) {
   return 1;
 }
 
-//Provides: caml_unix_getpwuid
+//Provides: caml_unix_geteuid
+//Alias: unix_geteuid
+function caml_unix_geteuid(unit) {
+  if (globalThis.process && globalThis.process.geteuid) {
+    return globalThis.process.geteuid();
+  }
+  return 1;
+}
+
+//Provides: caml_unix_getgid
+//Alias: unix_getgid
+function caml_unix_getgid(unit) {
+  if (globalThis.process && globalThis.process.getgid) {
+    return globalThis.process.getgid();
+  }
+  return 1;
+}
+
+//Provides: caml_unix_getegid
+//Alias: unix_getegid
+function caml_unix_getegid(unit) {
+  if (globalThis.process && globalThis.process.getegid) {
+    return globalThis.process.getegid();
+  }
+  return 1;
+}
+
+//Provides: caml_unix_getpwnam
 //Requires: caml_raise_not_found
+//Alias: unix_getpwnam
+//Alias: caml_unix_getpwuid
 //Alias: unix_getpwuid
-function caml_unix_getpwuid(unit) {
+//Alias: caml_unix_getgrnam
+//Alias: unix_getgrnam
+//Alias: caml_unix_getgrgid
+//Alias: unix_getgrgid
+function caml_unix_getpwnam(unit) {
   caml_raise_not_found();
 }
 
diff --git a/runtime/wasm/args.ml b/runtime/wasm/args.ml
index 16cd0418a5..3b34457fea 100644
--- a/runtime/wasm/args.ml
+++ b/runtime/wasm/args.ml
@@ -1,4 +1,4 @@
 let () =
   for i = 1 to Array.length Sys.argv - 1 do
-    Format.printf "%s@.%s@." Sys.argv.(i) (Filename.chop_suffix Sys.argv.(i) ".wat")
+    Format.printf "%s:%s@." (Filename.chop_suffix Sys.argv.(i) ".wat") Sys.argv.(i)
   done
diff --git a/runtime/wasm/array.wat b/runtime/wasm/array.wat
index 0be161fec1..6f9acbf4ff 100644
--- a/runtime/wasm/array.wat
+++ b/runtime/wasm/array.wat
@@ -24,7 +24,7 @@
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
 
-   (data $Array_make "Array.make")
+   (@string $Array_make "Array.make")
 
    (global $empty_array (ref eq)
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
@@ -34,10 +34,7 @@
       (local $sz i32) (local $b (ref $block)) (local $f f64)
       (local.set $sz (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (if (i32.ge_u (local.get $sz) (i32.const 0xfffffff))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $Array_make
-                               (i32.const 0) (i32.const 10)))))
+         (then (call $caml_invalid_argument (global.get $Array_make))))
       (if (i32.eqz (local.get $sz)) (then (return (global.get $empty_array))))
       (drop (block $not_float (result (ref eq))
          (local.set $f
@@ -56,10 +53,7 @@
       (local $sz i32) (local $f f64)
       (local.set $sz (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (if (i32.ge_u (local.get $sz) (i32.const 0x7ffffff))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $Array_make
-                               (i32.const 0) (i32.const 10)))))
+         (then (call $caml_invalid_argument (global.get $Array_make))))
       (if (i32.eqz (local.get $sz)) (then (return (global.get $empty_array))))
       (local.set $f
          (struct.get $float 0
@@ -73,10 +67,7 @@
       (local $sz i32)
       (local.set $sz (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (if (i32.ge_u (local.get $sz) (i32.const 0x7ffffff))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $Array_make
-                               (i32.const 0) (i32.const 10)))))
+         (then (call $caml_invalid_argument (global.get $Array_make))))
       (if (i32.eqz (local.get $sz)) (then (return (global.get $empty_array))))
       (array.new $float_array (f64.const 0) (local.get $sz)))
 
diff --git a/runtime/wasm/backtrace.wat b/runtime/wasm/backtrace.wat
index b411778759..62afca6fc1 100644
--- a/runtime/wasm/backtrace.wat
+++ b/runtime/wasm/backtrace.wat
@@ -38,14 +38,12 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
-   (data $raw_backtrace_slot_err
+   (@string $raw_backtrace_slot_err
       "Printexc.get_raw_backtrace_slot: index out of bounds")
 
    (func (export "caml_raw_backtrace_slot")
       (param (ref eq) (ref eq)) (result (ref eq))
-      (call $caml_invalid_argument
-          (array.new_data $bytes $raw_backtrace_slot_err
-             (i32.const 0) (i32.const 52)))
+      (call $caml_invalid_argument (global.get $raw_backtrace_slot_err))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_convert_raw_backtrace_slot")
diff --git a/runtime/wasm/bigarray.wat b/runtime/wasm/bigarray.wat
index 947f2e2bdb..c468ada3f6 100644
--- a/runtime/wasm/bigarray.wat
+++ b/runtime/wasm/bigarray.wat
@@ -16,6 +16,497 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+   (import "fail" "caml_bound_error" (func $caml_bound_error))
+   (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
+   (import "fail" "caml_invalid_argument"
+      (func $caml_invalid_argument (param (ref eq))))
+   (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
+   (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
+   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
+   (import "int32" "caml_copy_int32"
+      (func $caml_copy_int32 (param i32) (result (ref eq))))
+   (import "int32" "Int32_val"
+      (func $Int32_val (param (ref eq)) (result i32)))
+   (import "int32" "caml_copy_nativeint"
+      (func $caml_copy_nativeint (param i32) (result (ref eq))))
+   (import "int64" "caml_copy_int64"
+      (func $caml_copy_int64 (param i64) (result (ref eq))))
+   (import "int64" "Int64_val"
+      (func $Int64_val (param (ref eq)) (result i64)))
+   (import "obj" "double_array_tag" (global $double_array_tag i32))
+   (import "compare" "unordered" (global $unordered i32))
+   (import "hash" "caml_hash_mix_int"
+      (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
+   (import "hash" "caml_hash_mix_int64"
+      (func $caml_hash_mix_int64 (param i32) (param i64) (result i32)))
+   (import "hash" "caml_hash_mix_double"
+      (func $caml_hash_mix_double (param i32) (param f64) (result i32)))
+   (import "hash" "caml_hash_mix_float"
+      (func $caml_hash_mix_float (param i32) (param f32) (result i32)))
+   (import "hash" "caml_hash_mix_float16"
+      (func $caml_hash_mix_float16 (param i32) (param i32) (result i32)))
+   (import "marshal" "caml_serialize_int_1"
+      (func $caml_serialize_int_1 (param (ref eq)) (param i32)))
+   (import "marshal" "caml_serialize_int_2"
+      (func $caml_serialize_int_2 (param (ref eq)) (param i32)))
+   (import "marshal" "caml_serialize_int_4"
+      (func $caml_serialize_int_4 (param (ref eq)) (param i32)))
+   (import "marshal" "caml_serialize_int_8"
+      (func $caml_serialize_int_8 (param (ref eq)) (param i64)))
+   (import "marshal" "caml_deserialize_uint_1"
+      (func $caml_deserialize_uint_1 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_sint_1"
+      (func $caml_deserialize_sint_1 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_uint_2"
+      (func $caml_deserialize_uint_2 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_sint_2"
+      (func $caml_deserialize_sint_2 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_int_4"
+      (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
+   (import "marshal" "caml_deserialize_int_8"
+      (func $caml_deserialize_int_8 (param (ref eq)) (result i64)))
+
+(@if wasi
+(@then
+   (type $i32_array (array (mut i32)))
+   (type $i16_array (array (mut i16)))
+   (type $f32_array (array (mut f32)))
+
+   (type $data
+      (struct
+         (field $array (ref array))
+         (field $offset i32)
+         (field $len i32)))
+
+   (func $ta_create (export "ta_create")
+      (param $kind i32) (param $sz i32) (result (ref extern))
+      (local $a (ref array))
+      (local.set $a
+       (block $cont (result (ref array))
+        (block $f32
+         (block $f64
+          (block $i8
+           (block $i16
+            (block $i32
+             (br_table
+                $f32 $f64 $i8 $i8 $i16 $i16 $i32
+                $i32 $i32 $i32 $f32 $f64 $i8 $i16
+                (local.get $kind)))
+            ;; i32
+            (br $cont (array.new $i32_array (i32.const 0) (local.get $sz))))
+           ;; i16
+           (br $cont (array.new $i16_array (i32.const 0) (local.get $sz))))
+          ;; i8
+          (br $cont (array.new $bytes (i32.const 0) (local.get $sz))))
+         ;; f64
+         (br $cont (array.new $float_array (f64.const 0) (local.get $sz))))
+        ;; f32
+        (array.new $f32_array (f32.const 0) (local.get $sz))))
+      (extern.convert_any
+         (struct.new $data (local.get $a) (i32.const 0) (local.get $sz))))
+
+   (func $ta_length (export "ta_length") (param $b (ref extern)) (result i32)
+      (struct.get $data $len
+         (ref.cast (ref $data) (any.convert_extern (local.get $b)))))
+
+   (func $ta_get_f64 (param $b (ref extern)) (param $i i32) (result f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get $float_array
+         (ref.cast (ref $float_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_f32 (param $b (ref extern)) (param $i i32) (result f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (f64.promote_f32
+         (array.get $f32_array
+            (ref.cast (ref $f32_array) (struct.get $data $array (local.get $d)))
+            (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))))
+
+   (func $ta_get_i32 (export "ta_get_i32")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get $i32_array
+         (ref.cast (ref $i32_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_i16 (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_s $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_ui16 (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_u $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_i8 (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_s $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get_ui8 (export "ta_get_ui8")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.get_u $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))))
+
+   (func $ta_get32_ui8 (export "ta_get32_ui8")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data)) (local $s (ref $bytes))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (i32.or
+         (i32.or
+            (array.get_u $bytes (local.get $s) (local.get $i))
+            (i32.shl
+               (array.get_u $bytes (local.get $s)
+                  (i32.add (local.get $i) (i32.const 1)))
+               (i32.const 8)))
+         (i32.or
+            (i32.shl
+               (array.get_u $bytes (local.get $s)
+                  (i32.add (local.get $i) (i32.const 2)))
+               (i32.const 16))
+            (i32.shl
+               (array.get_u $bytes (local.get $s)
+                  (i32.add (local.get $i) (i32.const 3)))
+               (i32.const 24)))))
+
+   (func $ta_get16_ui8 (export "ta_get16_ui8")
+      (param $b (ref extern)) (param $i i32) (result i32)
+      (local $d (ref $data)) (local $s (ref $bytes))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (i32.or
+         (array.get_u $bytes (local.get $s) (local.get $i))
+         (i32.shl
+            (array.get_u $bytes (local.get $s)
+               (i32.add (local.get $i) (i32.const 1)))
+            (i32.const 8))))
+
+   (func $ta_set_f64 (param $b (ref extern)) (param $i i32) (param $v f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $float_array
+         (ref.cast (ref $float_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (local.get $v)))
+
+   (func $ta_set_f32 (param $b (ref extern)) (param $i i32) (param $v f64)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $f32_array
+         (ref.cast (ref $f32_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (f32.demote_f64 (local.get $v))))
+
+   (func $ta_set_i32 (export "ta_set_i32")
+      (param $b (ref extern)) (param $i i32) (param $v i32)
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $i32_array
+         (ref.cast (ref $i32_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (local.get $v)))
+
+   (func $ta_set_i16 (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set_ui16 (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $i16_array
+         (ref.cast (ref $i16_array) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set_i8 (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set_ui8 (export "ta_set_ui8")
+      (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (array.set $bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d)))
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i))
+         (i31.get_u (local.get $v))))
+
+   (func $ta_set16_ui8
+      (param $b (ref extern)) (param $i i32) (param $v (ref i31))
+      (local $d (ref $data)) (local $s (ref $bytes)) (local $j i32)
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $j (i31.get_u (local.get $v)))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (array.set $bytes (local.get $s) (local.get $i) (local.get $j))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 1))
+         (i32.shr_u (local.get $j) (i32.const 8))))
+
+   (func $ta_set32_ui8 (param $b (ref extern)) (param $i i32) (param $v i32)
+      (local $d (ref $data)) (local $s (ref $bytes))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $d))))
+      (local.set $i
+         (i32.add (struct.get $data $offset (local.get $d)) (local.get $i)))
+      (array.set $bytes (local.get $s) (local.get $i) (local.get $v))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 1))
+         (i32.shr_u (local.get $v) (i32.const 8)))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 2))
+         (i32.shr_u (local.get $v) (i32.const 16)))
+      (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 3))
+         (i32.shr_u (local.get $v) (i32.const 24))))
+
+   (func $ta_fill_int (param $b (ref extern)) (param $v i32)
+      (local $d (ref $data))
+      (local $a (ref array))
+      (local $a32 (ref $i32_array)) (local $a16 (ref $i16_array))
+      (local $a8 (ref $bytes))
+      (local $ofs i32) (local $i i32) (local $len i32)
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $a (struct.get $data $array (local.get $d)))
+      (local.set $ofs (struct.get $data $offset (local.get $d)))
+      (local.set $len (struct.get $data $len (local.get $d)))
+      (if (ref.test (ref $i32_array) (local.get $a))
+         (then
+            (local.set $a32 (ref.cast (ref $i32_array) (local.get $a)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i32_array (local.get $a32)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $v))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))
+      (else (if (ref.test (ref $i16_array) (local.get $a))
+         (then
+            (local.set $a16 (ref.cast (ref $i16_array) (local.get $a)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i16_array (local.get $a16)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $v))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))
+      (else
+         (local.set $a8 (ref.cast (ref $bytes) (local.get $a)))
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (local.get $len))
+               (then
+                  (array.set $bytes (local.get $a8)
+                     (i32.add (local.get $ofs) (local.get $i))
+                     (local.get $v))
+                  (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop)))))))))
+
+   (func $ta_fill_float (param $b (ref extern)) (param $f f64)
+      (local $d (ref $data))
+      (local $a (ref array))
+      (local $a64 (ref $float_array)) (local $a32 (ref $f32_array))
+      (local $f32 f32)
+      (local $ofs i32) (local $i i32) (local $len i32)
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $a (struct.get $data $array (local.get $d)))
+      (local.set $ofs (struct.get $data $offset (local.get $d)))
+      (local.set $len (struct.get $data $len (local.get $d)))
+      (if (ref.test (ref $float_array) (local.get $a))
+         (then
+            (local.set $a64 (ref.cast (ref $float_array) (local.get $a)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $float_array (local.get $a64)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $f))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))
+         (else
+            (local.set $a32 (ref.cast (ref $f32_array) (local.get $a)))
+            (local.set $f32 (f32.demote_f64 (local.get $f)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $f32_array (local.get $a32)
+                        (i32.add (local.get $ofs) (local.get $i))
+                        (local.get $f32))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))))
+
+   (func $ta_set (export "ta_set")
+      (param $d (ref extern)) (param $s (ref extern)) (param $do i32)
+      (local $sd (ref $data)) (local $sa (ref array)) (local $so i32)
+      (local $dd (ref $data)) (local $da (ref array))
+      (local $i i32) (local $len i32)
+      (local $sf64 (ref $float_array)) (local $df64 (ref $float_array))
+      (local $sf32 (ref $f32_array)) (local $df32 (ref $f32_array))
+      (local $si32 (ref $i32_array)) (local $di32 (ref $i32_array))
+      (local $si16 (ref $i16_array)) (local $di16 (ref $i16_array))
+      (local $si8 (ref $bytes)) (local $di8 (ref $bytes))
+      (local.set $sd (ref.cast (ref $data) (any.convert_extern (local.get $s))))
+      (local.set $sa (struct.get $data $array (local.get $sd)))
+      (local.set $so (struct.get $data $offset (local.get $sd)))
+      (local.set $len (struct.get $data $len (local.get $sd)))
+      (local.set $dd (ref.cast (ref $data) (any.convert_extern (local.get $d))))
+      (local.set $da (struct.get $data $array (local.get $dd)))
+      (local.set $do
+         (i32.add (struct.get $data $offset (local.get $dd)) (local.get $do)))
+      (if (ref.test (ref $float_array) (local.get $sa))
+         (then
+            (local.set $sf64 (ref.cast (ref $float_array) (local.get $sa)))
+            (local.set $df64 (ref.cast (ref $float_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $float_array (local.get $df64)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $float_array (local.get $sf64)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $f32_array) (local.get $sa))
+         (then
+            (local.set $sf32 (ref.cast (ref $f32_array) (local.get $sa)))
+            (local.set $df32 (ref.cast (ref $f32_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $f32_array (local.get $df32)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $f32_array (local.get $sf32)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $i32_array) (local.get $sa))
+         (then
+            (local.set $si32 (ref.cast (ref $i32_array) (local.get $sa)))
+            (local.set $di32 (ref.cast (ref $i32_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i32_array (local.get $di32)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $i32_array (local.get $si32)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $i16_array) (local.get $sa))
+         (then
+            (local.set $si16 (ref.cast (ref $i16_array) (local.get $sa)))
+            (local.set $di16 (ref.cast (ref $i16_array) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $i16_array (local.get $di16)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $i16_array (local.get $si16)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (if (ref.test (ref $bytes) (local.get $sa))
+         (then
+            (local.set $si8 (ref.cast (ref $bytes) (local.get $sa)))
+            (local.set $di8 (ref.cast (ref $bytes) (local.get $da)))
+            (loop $loop
+               (if (i32.lt_u (local.get $i) (local.get $len))
+                  (then
+                     (array.set $bytes (local.get $di8)
+                        (i32.add (local.get $do) (local.get $i))
+                        (array.get $bytes (local.get $si8)
+                           (i32.add (local.get $so) (local.get $i))))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))))))
+
+   (func $ta_blit (param $s (ref extern)) (param $d (ref extern))
+      (return_call $ta_set (local.get $d) (local.get $s) (i32.const 0)))
+
+   (func $ta_subarray (export "ta_subarray")
+      (param $b (ref extern)) (param $s i32) (param $e i32) (result (ref extern))
+      (local $d (ref $data))
+      (local.set $d (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (extern.convert_any
+         (struct.new $data
+            (struct.get $data $array (local.get $d))
+            (i32.add (struct.get $data $offset (local.get $d)) (local.get $s))
+            (i32.sub (local.get $e) (local.get $s)))))
+
+   (func $ta_blit_from_bytes (export "ta_blit_from_bytes")
+      (param $s (ref $bytes)) (param $so i32)
+      (param $b (ref extern)) (param $do i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local $d (ref $bytes))
+      (local $i i32)
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $d
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $do
+         (i32.add (local.get $do (struct.get $data $offset (local.get $data)))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $d)
+                  (i32.add (local.get $do) (local.get $i))
+                  (array.get $bytes (local.get $s)
+                     (i32.add (local.get $so) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func $ta_blit_to_bytes (export "ta_blit_to_bytes")
+      (param $b (ref extern)) (param $so i32)
+      (param $d (ref $bytes)) (param $do i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local $s (ref $bytes))
+      (local $i i32)
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $b))))
+      (local.set $s
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $so
+         (i32.add (local.get $so (struct.get $data $offset (local.get $data)))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $d)
+                  (i32.add (local.get $do) (local.get $i))
+                  (array.get $bytes (local.get $s)
+                     (i32.add (local.get $so) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+)
+(@else
    (import "bindings" "ta_create"
       (func $ta_create (param i32) (param i32) (result (ref extern))))
    (import "bindings" "ta_normalize"
@@ -77,55 +568,7 @@
       (func $ta_blit_to_bytes
          (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
          (param i32)))
-   (import "fail" "caml_bound_error" (func $caml_bound_error))
-   (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
-   (import "fail" "caml_invalid_argument"
-      (func $caml_invalid_argument (param (ref eq))))
-   (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
-   (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
-   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
-   (import "int32" "caml_copy_int32"
-      (func $caml_copy_int32 (param i32) (result (ref eq))))
-   (import "int32" "Int32_val"
-      (func $Int32_val (param (ref eq)) (result i32)))
-   (import "int32" "caml_copy_nativeint"
-      (func $caml_copy_nativeint (param i32) (result (ref eq))))
-   (import "int64" "caml_copy_int64"
-      (func $caml_copy_int64 (param i64) (result (ref eq))))
-   (import "int64" "Int64_val"
-      (func $Int64_val (param (ref eq)) (result i64)))
-   (import "obj" "double_array_tag" (global $double_array_tag i32))
-   (import "compare" "unordered" (global $unordered i32))
-   (import "hash" "caml_hash_mix_int"
-      (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
-   (import "hash" "caml_hash_mix_int64"
-      (func $caml_hash_mix_int64 (param i32) (param i64) (result i32)))
-   (import "hash" "caml_hash_mix_double"
-      (func $caml_hash_mix_double (param i32) (param f64) (result i32)))
-   (import "hash" "caml_hash_mix_float"
-      (func $caml_hash_mix_float (param i32) (param f32) (result i32)))
-   (import "hash" "caml_hash_mix_float16"
-      (func $caml_hash_mix_float16 (param i32) (param i32) (result i32)))
-   (import "marshal" "caml_serialize_int_1"
-      (func $caml_serialize_int_1 (param (ref eq)) (param i32)))
-   (import "marshal" "caml_serialize_int_2"
-      (func $caml_serialize_int_2 (param (ref eq)) (param i32)))
-   (import "marshal" "caml_serialize_int_4"
-      (func $caml_serialize_int_4 (param (ref eq)) (param i32)))
-   (import "marshal" "caml_serialize_int_8"
-      (func $caml_serialize_int_8 (param (ref eq)) (param i64)))
-   (import "marshal" "caml_deserialize_uint_1"
-      (func $caml_deserialize_uint_1 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_sint_1"
-      (func $caml_deserialize_sint_1 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_uint_2"
-      (func $caml_deserialize_uint_2 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_sint_2"
-      (func $caml_deserialize_sint_2 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_int_4"
-      (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
-   (import "marshal" "caml_deserialize_int_8"
-      (func $caml_deserialize_int_8 (param (ref eq)) (result i64)))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -155,10 +598,7 @@
 
    (global $bigarray_ops (export "bigarray_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $bytes 9 ;; "_bigarr02"
-            (i32.const 95) (i32.const 98) (i32.const 105) (i32.const 103)
-            (i32.const 97) (i32.const 114) (i32.const 114) (i32.const 48)
-            (i32.const 50))
+         (@string "_bigarr02")
          (ref.func $caml_ba_compare)
          (ref.null $compare)
          (ref.func $bigarray_hash)
@@ -596,7 +1036,7 @@
          (i32.mul (i32.add (i32.const 4) (local.get $num_dims)) (i32.const 4))
          (i32.mul (i32.add (i32.const 4) (local.get $num_dims)) (i32.const 8))))
 
-   (data $intern_overflow
+   (@string $intern_overflow
       "input_value: cannot read bigarray with 64-bit OCaml ints")
 
    (func $bigarray_deserialize
@@ -680,10 +1120,7 @@
                (br $done))
               ;; int
               (if (call $caml_deserialize_uint_1 (local.get $s))
-                 (then
-                    (call $caml_failwith
-                       (array.new_data $bytes $intern_overflow
-                          (i32.const 0) (i32.const 56))))))
+                 (then (call $caml_failwith (global.get $intern_overflow)))))
              ;; int32
              (loop $loop
                 (if (i32.lt_u (local.get $i) (local.get $len))
@@ -792,8 +1229,8 @@
 
    (global $CAML_BA_MAX_NUM_DIMS i32 (i32.const 16))
 
-   (data $ba_create_bad_dims "Bigarray.create: bad number of dimensions")
-   (data $ba_create_negative_dim "Bigarray.create: negative dimension")
+   (@string $ba_create_bad_dims "Bigarray.create: bad number of dimensions")
+   (@string $ba_create_negative_dim "Bigarray.create: negative dimension")
 
    (func (export "caml_ba_create")
       (param $vkind (ref eq)) (param $layout (ref eq)) (param $d (ref eq))
@@ -805,10 +1242,7 @@
       (local.set $vdim (ref.cast (ref $block) (local.get $d)))
       (local.set $num_dims (i32.sub (array.len (local.get $vdim)) (i32.const 1)))
       (if (i32.gt_u (local.get $num_dims) (global.get $CAML_BA_MAX_NUM_DIMS))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $ba_create_bad_dims
-                  (i32.const 0) (i32.const 41)))))
+         (then (call $caml_invalid_argument (global.get $ba_create_bad_dims))))
       (local.set $dim
          (array.new $int_array (i32.const 0) (local.get $num_dims)))
       (local.set $i (i32.const 0))
@@ -823,8 +1257,7 @@
                (if (i32.lt_s (local.get $n) (i32.const 0))
                   (then
                      (call $caml_invalid_argument
-                        (array.new_data $bytes $ba_create_negative_dim
-                              (i32.const 0) (i32.const 35)))))
+                        (global.get $ba_create_negative_dim))))
                (array.set $int_array
                   (local.get $dim) (local.get $i) (local.get $n))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
@@ -838,9 +1271,11 @@
          (local.get $kind)
          (i31.get_s (ref.cast (ref i31) (local.get $layout)))))
 
-   (data $ta_unsupported_kind "Typed_array.to_genarray: unsupported kind")
-   (data $ta_too_large "Typed_array.to_genarray: too large")
+   (@string $ta_unsupported_kind "Typed_array.to_genarray: unsupported kind")
+   (@string $ta_too_large "Typed_array.to_genarray: too large")
 
+(@if (not wasi)
+(@then
    (func (export "caml_ba_from_typed_array") (param (ref eq)) (result (ref eq))
       (local $data (ref extern))
       (local $kind i32)
@@ -850,18 +1285,12 @@
             (ref.as_non_null (extern.convert_any (call $unwrap (local.get 0))))))
       (local.set $kind (call $ta_kind (local.get $data)))
       (if (i32.lt_s (local.get $kind) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $ta_unsupported_kind
-                  (i32.const 0) (i32.const 41)))))
+         (then (call $caml_invalid_argument (global.get $ta_unsupported_kind))))
       (if (i32.eq (local.get $kind) (i32.const 14)) ;; Uint8ClampedArray
          (then (local.set $kind (i32.const 3))))
       (local.set $len (call $ta_length (local.get $data)))
       (if (i32.lt_s (local.get $len) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $ta_too_large
-                  (i32.const 0) (i32.const 34)))))
+         (then (call $caml_invalid_argument (global.get $ta_too_large))))
       (struct.new $bigarray
          (global.get $bigarray_ops)
          (local.get $data)
@@ -875,6 +1304,7 @@
          (any.convert_extern
             (struct.get $bigarray $ba_data
                (ref.cast (ref $bigarray) (local.get 0))))))
+))
 
    (func $caml_ba_get_at_offset
       (param $ba (ref $bigarray)) (param $i i32) (result (ref eq))
@@ -1050,7 +1480,7 @@
          (struct.get $float 0 (ref.cast (ref $float) (local.get $v))))
       (return))
 
-   (data $Bigarray_dim "Bigarray.dim")
+   (@string $Bigarray_dim "Bigarray.dim")
 
    (func $caml_ba_dim (export "caml_ba_dim")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
@@ -1061,9 +1491,7 @@
             (ref.cast (ref $bigarray) (local.get 0))))
       (local.set $i (i31.get_s (ref.cast (ref i31) (local.get 1))))
       (if (i32.ge_u (local.get $i) (array.len (local.get $dim)))
-         (then (call $caml_invalid_argument
-                  (array.new_data $bytes $Bigarray_dim
-                     (i32.const 0) (i32.const 12)))))
+         (then (call $caml_invalid_argument (global.get $Bigarray_dim))))
       (ref.i31 (array.get $int_array (local.get $dim) (local.get $i))))
 
    (func (export "caml_ba_dim_1") (param (ref eq)) (result (ref eq))
@@ -1409,7 +1837,7 @@
          (local.get $v))
       (ref.i31 (i32.const 0)))
 
-   (data $too_many_indices "Bigarray.slice: too many indices")
+   (@string $too_many_indices "Bigarray.slice: too many indices")
 
    (func (export "caml_ba_slice")
       (param $vb (ref eq)) (param $vind (ref eq)) (result (ref eq))
@@ -1425,10 +1853,7 @@
       (local.set $num_dims (struct.get $bigarray $ba_num_dims (local.get $b)))
       (if (i32.gt_u (local.get $num_inds)
              (struct.get $bigarray $ba_num_dims (local.get $b)))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $too_many_indices
-                  (i32.const 0) (i32.const 32)))))
+         (then (call $caml_invalid_argument (global.get $too_many_indices))))
       (local.set $sub_dim
          (array.new $int_array (i32.const 0)
             (i32.sub (local.get $num_dims) (local.get $num_inds))))
@@ -1492,7 +1917,7 @@
          (struct.get $bigarray $ba_kind (local.get $b))
          (struct.get $bigarray $ba_layout (local.get $b))))
 
-   (data $bad_subarray "Bigarray.sub: bad sub-array")
+   (@string $bad_subarray "Bigarray.sub: bad sub-array")
 
    (func (export "caml_ba_sub")
       (param $vba (ref eq)) (param $vofs (ref eq)) (param $vlen (ref eq))
@@ -1542,10 +1967,7 @@
              (i32.gt_s (i32.add (local.get $ofs) (local.get $len))
                 (array.get $int_array (local.get $dim)
                    (local.get $changed_dim))))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $bad_subarray
-                  (i32.const 0) (i32.const 27)))))
+         (then (call $caml_invalid_argument (global.get $bad_subarray))))
       (local.set $new_dim
          (array.new $int_array (i32.const 0) (local.get $num_dims)))
       (array.copy $int_array $int_array
@@ -1658,7 +2080,7 @@
           (struct.get $float 0 (ref.cast (ref $float) (local.get $v))))
        (return (ref.i31 (i32.const 0))))
 
-   (data $dim_mismatch "Bigarray.blit: dimension mismatch")
+   (@string $dim_mismatch "Bigarray.blit: dimension mismatch")
 
    (func (export "caml_ba_blit")
       (param $vsrc (ref eq)) (param $vdst (ref eq)) (result (ref eq))
@@ -1672,10 +2094,7 @@
       (local.set $len (struct.get $bigarray $ba_num_dims (local.get $dst)))
       (if (i32.ne (local.get $len)
              (struct.get $bigarray $ba_num_dims (local.get $src)))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $dim_mismatch
-                  (i32.const 0) (i32.const 33)))))
+         (then (call $caml_invalid_argument (global.get $dim_mismatch))))
       (local.set $sdim (struct.get $bigarray $ba_dim (local.get $src)))
       (local.set $ddim (struct.get $bigarray $ba_dim (local.get $dst)))
       (loop $loop
@@ -1685,9 +2104,7 @@
                       (array.get $int_array (local.get $sdim) (local.get $i))
                       (array.get $int_array (local.get $ddim) (local.get $i)))
                   (then
-                     (call $caml_invalid_argument
-                        (array.new_data $bytes $dim_mismatch
-                           (i32.const 0) (i32.const 33)))))
+                     (call $caml_invalid_argument (global.get $dim_mismatch))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop))))
       (call $ta_blit
@@ -1695,9 +2112,9 @@
          (struct.get $bigarray $ba_data (local.get $dst)))
      (ref.i31 (i32.const 0)))
 
-   (data $bad_number_dim "Bigarray.reshape: bad number of dimensions")
-   (data $negative_dim "Bigarray.reshape: negative dimension")
-   (data $size_mismatch "Bigarray.reshape: size mismatch")
+   (@string $bad_number_dim "Bigarray.reshape: bad number of dimensions")
+   (@string $negative_dim "Bigarray.reshape: negative dimension")
+   (@string $size_mismatch "Bigarray.reshape: size mismatch")
 
    (func (export "caml_ba_reshape")
       (param $vb (ref eq)) (param $vd (ref eq)) (result (ref eq))
@@ -1709,10 +2126,7 @@
       (local.set $num_dims (i32.sub (array.len (local.get $vdim)) (i32.const 1)))
       (local.set $b (ref.cast (ref $bigarray) (local.get $vb)))
       (if (i32.gt_u (local.get $num_dims) (global.get $CAML_BA_MAX_NUM_DIMS))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $bad_number_dim
-                  (i32.const 0) (i32.const 42)))))
+         (then (call $caml_invalid_argument (global.get $bad_number_dim))))
       (local.set $num_elts (i64.const 1))
       (local.set $dim (array.new $int_array (i32.const 0) (local.get $num_dims)))
       (loop $loop
@@ -1725,9 +2139,7 @@
                            (i32.add (local.get $i) (i32.const 1))))))
                (if (i32.lt_s (local.get $d) (i32.const 0))
                   (then
-                     (call $caml_invalid_argument
-                        (array.new_data $bytes $negative_dim
-                           (i32.const 0) (i32.const 36)))))
+                     (call $caml_invalid_argument (global.get $negative_dim))))
                (array.set $int_array (local.get $dim) (local.get $i)
                   (local.get $d))
                (local.set $num_elts
@@ -1741,10 +2153,7 @@
       (if (i32.ne (i32.wrap_i64 (local.get $num_elts))
              (call $caml_ba_get_size
                 (struct.get $bigarray $ba_dim (local.get $b))))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $size_mismatch
-                  (i32.const 0) (i32.const 31)))))
+         (then (call $caml_invalid_argument (global.get $size_mismatch))))
       (struct.new $bigarray
          (global.get $bigarray_ops)
          (struct.get $bigarray $ba_data (local.get $b))
@@ -2158,6 +2567,8 @@
          (i32.wrap_i64 (i64.shr_u (local.get $d) (i64.const 32))))
       (ref.i31 (i32.const 0)))
 
+(@if (not wasi)
+(@then
    (export "caml_bytes_of_uint8_array" (func $caml_string_of_uint8_array))
    (func $caml_string_of_uint8_array (export "caml_string_of_uint8_array")
       (param (ref eq)) (result (ref eq))
@@ -2189,6 +2600,7 @@
          (local.get $s) (i32.const 0) (local.get $ta) (i32.const 0)
          (local.get $len))
       (call $wrap (any.convert_extern (local.get $ta))))
+))
 
    (func (export "caml_ba_get_kind") (param (ref eq)) (result i32)
       (struct.get $bigarray $ba_kind (ref.cast (ref $bigarray) (local.get 0))))
diff --git a/runtime/wasm/bigstring.wat b/runtime/wasm/bigstring.wat
index 3c816953ac..ec3b903b02 100644
--- a/runtime/wasm/bigstring.wat
+++ b/runtime/wasm/bigstring.wat
@@ -31,6 +31,31 @@
       (func $caml_ba_fill (param (ref eq)) (param (ref eq)) (result (ref eq))))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
+(@if wasi
+(@then
+   (import "bigarray" "ta_get_ui8"
+      (func $ta_get_ui8 (param (ref extern)) (param i32) (result i32)))
+   (import "bigarray" "ta_get32_ui8"
+      (func $ta_get32_ui8 (param (ref extern)) (param i32) (result i32)))
+   (import "bigarray" "ta_set_ui8"
+      (func $ta_set_ui8 (param (ref extern)) (param i32) (param (ref i31))))
+   (import "bigarray" "ta_subarray"
+      (func $ta_subarray
+         (param (ref extern)) (param i32) (param i32) (result (ref extern))))
+   (import "bigarray" "ta_set"
+      (func $ta_set (param (ref extern)) (param (ref extern)) (param i32)))
+   (import "bigarray" "ta_length"
+      (func $ta_length (param (ref extern)) (result i32)))
+   (import "bigarray" "ta_blit_from_bytes"
+      (func $ta_blit_from_bytes
+         (param (ref $bytes)) (param i32) (param (ref extern)) (param i32)
+         (param i32)))
+   (import "bigarray" "ta_blit_to_bytes"
+      (func $ta_blit_to_bytes
+         (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
+         (param i32)))
+)
+(@else
    (import "bindings" "ta_create"
       (func $ta_create (param i32) (param anyref) (result anyref)))
    (import "bindings" "ta_get_ui8"
@@ -56,6 +81,7 @@
       (func $ta_blit_to_bytes
          (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
          (param i32)))
+))
    (import "hash" "caml_hash_mix_int"
       (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
 
@@ -98,13 +124,15 @@
          (local.set $h (call $caml_hash_mix_int (local.get $h) (local.get $w))))
       (i32.xor (local.get $h) (local.get $len)))
 
-   (data $buffer "buffer")
+(@if (not wasi)
+(@then
+   (@string $buffer "buffer")
 
    (func (export "bigstring_to_array_buffer")
       (param $bs (ref eq)) (result (ref eq))
       (return_call $caml_js_get
          (call $caml_ba_to_typed_array (local.get $bs))
-         (array.new_data $bytes $buffer (i32.const 0) (i32.const 6))))
+         (global.get $buffer)))
 
    (export "bigstring_to_typed_array" (func $caml_ba_to_typed_array))
 
@@ -116,6 +144,7 @@
    (func (export "bigstring_of_typed_array") (param (ref eq)) (result (ref eq))
        (return_call $caml_ba_from_typed_array
           (call $wrap (call $ta_bytes (call $unwrap (local.get 0))))))
+))
 
    (func (export "caml_bigstring_memset")
       (param $s (ref eq)) (param $pos (ref eq)) (param $len (ref eq))
diff --git a/runtime/wasm/compare.wat b/runtime/wasm/compare.wat
index e17b223dbc..4b8805a4a7 100644
--- a/runtime/wasm/compare.wat
+++ b/runtime/wasm/compare.wat
@@ -215,9 +215,9 @@
       (call $clear_compare_stack)
       (local.get $res))
 
-   (data $abstract_value "compare: abstract value")
-   (data $functional_value "compare: functional value")
-   (data $continuation_value "compare: continuation value")
+   (@string $abstract_value "compare: abstract value")
+   (@string $functional_value "compare: functional value")
+   (@string $continuation_value "compare: continuation value")
 
    (func $do_compare_val
       (param $stack (ref $compare_stack))
@@ -477,10 +477,10 @@
                      (br_if $next_item (i32.eqz (local.get $res)))
                      (return (local.get $res)))
                   (call $clear_compare_stack)
-                  (call $caml_invalid_argument
-                     (array.new_data $bytes $abstract_value
-                        (i32.const 0) (i32.const 23)))
+                  (call $caml_invalid_argument (global.get $abstract_value))
                   (ref.i31 (i32.const 0))))
+(@if (not wasi)
+(@then
                (drop (block $v1_not_js (result (ref eq))
                   (local.set $js1
                      (struct.get $js 0
@@ -508,14 +508,14 @@
                            (call $equals (local.get $js1) (local.get $js2)))
                         (return (global.get $unordered))))
                   (br $heterogeneous (ref.i31 (i32.const 0)))))
+))
                (if (call $caml_is_closure (local.get $v1))
                   (then
                      (drop (br_if $heterogeneous (ref.i31 (i32.const 0))
                               (i32.eqz (call $caml_is_closure (local.get $v2)))))
                      (call $clear_compare_stack)
                      (call $caml_invalid_argument
-                        (array.new_data $bytes $functional_value
-                           (i32.const 0) (i32.const 25)))))
+                        (global.get $functional_value))))
                (if (call $caml_is_continuation (local.get $v1))
                   (then
                      (drop (br_if $heterogeneous(ref.i31 (i32.const 0))
@@ -523,8 +523,7 @@
                                  (call $caml_is_continuation (local.get $v2)))))
                      (call $clear_compare_stack)
                      (call $caml_invalid_argument
-                        (array.new_data $bytes $continuation_value
-                           (i32.const 0) (i32.const 27)))))
+                        (global.get $continuation_value))))
                (ref.i31 (i32.const 0)))) ;; fall through
             ;; heterogeneous comparison
             (local.set $t1
@@ -549,9 +548,7 @@
             (if (i32.eqz (local.get $res))
                (then
                   (call $clear_compare_stack)
-                  (call $caml_invalid_argument
-                     (array.new_data $bytes $abstract_value
-                        (i32.const 0) (i32.const 23)))))
+                  (call $caml_invalid_argument (global.get $abstract_value))))
             (return (local.get $res)))
          (if (call $compare_stack_is_not_empty (local.get $stack))
             (then
diff --git a/runtime/wasm/deps-wasi.json b/runtime/wasm/deps-wasi.json
new file mode 100644
index 0000000000..0a49660901
--- /dev/null
+++ b/runtime/wasm/deps-wasi.json
@@ -0,0 +1,15 @@
+[
+  {
+    "name": "root",
+    "reaches": ["start", "memory"],
+    "root": true
+  },
+  {
+    "name": "start",
+    "export": "_start"
+  },
+  {
+    "name": "memory",
+    "export": "memory"
+  }
+]
diff --git a/runtime/wasm/deps.json b/runtime/wasm/deps.json
index cb37130f87..4a2173c983 100644
--- a/runtime/wasm/deps.json
+++ b/runtime/wasm/deps.json
@@ -72,6 +72,15 @@
     "import": ["bindings", "wrap_meth_callback_unsafe"],
     "reaches": ["callback"]
   },
+  {
+    "name": "alloc_times",
+    "export": "caml_alloc_times"
+  },
+  {
+    "name": "times",
+    "import": ["bindings", "times"],
+    "reaches": ["alloc_times"]
+  },
   {
     "name": "alloc_tm",
     "export": "caml_alloc_tm"
diff --git a/runtime/wasm/domain.wat b/runtime/wasm/domain.wat
index 2c53926868..a4d46414ec 100644
--- a/runtime/wasm/domain.wat
+++ b/runtime/wasm/domain.wat
@@ -16,6 +16,12 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+   (import "obj" "caml_callback_1"
+      (func $caml_callback_1
+         (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "sync" "caml_ml_mutex_unlock"
+      (func $caml_ml_mutex_unlock (param (ref eq)) (result (ref eq))))
+
    (type $block (array (mut (ref eq))))
    (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
    (type $closure (sub (struct (;(field i32);) (field (ref $function_1)))))
@@ -99,6 +105,47 @@
    (global $caml_domain_latest_id (export "caml_domain_latest_id") (mut i32)
       (i32.const 1))
 
+(@if (>= ocaml_version (5 2 0))
+(@then
+   (func (export "caml_domain_spawn")
+      (param $f (ref eq)) (param $term_sync_v (ref eq)) (result (ref eq))
+      (local $id i32) (local $old i32) (local $ts (ref $block)) (local $res (ref eq))
+      (local.set $id (global.get $caml_domain_latest_id))
+      (global.set $caml_domain_latest_id
+         (i32.add (local.get $id) (i32.const 1)))
+      (local.set $old (global.get $caml_domain_id))
+      (local.set $res
+         (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
+      (global.set $caml_domain_id (local.get $old))
+      (local.set $ts (ref.cast (ref $block) (local.get $term_sync_v)))
+      (drop (call $caml_ml_mutex_unlock (array.get $block (local.get $ts) (i32.const 2))))
+      ;; TODO: fix exn case
+      (array.set
+         $block
+         (local.get $ts)
+         (i32.const 1)
+         (array.new_fixed
+            $block
+            2
+            (ref.i31 (i32.const 0))
+            (array.new_fixed $block 2 (ref.i31 (i32.const 0)) (local.get $res))))
+      (ref.i31 (local.get $id)))
+)
+(@else
+   (func (export "caml_domain_spawn")
+      (param $f (ref eq)) (param $mutex (ref eq)) (result (ref eq))
+      (local $id i32) (local $old i32)
+      (local.set $id (global.get $caml_domain_latest_id))
+      (global.set $caml_domain_latest_id
+         (i32.add (local.get $id) (i32.const 1)))
+      (local.set $old (global.get $caml_domain_id))
+      (drop (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
+      (global.set $caml_domain_id (local.get $old))
+      (drop (call $caml_ml_mutex_unlock (local.get $mutex)))
+      (ref.i31 (local.get $id)))
+))
+
+
    (func (export "caml_ml_domain_id") (export "caml_ml_domain_index")
       (param (ref eq)) (result (ref eq))
       (ref.i31 (global.get $caml_domain_id)))
diff --git a/runtime/wasm/dune b/runtime/wasm/dune
index 3bbc0a5fc6..2a7e709d95 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -1,72 +1,42 @@
 (install
  (section lib)
  (package wasm_of_ocaml-compiler)
- (files runtime.wasm runtime.js))
+ (files
+  (glob_files *.wat)
+  (glob_files runtime-*.wasm)
+  runtime.js))
 
 (rule
- (target version-dependent.wat)
- (deps version-dependent/post-5.2.wat)
- (enabled_if
-  (>= %{ocaml_version} 5.2.0))
- (action
-  (copy %{deps} %{target})))
-
-(rule
- (target version-dependent.wat)
- (deps version-dependent/post-5.1.wat)
- (enabled_if
-  (and
-   (>= %{ocaml_version} 5.1.0)
-   (< %{ocaml_version} 5.2.0)))
- (action
-  (copy %{deps} %{target})))
-
-(rule
- (target version-dependent.wat)
- (deps version-dependent/pre-5.1.wat)
- (enabled_if
-  (< %{ocaml_version} 5.1.0))
- (action
-  (copy %{deps} %{target})))
-
-(rule
- (target runtime.wasm)
- (deps runtime.merged.wasm)
+ (target runtime-standard.wasm)
+ (deps
+  args
+  (glob_files *.wat))
  (action
   (run
-   wasm-opt
-   -g
-   --enable-gc
-   --enable-exception-handling
-   --enable-reference-types
-   --enable-tail-call
-   --enable-strings
-   --enable-multivalue
-   --enable-bulk-memory
-   %{deps}
-   -O3
-   -o
-   %{target})))
+   ../../compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.exe
+   --binaryen=-g
+   --binaryen-opt=-O3
+   --disable
+   wasi
+   %{target}
+   %{read-lines:args})))
 
 (rule
- (target runtime.merged.wasm)
+ (target runtime-wasi.wasm)
  (deps
   args
-  (glob_files *.wat))
+  (glob_files *.wat)
+  libc.wasm)
  (action
   (run
-   wasm-merge
-   -g
-   --enable-gc
-   --enable-exception-handling
-   --enable-reference-types
-   --enable-tail-call
-   --enable-strings
-   --enable-multivalue
-   --enable-bulk-memory
-   %{read-lines:args}
-   -o
-   %{target})))
+   ../../compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.exe
+   --binaryen=-g
+   --binaryen-opt=-O3
+   --enable
+   wasi
+   %{target}
+   libc:libc.wasm
+   %{read-lines:args})))
 
 (rule
  (target args)
@@ -77,3 +47,34 @@
   (with-stdout-to
    %{target}
    (run ocaml %{deps}))))
+
+(rule
+ (target libc.new.wasm)
+ (deps libc.c)
+ (enabled_if
+  (not %{env:CI=false}))
+ (mode promote)
+ (action
+  (with-stdout-to
+   %{target}
+   (pipe-stdout
+    (run
+     docker
+     run
+     -v
+     .:/src
+     -w
+     /src
+     ghcr.io/webassembly/wasi-sdk
+     /opt/wasi-sdk/bin/clang
+     -O2
+     libc.c
+     -flto
+     -o
+     -)
+    (run wasm-opt -Oz --strip-debug --strip-dwarf - -o -)))))
+
+(rule
+ (alias recompile-libc)
+ (action
+  (cmp libc.wasm libc.new.wasm)))
diff --git a/runtime/wasm/effect-native.wat b/runtime/wasm/effect-native.wat
new file mode 100644
index 0000000000..aca6344883
--- /dev/null
+++ b/runtime/wasm/effect-native.wat
@@ -0,0 +1,242 @@
+(module
+(@if wasi
+(@then
+   (import "fail" "caml_raise_constant"
+      (func $caml_raise_constant (param (ref eq))))
+   (import "fail" "caml_raise_with_arg"
+      (func $caml_raise_with_arg (param $tag (ref eq)) (param $arg (ref eq))))
+   (import "obj" "caml_fresh_oo_id"
+     (func $caml_fresh_oo_id (param (ref eq)) (result (ref eq))))
+   (import "obj" "cont_tag" (global $cont_tag i32))
+   (import "stdlib" "caml_named_value"
+      (func $caml_named_value (param (ref eq)) (result (ref null eq))))
+   (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
+   (import "fail" "javascript_exception"
+      (tag $javascript_exception (param externref)))
+   (import "jslib" "caml_wrap_exception"
+      (func $caml_wrap_exception (param externref) (result (ref eq))))
+   (import "stdlib" "caml_main_wrapper"
+      (global $caml_main_wrapper (mut (ref null $wrapper_func))))
+   (import "effect" "effect_allowed" (global $effect_allowed (mut i32)))
+
+   (type $block (array (mut (ref eq))))
+   (type $bytes (array (mut i8)))
+   (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
+   (type $closure (sub (struct (;(field i32);) (field (ref $function_1)))))
+   (type $function_3
+      (func (param (ref eq) (ref eq) (ref eq) (ref eq)) (result (ref eq))))
+   (type $closure_3
+      (sub $closure
+         (struct (field (ref $function_1)) (field (ref $function_3)))))
+
+   ;; Effect types
+
+   (tag $effect (param (ref eq)) (result (ref eq) (ref eq)))
+
+   (type $cont_function (func (param (ref eq) (ref eq)) (result (ref eq))))
+
+   (type $cont (cont $cont_function))
+
+   (type $handlers
+      (struct
+         (field $value (ref eq))
+         (field $exn (ref eq))
+         (field $effect (ref eq))))
+
+   (type $fiber
+      (struct
+         (field $handlers (mut (ref $handlers)))
+         (field $cont (ref $cont))))
+
+   ;; Unhandled effects
+
+   (@string $effect_unhandled "Effect.Unhandled")
+
+   (func $raise_unhandled
+      (param $eff (ref eq)) (param (ref eq)) (result (ref eq))
+      (block $null
+         (call $caml_raise_with_arg
+            (br_on_null $null
+               (call $caml_named_value (global.get $effect_unhandled)))
+            (local.get $eff)))
+      (call $caml_raise_constant
+         (array.new_fixed $block 3 (ref.i31 (i32.const 248))
+            (global.get $effect_unhandled)
+            (call $caml_fresh_oo_id (ref.i31 (i32.const 0)))))
+      (ref.i31 (i32.const 0)))
+
+   (global $raise_unhandled (ref $closure)
+      (struct.new $closure (ref.func $raise_unhandled)))
+
+   (type $func (func (result (ref eq))))
+   (type $wrapper_func (func (param (ref $func))))
+   (type $func_closure (struct (field (ref $func))))
+
+   (func $wrapper_cont
+      (param $f (ref eq)) (param (ref eq)) (result (ref eq))
+      (return_call_ref $func
+         (local.get $f)
+         (struct.get $func_closure 0
+            (ref.cast (ref $func_closure) (local.get $f)))))
+
+   (func $unhandled_effect_wrapper (param $start (ref $func))
+      (local $cont (ref $cont))
+      (local $f (ref eq)) (local $v (ref eq))
+      (local $resume_res (tuple (ref eq) (ref $cont)))
+      (local.set $cont (cont.new $cont (ref.func $wrapper_cont)))
+      (local.set $f (struct.new $func_closure (local.get $start)))
+      (local.set $v (ref.i31 (i32.const 0)))
+      (loop $loop
+         (local.set $resume_res
+            (block $handle_effect (result (ref eq) (ref $cont))
+               (resume $cont (on $effect $handle_effect)
+                  (local.get $f) (local.get $v) (local.get $cont))
+               (return)))
+         (local.set $cont (tuple.extract 2 1 (local.get $resume_res)))
+         (local.set $v (tuple.extract 2 0 (local.get $resume_res)))
+         (local.set $f (global.get $raise_unhandled))
+         (br $loop)))
+
+   (func $init
+      (global.set $caml_main_wrapper (ref.func $unhandled_effect_wrapper)))
+
+   (start $init)
+
+   ;; Resume
+
+   (@string $already_resumed "Effect.Continuation_already_resumed")
+
+   (func $resume (export "%resume")
+      (param $vfiber (ref eq)) (param $f (ref eq)) (param $v (ref eq))
+      (param $tail (ref eq)) (result (ref eq))
+      (local $fiber (ref $fiber))
+      (local $res (ref eq))
+      (local $exn (ref eq))
+      (local $resume_res (tuple (ref eq) (ref $cont)))
+      (if (ref.eq (local.get $vfiber) (ref.i31 (i32.const 0)))
+         (then
+            (call $caml_raise_constant
+               (ref.as_non_null
+                  (call $caml_named_value (global.get $already_resumed))))))
+      (local.set $fiber (ref.cast (ref $fiber) (local.get $vfiber)))
+      (local.set $exn
+         (block $handle_exception (result (ref eq))
+            (local.set $resume_res
+               (block $handle_effect (result (ref eq) (ref $cont))
+                  (local.set $res
+                     (try (result (ref eq))
+                        (do
+                           (resume $cont
+                               (on $effect $handle_effect)
+                               (local.get $f) (local.get $v)
+                               (struct.get $fiber $cont (local.get $fiber))))
+(@if (not wasi)
+(@then
+                        (catch $javascript_exception
+                           (br $handle_exception
+                              (call $caml_wrap_exception (pop externref))))
+))
+                        (catch $ocaml_exception
+                           (br $handle_exception (pop (ref eq))))))
+                  ;; handle return
+                  (return_call_ref $function_1 (local.get $res)
+                     (local.tee $f
+                        (struct.get $handlers $value
+                           (struct.get $fiber $handlers (local.get $fiber))))
+                     (struct.get $closure 0
+                        (ref.cast (ref $closure) (local.get $f))))))
+            ;; handle effect
+            (return_call_ref $function_3
+               (tuple.extract 2 0 (local.get $resume_res))
+               (array.new_fixed $block 3 (ref.i31 (global.get $cont_tag))
+                  (struct.new $fiber
+                     (struct.get $fiber $handlers (local.get $fiber))
+                     (tuple.extract 2 1 (local.get $resume_res)))
+                  (ref.i31 (i32.const 0)))
+               (local.get $tail)
+               (local.tee $f
+                  (struct.get $handlers $effect
+                     (struct.get $fiber $handlers (local.get $fiber))))
+               (struct.get $closure_3 1
+                  (ref.cast (ref $closure_3) (local.get $f))))))
+      ;; handle exception
+      (return_call_ref $function_1 (local.get $exn)
+         (local.tee $f
+            (struct.get $handlers $exn
+               (struct.get $fiber $handlers (local.get $fiber))))
+         (struct.get $closure 0 (ref.cast (ref $closure) (local.get $f)))))
+
+   ;; Perform
+
+   (func (export "%reperform")
+      (param $eff (ref eq)) (param $cont (ref eq)) (param $tail (ref eq))
+      (result (ref eq))
+      (local $res (tuple (ref eq) (ref eq)))
+      (local.set $res (suspend $effect (local.get $eff)))
+      (return_call $resume
+         (ref.as_non_null
+            (array.get $block
+               (ref.cast (ref $block) (local.get $cont))
+               (i32.const 1)))
+         (tuple.extract 2 0 (local.get $res))
+         (tuple.extract 2 1 (local.get $res))
+         (local.get $tail)))
+
+   (func (export "%perform") (param $eff (ref eq)) (result (ref eq))
+      (local $res (tuple (ref eq) (ref eq)))
+      (if (i32.eqz (global.get $effect_allowed))
+         (then
+            (return_call $raise_unhandled
+               (local.get $eff) (ref.i31 (i32.const 0)))))
+      (local.set $res (suspend $effect (local.get $eff)))
+      (return_call_ref $function_1 (tuple.extract 2 1 (local.get $res))
+         (tuple.extract 2 0 (local.get $res))
+         (struct.get $closure 0
+            (ref.cast (ref $closure) (tuple.extract 2 0 (local.get $res))))))
+
+   ;; Allocate a stack
+
+   (func $initial_cont
+      (param $f (ref eq)) (param $x (ref eq)) (result (ref eq))
+      (return_call_ref $function_1 (local.get $x)
+         (local.get $f)
+         (struct.get $closure 0 (ref.cast (ref $closure) (local.get $f)))))
+
+   (func (export "caml_alloc_stack")
+      (param $hv (ref eq)) (param $hx (ref eq)) (param $hf (ref eq))
+      (result (ref eq))
+      (struct.new $fiber
+         (struct.new $handlers (local.get $hv) (local.get $hx) (local.get $hf))
+         (cont.new $cont (ref.func $initial_cont))))
+
+   ;; Other functions
+
+   (func $caml_continuation_use_noexc (export "caml_continuation_use_noexc")
+      (param (ref eq)) (result (ref eq))
+      (local $cont (ref $block))
+      (local $stack (ref eq))
+      (drop (block $used (result (ref eq))
+         (local.set $cont (ref.cast (ref $block) (local.get 0)))
+         (local.set $stack
+            (br_on_cast_fail $used (ref eq) (ref $fiber)
+               (array.get $block (local.get $cont) (i32.const 1))))
+         (array.set $block (local.get $cont) (i32.const 1)
+            (ref.i31 (i32.const 0)))
+         (return (local.get $stack))))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "caml_continuation_use_and_update_handler_noexc")
+      (param $cont (ref eq)) (param $hval (ref eq)) (param $hexn (ref eq))
+      (param $heff (ref eq)) (result (ref eq))
+      (local $stack (ref eq))
+      (local.set $stack (call $caml_continuation_use_noexc (local.get $cont)))
+      (drop (block $used (result (ref eq))
+         (struct.set $fiber $handlers
+            (br_on_cast_fail $used (ref eq) (ref $fiber)
+               (local.get $stack))
+            (struct.new $handlers
+               (local.get $hval) (local.get $hexn) (local.get $heff)))
+         (ref.i31 (i32.const 0))))
+      (local.get $stack))
+))
+)
diff --git a/runtime/wasm/effect.wat b/runtime/wasm/effect.wat
index 9b9430a871..4d144bdae4 100644
--- a/runtime/wasm/effect.wat
+++ b/runtime/wasm/effect.wat
@@ -23,11 +23,20 @@
    (import "obj" "caml_fresh_oo_id"
      (func $caml_fresh_oo_id (param (ref eq)) (result (ref eq))))
    (import "obj" "cont_tag" (global $cont_tag i32))
+   (import "obj" "caml_callback_1"
+      (func $caml_callback_1
+         (param (ref eq)) (param (ref eq)) (result (ref eq))))
    (import "stdlib" "caml_named_value"
       (func $caml_named_value (param (ref eq)) (result (ref null eq))))
    (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
+(@if wasi
+(@then
+   (func $caml_wrap_exception (param externref) (result (ref eq))
+      (unreachable))
+)
+(@else
    (import "jslib" "caml_wrap_exception"
       (func $caml_wrap_exception (param externref) (result (ref eq))))
    (import "bindings" "start_fiber" (func $start_fiber (param (ref eq))))
@@ -36,9 +45,7 @@
          (param $f funcref) (param $env eqref) (result anyref)))
    (import "bindings" "resume_fiber"
       (func $resume_fiber (param externref) (param (ref eq))))
-   (import "obj" "caml_callback_1"
-      (func $caml_callback_1
-         (param (ref eq)) (param (ref eq)) (result (ref eq))))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -50,6 +57,37 @@
       (sub $closure
          (struct (field (ref $function_1)) (field (ref $function_3)))))
 
+   ;; Generic fibers
+
+   (type $handlers
+      (struct
+         (field $value (ref eq))
+         (field $exn (ref eq))
+         (field $effect (ref eq))))
+
+   (type $generic_fiber (sub (struct (field $handlers (mut (ref $handlers))))))
+
+   (@string $already_resumed "Effect.Continuation_already_resumed")
+
+   (@string $effect_unhandled "Effect.Unhandled")
+
+   (func $raise_unhandled
+      (param $eff (ref eq)) (param (ref eq)) (result (ref eq))
+      (block $null
+         (call $caml_raise_with_arg
+            (br_on_null $null
+               (call $caml_named_value (global.get $effect_unhandled)))
+            (local.get $eff)))
+      (call $caml_raise_constant
+         (array.new_fixed $block 3 (ref.i31 (i32.const 248))
+            (global.get $effect_unhandled)
+            (call $caml_fresh_oo_id (ref.i31 (i32.const 0)))))
+      (ref.i31 (i32.const 0)))
+
+   (global $effect_allowed (export "effect_allowed") (mut i32) (i32.const 1))
+
+(@if (not wasi)
+(@then
    ;; Apply a function f to a value v, both contained in a pair (f, v)
 
    (type $pair (struct (field (ref eq)) (field (ref eq))))
@@ -106,14 +144,6 @@
 
    ;; Stack of fibers
 
-   (type $handlers
-      (struct
-         (field $value (ref eq))
-         (field $exn (ref eq))
-         (field $effect (ref eq))))
-
-   (type $generic_fiber (sub (struct (field $handlers (mut (ref $handlers))))))
-
    (type $fiber
       (sub final $generic_fiber
          (struct
@@ -121,26 +151,6 @@
             (field $cont (ref $cont))
             (field $next (ref null $fiber)))))
 
-   (data $effect_unhandled "Effect.Unhandled")
-
-   (func $raise_unhandled
-      (param $eff (ref eq)) (param (ref eq)) (result (ref eq))
-      (local $effect_unhandled (ref $bytes))
-      (local.set $effect_unhandled
-         (array.new_data $bytes $effect_unhandled
-            (i32.const 0) (i32.const 16)))
-      (block $null
-         (call $caml_raise_with_arg
-            (br_on_null $null
-               (call $caml_named_value
-                  (local.get $effect_unhandled)))
-            (local.get $eff)))
-      (call $caml_raise_constant
-         (array.new_fixed $block 3 (ref.i31 (i32.const 248))
-            (local.get $effect_unhandled)
-            (call $caml_fresh_oo_id (ref.i31 (i32.const 0)))))
-      (ref.i31 (i32.const 0)))
-
    (func $uncaught_effect_handler
       (param $eff (ref eq)) (param $cont (ref eq)) (param $k (ref eq))
       (param (ref eq)) (result (ref eq))
@@ -218,8 +228,6 @@
       (return_call_ref $cont_func (local.get $p) (local.get $k)
          (struct.get $cont $cont_func (local.get $k))))
 
-   (data $already_resumed "Effect.Continuation_already_resumed")
-
    (func (export "%resume")
       (param $stack (ref eq)) (param $f (ref eq)) (param $v (ref eq)) (param $tail (ref eq))
       (result (ref eq))
@@ -229,9 +237,7 @@
          (then
             (call $caml_raise_constant
                (ref.as_non_null
-                  (call $caml_named_value
-                     (array.new_data $bytes $already_resumed
-                        (i32.const 0) (i32.const 35)))))))
+                  (call $caml_named_value (global.get $already_resumed))))))
       (return_call $capture_continuation
          (ref.func $do_resume)
           (struct.new $pair
@@ -306,8 +312,6 @@
          (ref.func $do_perform)
          (struct.new $pair (local.get $eff) (local.get $cont))))
 
-   (global $effect_allowed (mut i32) (i32.const 1))
-
    (func (export "%perform") (param $eff (ref eq)) (result (ref eq))
       (if (i32.eqz (global.get $effect_allowed))
          (then
@@ -392,6 +396,7 @@
                (local.get $hval) (local.get $hexn) (local.get $heff)))
          (ref.i31 (i32.const 0))))
       (local.get $stack))
+))
 
    (func (export "caml_get_continuation_callstack")
       (param (ref eq) (ref eq)) (result (ref eq))
@@ -648,9 +653,7 @@
          (return (local.get $k))))
       (call $caml_raise_constant
          (ref.as_non_null
-            (call $caml_named_value
-               (array.new_data $bytes $already_resumed
-                  (i32.const 0) (i32.const 35)))))
+            (call $caml_named_value (global.get $already_resumed))))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_perform_effect")
diff --git a/runtime/wasm/fail.wat b/runtime/wasm/fail.wat
index 2661139d01..2aa44adf42 100644
--- a/runtime/wasm/fail.wat
+++ b/runtime/wasm/fail.wat
@@ -18,7 +18,13 @@
 (module
    (import "stdlib" "caml_global_data"
       (global $caml_global_data (mut (ref $block))))
+(@if wasi
+(@then
+   (tag $javascript_exception (param externref))
+)
+(@else
    (import "bindings" "jstag" (tag $javascript_exception (param externref)))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -71,12 +77,10 @@
               (global.get $INVALID_EXN))
            (local.get 0)))
 
-   (data $index_out_of_bounds "index out of bounds")
+   (@string $index_out_of_bounds "index out of bounds")
 
    (func (export "caml_bound_error")
-      (return_call $caml_invalid_argument
-         (array.new_data $bytes $index_out_of_bounds
-            (i32.const 0) (i32.const 19))))
+      (return_call $caml_invalid_argument (global.get $index_out_of_bounds)))
 
    (global $END_OF_FILE_EXN i32 (i32.const 4))
 
diff --git a/runtime/wasm/float.wat b/runtime/wasm/float.wat
index 527581bbe0..c21cdc7551 100644
--- a/runtime/wasm/float.wat
+++ b/runtime/wasm/float.wat
@@ -16,12 +16,36 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
+(@if wasi
+(@then
+   (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "libc" "format_float"
+      (func $format_float (param i32 i32 i32 f64) (result i32)))
+   (import "libc" "strtod" (func $strtod (param i32) (param i32) (result f64)))
+   (import "libc" "exp" (func $exp (param f64) (result f64)))
+   (import "wasi_memory" "checked_malloc"
+     (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "release_memory"
+      (func $release_memory (param i32 i32)))
+   (import "wasi_memory" "blit_string_to_memory"
+      (func $blit_string_to_memory (param i32 (ref $bytes))))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+)
+(@else
    (import "bindings" "format_float"
       (func $format_float
          (param i32) (param i32) (param i32) (param f64) (result anyref)))
    (import "bindings" "identity"
       (func $parse_float (param anyref) (result f64)))
    (import "Math" "exp" (func $exp (param f64) (result f64)))
+))
    (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param (ref eq))))
@@ -194,7 +218,7 @@
                      (local.get $style))))))
       (local.get $s))
 
-   (data $format_error "format_float: bad format")
+   (@string $format_error "format_float: bad format")
 
    (func $parse_format
       (param $s (ref $bytes)) (result i32 i32 i32 i32)
@@ -242,9 +266,7 @@
                   (i32.and (local.get $c) (i32.const 0xdf))
                   (i32.const 69))) ;; 'E'
             (br_if $return (i32.le_u (local.get $conversion) (i32.const 2))))
-         (call $caml_invalid_argument
-            (array.new_data $bytes $format_error
-               (i32.const 0) (i32.const 22))))
+         (call $caml_invalid_argument (global.get $format_error)))
       (tuple.make 4
          (local.get $sign_style)
          (local.get $precision)
@@ -255,6 +277,49 @@
       (array.new_fixed $chars 3
          (i32.const 105) (i32.const 110) (i32.const 102)))
 
+(@if wasi
+(@then
+   (func (export "caml_format_float")
+      (param $vfmt (ref eq)) (param $arg (ref eq)) (result (ref eq))
+      (local $fmt (ref $bytes)) (local $res (ref $bytes))
+      (local $d f64)
+      (local $buffer i32) (local $out_buffer i32)
+      (local $fmt_len i32) (local $avail i32) (local $len i32)
+      (local.set $fmt (ref.cast (ref $bytes) (local.get $vfmt)))
+      (local.set $d
+         (struct.get $float 0 (ref.cast (ref $float) (local.get $arg))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $fmt_len (array.len (local.get $fmt)))
+      (call $blit_string_to_memory (local.get $buffer) (local.get $fmt))
+      (i32.store8
+         (i32.add (local.get $buffer) (local.get $fmt_len)) (i32.const 0))
+      (local.set $out_buffer
+         (i32.add (local.get $buffer)
+            (i32.add (local.get $fmt_len) (i32.const 1))))
+      (local.set $avail
+         (i32.sub (global.get $IO_BUFFER_SIZE) (local.get $fmt_len)))
+      (local.set $len
+         (call $format_float
+            (local.get $out_buffer) (local.get $avail)
+            (local.get $buffer) (local.get $d)))
+      (if (i32.ge_u (local.get $len) (local.get $avail))
+         (then
+            (local.set $out_buffer
+               (call $checked_malloc (i32.add (local.get $len) (i32.const 1))))
+            (drop
+               (call $format_float
+                  (local.get $out_buffer)
+                  (i32.add (local.get $len) (i32.const 1))
+                  (local.get $buffer) (local.get $d)))))
+      (local.set $res
+         (call $blit_memory_to_string (local.get $out_buffer) (local.get $len)))
+      (if (i32.ge_u (local.get $len) (local.get $avail))
+         (then
+            (call $free (local.get $out_buffer))))
+      (local.get $res)
+  )
+)
+(@else
    (func (export "caml_format_float")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
       (local $f f64) (local $b i64) (local $format (tuple i32 i32 i32 i32))
@@ -338,8 +403,9 @@
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $uppercase (i32.lt_u (local.get $i) (local.get $len))))))
       (local.get $s))
+))
 
-   (data $float_of_string "float_of_string")
+   (@string $float_of_string "float_of_string")
 
    (func $caml_float_of_hex (param $s (ref $bytes)) (param $i i32) (result f64)
       (local $len i32) (local $c i32) (local $d i32) (local $m i64)
@@ -480,8 +546,7 @@
          (if (local.get $exp)
             (then (local.set $f (call $ldexp (local.get $f) (local.get $exp)))))
          (return (local.get $f)))
-      (call $caml_failwith
-         (array.new_data $bytes $float_of_string (i32.const 0) (i32.const 15)))
+      (call $caml_failwith (global.get $float_of_string))
       (f64.const 0))
 
    (func $on_whitespace (param $s (ref $bytes)) (param $i i32) (result i32)
@@ -495,6 +560,7 @@
       (local $s' (ref $bytes))
       (local $negative i32) (local $c i32)
       (local $f f64)
+      (local $buffer i32) (local $buf i32)
       (local.set $s (ref.cast (ref $bytes) (local.get 0)))
       (local.set $len (array.len (local.get $s)))
       (loop $count
@@ -661,12 +727,28 @@
                                                 (f64.const inf)
                                                 (local.get $negative))))
                                        ))))))))))))))))))
+(@if wasi
+(@then
+         (local.set $buffer (call $get_buffer))
+         (local.set $buf
+            (call $write_string_to_memory
+               (i32.add (local.get $buffer) (i32.const 4))
+               (global.get $IO_BUFFER_SIZE)
+               (local.get $s)))
+         (local.set $f (call $strtod (local.get $buf) (local.get $buffer)))
+         (call $release_memory (i32.add (local.get $buffer) (i32.const 4))
+            (local.get $buf))
+         (br_if $error
+            (i32.ne (i32.load (local.get $buffer))
+              (i32.add (local.get $buf) (local.get $len))))
+)
+(@else
          (local.set $f
             (call $parse_float (call $jsstring_of_bytes (local.get $s))))
          (br_if $error (f64.ne (local.get $f) (local.get $f)))
+))
          (return (struct.new $float (local.get $f))))
-      (call $caml_failwith
-         (array.new_data $bytes $float_of_string (i32.const 0) (i32.const 15)))
+      (call $caml_failwith (global.get $float_of_string))
       (return (ref.i31 (i32.const 0))))
 
    (func (export "caml_nextafter_float")
diff --git a/runtime/wasm/fs.wat b/runtime/wasm/fs.wat
index e4ba3664a4..5301f33de4 100644
--- a/runtime/wasm/fs.wat
+++ b/runtime/wasm/fs.wat
@@ -16,18 +16,57 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if wasi
+(@then
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "wasi_memory" "checked_malloc"
+      (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+   (import "wasi_snapshot_preview1" "fd_prestat_get"
+      (func $fd_prestat_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_prestat_dir_name"
+      (func $fd_prestat_dir_name (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_rename"
+      (func $path_rename (param i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_unlink_file"
+      (func $path_unlink_file (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_create_directory"
+      (func $path_create_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_remove_directory"
+      (func $path_remove_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_filestat_get"
+      (func $path_filestat_get (param i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_open"
+      (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_readdir"
+      (func $fd_readddir (param i32 i32 i32 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_close"
+      (func $fd_close (param i32) (result i32)))
+   (import "sys" "caml_handle_sys_error"
+      (func $caml_handle_sys_error (param (ref eq) i32)))
+)
+(@else
    (import "bindings" "getcwd" (func $getcwd (result anyref)))
    (import "bindings" "chdir" (func $chdir (param anyref)))
    (import "bindings" "mkdir" (func $mkdir (param anyref) (param i32)))
    (import "bindings" "rmdir" (func $rmdir (param anyref)))
    (import "bindings" "unlink" (func $unlink (param anyref)))
-   (import "bindings" "readdir"
-      (func $readdir (param anyref) (result (ref extern))))
+   (import "bindings" "read_dir"
+      (func $read_dir (param anyref) (result (ref extern))))
    (import "bindings" "file_exists"
       (func $file_exists (param anyref) (result (ref eq))))
    (import "bindings" "is_directory"
       (func $is_directory (param anyref) (result (ref eq))))
+   (import "bindings" "is_file"
+      (func $is_file (param anyref) (result (ref eq))))
    (import "bindings" "rename" (func $rename (param anyref) (param anyref)))
+   (import "fail" "javascript_exception"
+      (tag $javascript_exception (param externref)))
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "caml_string_of_jsstring"
@@ -36,19 +75,334 @@
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jslib" "caml_js_to_string_array"
       (func $caml_js_to_string_array (param $a (ref extern)) (result (ref eq))))
-   (import "fail" "caml_raise_sys_error"
-      (func $caml_raise_sys_error (param (ref eq))))
-   (import "fail" "javascript_exception"
-      (tag $javascript_exception (param externref)))
    (import "sys" "caml_handle_sys_error"
       (func $caml_handle_sys_error (param externref)))
+))
+   (import "string" "caml_string_concat"
+      (func $caml_string_concat (param (ref eq) (ref eq)) (result (ref eq))))
+   (import "fail" "caml_raise_sys_error"
+      (func $caml_raise_sys_error (param (ref eq))))
 
    (type $bytes (array (mut i8)))
+   (type $block (array (mut (ref eq))))
+
+(@if wasi
+(@then
+   (type $preopen
+      (struct
+         (field $prefix (ref $bytes))
+         (field $fd i32)
+         (field $next (ref null $preopen))))
+
+   (global $preopens (mut (ref null $preopen)) (ref.null $preopen))
+
+   (global $preopens_initialized (mut i32) (i32.const 0))
+
+   (func $normalize_prefix (param $prefix (ref $bytes)) (result (ref $bytes))
+      (local $i i32) (local $len i32) (local $c i32) (local $res (ref $bytes))
+      (local.set $len (array.len (local.get $prefix)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (local.set $c
+                  (array.get $bytes (local.get $prefix) (local.get $i)))
+               (if (i32.eq (local.get $c) (i32.const 47)) ;; '/'
+                  (then
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))
+               (if (i32.eq (local.get $c) (i32.const 46)) ;; '.'
+                  (then
+                     (if (i32.eq (local.get $i)
+                            (i32.sub (local.get $len) (i32.const 1)))
+                        (then
+                           (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                           (br $loop))
+                        (else
+                           (local.set $c
+                              (array.get $bytes (local.get $prefix)
+                                 (i32.add (local.get $i) (i32.const 1))))
+                           (if (i32.eq (local.get $c) (i32.const 47)) ;; '/'
+                              (then
+                                 (local.set $i
+                                    (i32.add (local.get $i) (i32.const 2)))
+                                 (br $loop))))))))))
+      (if (i32.eq (local.get $i) (local.get $len))
+         (then (return (@string ""))))
+      (local.set $i (i32.sub (local.get $i) (i32.const 1)))
+      (if (i32.gt_u (local.get $i) (i32.const 0))
+         (then
+            (local.set $res
+               (array.new $bytes (i32.const 0)
+                  (i32.sub (local.get $len) (local.get $i))))
+            (array.copy $bytes $bytes
+               (local.get $res) (i32.const 0)
+               (local.get $prefix) (local.get $i)
+               (i32.sub (local.get $len) (local.get $i)))
+            (return (local.get $res))))
+      (return (local.get $prefix)))
+
+   (func $get_preopens (result (ref null $preopen))
+      (local $fd i32) (local $buffer i32) (local $res i32) (local $len i32)
+      (if $done (i32.eqz (global.get $preopens_initialized))
+         (then
+            (local.set $buffer (call $get_buffer))
+            (local.set $fd (i32.const 3))
+            (loop $loop
+               (local.set $res
+                  (call $fd_prestat_get (local.get $fd) (local.get $buffer)))
+               (br_if $done (i32.eq (local.get $res) (i32.const 8))) ;; EBADF
+               (block $skip
+                  (br_if $skip
+                     (i32.eqz
+                        (i32.and (i32.eqz (local.get $res))
+                           (i32.eqz (i32.load8_u (local.get $buffer))))))
+                  (local.set $len (i32.load offset=4 (local.get $buffer)))
+                  (local.set $res
+                     (call $fd_prestat_dir_name
+                        (local.get $fd) (local.get $buffer) (local.get $len)))
+                  (br_if $skip (local.get $res))
+                  (global.set $preopens
+                     (struct.new $preopen
+                        (call $normalize_prefix
+                           (call $blit_memory_to_string
+                              (local.get $buffer) (local.get $len)))
+                        (local.get $fd)
+                        (global.get $preopens))))
+               (local.set $fd (i32.add (local.get $fd) (i32.const 1)))
+               (br $loop))
+            (global.set $preopens_initialized (i32.const 1))))
+      (global.get $preopens))
+
+   (global $current_dir (mut (ref $bytes)) (@string ""))
+
+   (@string $root_dir "/")
+
+   (func $make_absolute
+      (param $path (ref $bytes)) (result (ref $bytes))
+      (local $need_slash i32) (local $i i32) (local $abs_path (ref $bytes))
+      (if (i32.eqz (array.len (local.get $path)))
+         (then ;; empty path
+            (return (global.get $current_dir))))
+      (if (i32.eq (i32.const 47) ;; '/'
+             (array.get_u $bytes (local.get $path) (i32.const 0)))
+         (then ;; absolute path
+            (return (local.get $path))))
+      (if (i32.and
+             (i32.eq (i32.const 46) ;; '.'
+                (array.get_u $bytes (local.get $path) (i32.const 0)))
+             (i32.eq (array.len (local.get $path)) (i32.const 1)))
+         (then
+            ;; "."
+            (return (global.get $current_dir))))
+      (if (i32.ge_u (array.len (local.get $path)) (i32.const 2))
+         (then
+            (if (i32.and
+                   (i32.eq (i32.const 46) ;; '.'
+                      (array.get_u $bytes (local.get $path) (i32.const 0)))
+                   (i32.eq (i32.const 47) ;; '/'
+                      (array.get_u $bytes (local.get $path) (i32.const 1))))
+               (then ;; starts with "./"
+                  (local.set $i (i32.const 2))))))
+      (if (i32.eq (local.get $i) (array.len (local.get $path)))
+         (then ;; "./"
+            (return (global.get $current_dir))))
+      (local.set $need_slash
+         (if (result i32) (array.len (global.get $current_dir))
+            (then
+               (i32.ne (i32.const 47) ;; '/'
+                  (array.get_u $bytes (global.get $current_dir)
+                     (i32.sub (array.len (global.get $current_dir))
+                        (i32.const 1)))))
+            (else
+               (i32.const 1))))
+      (local.set $abs_path
+         (array.new $bytes (i32.const 0)
+            (i32.add (array.len (global.get $current_dir))
+               (i32.add (i32.sub (local.get $need_slash) (local.get $i))
+                  (array.len (local.get $path))))))
+      (array.copy $bytes $bytes
+         (local.get $abs_path) (i32.const 0)
+         (global.get $current_dir) (i32.const 0)
+         (array.len (global.get $current_dir)))
+      (array.set $bytes (local.get $abs_path)
+         (array.len (global.get $current_dir))
+         (i32.const 47)) ;; '/'
+      (array.copy $bytes $bytes
+         (local.get $abs_path)
+         (i32.add (array.len (global.get $current_dir))
+            (local.get $need_slash))
+         (local.get $path) (local.get $i)
+         (i32.sub (array.len (local.get $path)) (local.get $i)))
+      (local.get $abs_path))
+
+   (func $wasi_chdir (export "wasi_chdir") (param $name (ref eq))
+      (local $abs_path (ref $bytes)) (local $path (ref $bytes)) (local $i i32)
+      (local.set $abs_path
+         (call $make_absolute (ref.cast (ref $bytes) (local.get $name))))
+      (local.set $i (i32.sub (array.len (local.get $abs_path)) (i32.const 1)))
+      ;; remove trailing slashes
+      (loop $loop
+         (if (i32.ge_s (local.get $i) (i32.const 0))
+            (then
+               (if (i32.eq (i32.const 47) ;; '/'
+                      (array.get $bytes (local.get $abs_path) (local.get $i)))
+                  (then
+                     (local.set $i (i32.sub (local.get $i) (i32.const 1)))
+                     (br $loop))))))
+      (local.set $i (i32.add (local.get $i) (i32.const 1)))
+      (if (i32.lt_u (local.get $i) (array.len (local.get $abs_path)))
+         (then
+            (local.set $path (array.new $bytes (i32.const 0) (local.get $i)))
+            (array.copy $bytes $bytes
+               (local.get $path) (i32.const 0)
+               (local.get $abs_path) (i32.const 0)
+               (local.get $i))
+            (local.set $abs_path (local.get $path))))
+      (global.set $current_dir (local.get $abs_path)))
+
+   (func $prefix_match
+      (param $prefix (ref $bytes)) (param $path (ref $bytes)) (result i32)
+      (local $i i32) (local $len i32)
+      (local.set $len (array.len (local.get $prefix)))
+      (if (i32.lt_u (array.len (local.get $path)) (local.get $len))
+         (then (return (i32.const 0))))
+      (if (i32.gt_u (array.len (local.get $path)) (local.get $len))
+         (then
+            (if (i32.ne (array.get_u $bytes (local.get $path) (local.get $i))
+                   (i32.const 47))
+               (then (return (i32.const 0))))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (if (i32.ne (array.get_u $bytes (local.get $path) (local.get $i))
+                      (array.get_u $bytes (local.get $prefix) (local.get $i)))
+                  (then (return (i32.const 0))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (return (i32.const 1)))
+
+   (func $resolve_abs_path
+      (param $path (ref $bytes)) (result i32 (ref $bytes))
+      (local $fd i32) (local $len i32) (local $i i32)
+      (local $preopens (ref null $preopen)) (local $current (ref $preopen))
+      (local $prefix (ref $bytes)) (local $rel_path (ref $bytes))
+      (local.set $preopens (call $get_preopens))
+      (local.set $i (i32.const -1))
+      (block $done
+         (loop $loop
+            (local.set $current (br_on_null $done (local.get $preopens)))
+            (local.set $prefix
+               (struct.get $preopen $prefix (local.get $current)))
+            (if (i32.and
+                   (i32.gt_s (array.len (local.get $prefix)) (local.get $i))
+                   (call $prefix_match (local.get $prefix) (local.get $path)))
+               (then
+                  (local.set $fd (struct.get $preopen $fd (local.get $current)))
+                  (local.set $i (array.len (local.get $prefix)))))
+            (local.set $preopens
+               (struct.get $preopen $next (local.get $current)))
+            (br $loop)))
+      (if (i32.eq (local.get $i) (i32.const -1))
+         (then ;; not found
+            (return (tuple.make 2 (i32.const -1) (@string "")))))
+      ;; skip leading slashes
+      (local.set $len (local.get $i))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (array.len (local.get $path)))
+            (then
+               (if (i32.eq (array.get_u $bytes (local.get $path) (local.get $i))
+                      (i32.const 47)) ;; 47
+                  (then
+                     (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                     (br $loop)))
+               (local.set $rel_path
+                  (array.new $bytes (i32.const 0)
+                     (i32.sub (array.len (local.get $path)) (local.get $i))))
+               (array.copy $bytes $bytes
+                  (local.get $rel_path) (i32.const 0)
+                  (local.get $path) (local.get $i)
+                  (i32.sub (array.len (local.get $path)) (local.get $i)))
+               (return
+                  (tuple.make 2 (local.get $fd) (local.get $rel_path))))))
+      (return (tuple.make 2 (local.get $fd) (@string "."))))
 
+   (func (export "wasi_resolve_path")
+      (param $vpath (ref eq))
+      (result (;fd;) i32 (;address;) i32 (;length;) i32)
+      (local $res (tuple i32 (ref $bytes)))
+      (local $p i32)
+      (local.set $res
+         (call $resolve_abs_path
+            (call $make_absolute
+               (ref.cast (ref $bytes) (local.get $vpath)))))
+      (if (i32.ge_u (tuple.extract 2 0 (local.get $res)) (i32.const 0))
+         (then
+            (local.set $p
+               (call $write_string_to_memory
+                  (i32.const 0) (i32.const 0)
+                  (tuple.extract 2 1 (local.get $res))))))
+      (return
+         (tuple.make 3
+            (tuple.extract 2 0 (local.get $res))
+            (local.get $p)
+            (array.len (tuple.extract 2 1 (local.get $res))))))
+
+   (func $caml_sys_resolve_path (export "caml_sys_resolve_path")
+      (param $path (ref eq)) (result i32 i32 i32)
+      (local $res (tuple i32 i32 i32))
+      (local.set $res (call $wasi_resolve_path (local.get $path)))
+      (if (i32.lt_s (tuple.extract 3 0 (local.get $res)) (i32.const 0))
+         (then ;; ENOENT
+            (call $caml_handle_sys_error (local.get $path) (i32.const 44))))
+      (local.get $res))
+))
+
+(@if wasi
+(@then
+   (func (export "caml_sys_getcwd")
+      (export "unix_getcwd") (export "caml_unix_getcwd")
+      (param (ref eq)) (result (ref eq))
+      (if (array.len (global.get $current_dir))
+         (then (return (global.get $current_dir))))
+      (global.get $root_dir))
+)
+(@else
    (func (export "caml_sys_getcwd")
       (param (ref eq)) (result (ref eq))
-      (return_call $caml_string_of_jsstring (call $wrap (call $getcwd))))
+      (try (result (ref eq))
+         (do
+            (call $caml_string_of_jsstring (call $wrap (call $getcwd))))
+         (catch $javascript_exception
+            (call $caml_handle_sys_error (pop externref))
+            (ref.i31 (i32.const 0)))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_chdir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32) (local $kind i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (local.set $kind (i32.load8_u offset=16 (local.get $buffer)))
+      (if (i32.ne (local.get $kind) (i32.const 3))
+         (then
+            (call $caml_handle_sys_error
+               (local.get $name) (i32.const 54)))) ;; ENOTDIR
+      (call $wasi_chdir (local.get $name))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_chdir")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -58,7 +412,26 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_mkdir")
+      (param $name (ref eq)) (param $perm (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $res
+         (call $path_create_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_mkdir")
       (param $name (ref eq)) (param $perm (ref eq)) (result (ref eq))
       (try
@@ -69,20 +442,160 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_read_directory")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32) (local $fd i32)
+      (local $buf i32) (local $new_buf i32)
+      (local $size i32) (local $pos i32) (local $available i32)
+      (local $left i32) (local $namelen i32)
+      (local $entry i32) (local $entry_size i32)
+      (local $cookie i64) (local $tbl (ref $block)) (local $new_tbl (ref $block))
+      (local $i i32) (local $s (ref $bytes))
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (i32.const 2) ;; O_DIRECTORY
+            (i64.const 0x4000) ;; allow fd_readdir
+            (i64.const 0)
+            (i32.const 0)
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (local.set $fd (i32.load (local.get $buffer)))
+      (local.set $buf (call $checked_malloc (i32.const 512)))
+      (local.set $size (i32.const 512))
+      (local.set $tbl (array.new $block (ref.i31 (i32.const 0)) (i32.const 50)))
+      (local.set $i (i32.const 1))
+      (loop $loop
+         (block $refill
+            (local.set $left (i32.sub (local.get $available) (local.get $pos)))
+            (br_if $refill (i32.lt_u (local.get $left) (i32.const 24)))
+            (local.set $entry (i32.add (local.get $buf) (local.get $pos)))
+            (local.set $namelen (i32.load offset=16 (local.get $entry)))
+            (local.set $entry_size (i32.add (local.get $namelen) (i32.const 24)))
+            (br_if $refill (i32.lt_u (local.get $left) (local.get $entry_size)))
+            (local.set $pos (i32.add (local.get $pos) (local.get $entry_size)))
+            (local.set $cookie (i64.load (local.get $entry)))
+            (if (i32.eq (local.get $i) (array.len (local.get $tbl)))
+               (then
+                  (local.set $new_tbl
+                     (array.new $block (ref.i31 (i32.const 0))
+                        (i32.shl (local.get $i) (i32.const 1))))
+                  (array.copy $block $block
+                     (local.get $new_tbl) (i32.const 0)
+                     (local.get $tbl) (i32.const 0) (local.get $i))
+                  (local.set $tbl (local.get $new_tbl))))
+            (local.set $s
+               (call $blit_memory_to_string
+                  (i32.add (local.get $entry) (i32.const 24))
+                  (local.get $namelen)))
+            ;; skip "." and ".."
+            (if (i32.eq (local.get $namelen) (i32.const 2))
+               (then
+                  (br_if $loop
+                     (i32.and
+                        (i32.eq (i32.const 46)
+                           (array.get_u $bytes (local.get $s) (i32.const 0)))
+                        (i32.eq (i32.const 46)
+                           (array.get_u $bytes (local.get $s) (i32.const 1))))))
+               (else
+                  (if (i32.eq (local.get $namelen) (i32.const 2))
+                     (then
+                        (br_if $loop
+                           (i32.eq
+                              (array.get_u $bytes (local.get $s) (i32.const 0))
+                              (i32.const 46)))))))
+            (array.set $block (local.get $tbl) (local.get $i) (local.get $s))
+            (local.set $i (i32.add (local.get $i) (i32.const 1)))
+            (br $loop))
+         ;; refill
+         (if (i32.lt_u (local.get $size) (local.get $entry_size))
+            (then
+               ;; the entry does not fit
+               (local.set $new_buf
+                  (call $checked_malloc (local.get $entry_size)))
+               (call $free (local.get $buf))
+               (local.set $buf (local.get $new_buf))
+               (local.set $size (local.get $entry_size))))
+         (block $done
+            (br_if $done
+               (i32.and
+                  (i32.ne (i32.const 0) (local.get $available))
+                  (i32.lt_u (local.get $available) (local.get $size))))
+            (local.set $res
+               (call $fd_readddir
+                  (local.get $fd)
+                  (local.get $buffer)
+                  (local.get $size)
+                  (local.get $cookie)
+                  (local.get $buffer)))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (local.get $name) (local.get $res))))
+            (local.set $available (i32.load (local.get $buffer)))
+            (br_if $done (i32.eqz (local.get $available)))
+            (local.set $pos (i32.const 0))
+            (br $loop)))
+      ;; done
+      (call $free (local.get $buf))
+      (local.set $res (call $fd_close (local.get $fd)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (if (i32.eq (local.get $i) (array.len (local.get $tbl)))
+         (then (return (local.get $tbl))))
+      (local.set $new_tbl
+         (array.new $block (ref.i31 (i32.const 0)) (local.get $i)))
+      (array.copy $block $block
+         (local.get $new_tbl) (i32.const 0)
+         (local.get $tbl) (i32.const 0) (local.get $i))
+      (local.get $new_tbl))
+)
+(@else
    (func (export "caml_sys_read_directory")
       (param $name (ref eq)) (result (ref eq))
       (try
          (do
             (return
                (call $caml_js_to_string_array
-                  (call $readdir
+                  (call $read_dir
                      (call $unwrap
                         (call $caml_jsstring_of_string (local.get $name)))))))
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))
             (return (ref.i31 (i32.const 0))))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_rmdir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $res
+         (call $path_remove_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_rmdir")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -92,7 +605,26 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_remove")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $res
+         (call $path_unlink_file
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_remove")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -102,7 +634,32 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_rename")
+      (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
+      (local $op (tuple i32 i32 i32))
+      (local $np (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $op (call $caml_sys_resolve_path (local.get $o)))
+      (local.set $np (call $caml_sys_resolve_path (local.get $n)))
+      (local.set $res
+         (call $path_rename
+            (tuple.extract 3 0 (local.get $op))
+            (tuple.extract 3 1 (local.get $op))
+            (tuple.extract 3 2 (local.get $op))
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (tuple.extract 3 1 (local.get $op)))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $o) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "caml_sys_rename")
       (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
       (try
@@ -113,31 +670,38 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_file_exists")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (ref.i31 (i32.eqz (local.get $res))))
+)
+(@else
    (func (export "caml_sys_file_exists")
       (param $name (ref eq)) (result (ref eq))
       (return_call $file_exists
          (call $unwrap (call $caml_jsstring_of_string (local.get $name)))))
+))
 
-   (data $no_such_file ": No such file or directory")
+   (@string $no_such_file ": No such file or directory")
 
-   (func $caml_raise_no_such_file (param $vname (ref eq))
-      (local $name (ref $bytes)) (local $msg (ref $bytes))
-      (local $len i32)
-      (local.set $name (ref.cast (ref $bytes) (local.get $vname)))
-      (local.set $len (array.len (local.get $name)))
-      (local.set $msg
-         (array.new $bytes (i32.const 0)
-            (i32.add (local.get $len) (i32.const 27))))
-      (array.copy $bytes $bytes
-         (local.get $msg) (i32.const 0)
-         (local.get $name) (i32.const 0)
-         (local.get $len))
-      (array.init_data $bytes $no_such_file
-         (local.get $msg) (local.get $len) (i32.const 0) (i32.const 27))
-      (call $caml_raise_sys_error (local.get $msg)))
-
-   (data $caml_read_file_content "caml_read_file_content")
+   (func $caml_raise_no_such_file (param $name (ref eq))
+      (call $caml_raise_sys_error
+         (call $caml_string_concat (local.get $name)
+            (global.get $no_such_file))))
 
    (func (export "caml_read_file_content")
       (param (ref eq)) (result (ref eq))
@@ -147,8 +711,30 @@
    (func (export "caml_fs_init") (result (ref eq))
       (ref.i31 (i32.const 0)))
 
-   (data $caml_sys_is_directory "caml_sys_is_directory")
+(@if wasi
+(@then
+   (func $caml_sys_file_mode (param $name (ref eq)) (result i32)
+      (local $p (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $p (call $caml_sys_resolve_path (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then (call $caml_handle_sys_error (local.get $name) (local.get $res))))
+      (i32.load8_u offset=16 (local.get $buffer)))
 
+   (func (export "caml_sys_is_directory")
+      (param $name (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.eq (call $caml_sys_file_mode (local.get $name)) (i32.const 3))))
+)
+(@else
    (func (export "caml_sys_is_directory")
       (param $name (ref eq)) (result (ref eq))
       (try
@@ -160,4 +746,26 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))
             (return (ref.i31 (i32.const 0))))))
+))
+
+(@if wasi
+(@then
+   (func (export "caml_sys_is_regular_file")
+      (param $name (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.eq (call $caml_sys_file_mode (local.get $name)) (i32.const 4))))
+)
+(@else
+   (func (export "caml_sys_is_regular_file")
+      (param $name (ref eq)) (result (ref eq))
+      (try
+         (do
+            (return
+               (call $is_file
+                  (call $unwrap
+                     (call $caml_jsstring_of_string (local.get $name))))))
+         (catch $javascript_exception
+            (call $caml_handle_sys_error (pop externref))))
+      (return (ref.i31 (i32.const 0))))
+))
 )
diff --git a/runtime/wasm/hash.wat b/runtime/wasm/hash.wat
index a7a78c9e49..ad9fd4d628 100644
--- a/runtime/wasm/hash.wat
+++ b/runtime/wasm/hash.wat
@@ -304,6 +304,8 @@
                                           (local.get $v))))))))
                      (local.set $num (i32.sub (local.get $num) (i32.const 1)))
                      (br $loop)))
+(@if (not wasi)
+(@then
                   (drop (block $not_jsstring (result anyref)
                      (local.set $str
                         (struct.get $js 0
@@ -315,6 +317,7 @@
                      (local.set $h
                         (call $jsstring_hash (local.get $h) (local.get $str)))
                      (ref.i31 (i32.const 0))))
+))
                   ;; closures and continuations and other js values are ignored
                   (br $loop)))))
       ;; clear the queue to avoid a memory leak
diff --git a/runtime/wasm/int32.wat b/runtime/wasm/int32.wat
index 089c601964..bb3126fb53 100644
--- a/runtime/wasm/int32.wat
+++ b/runtime/wasm/int32.wat
@@ -18,7 +18,7 @@
 (module
    (import "ints" "parse_int"
       (func $parse_int
-         (param (ref eq)) (param i32) (param (ref $bytes)) (result i32)))
+         (param (ref eq)) (param i32) (param (ref eq)) (result i32)))
    (import "ints" "format_int"
       (func $format_int
          (param (ref eq)) (param i32) (param i32) (result (ref eq))))
@@ -56,7 +56,7 @@
 
    (global $int32_ops (export "int32_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $bytes 2 (i32.const 95) (i32.const 105)) ;; "_i"
+         (@string "_i")
          (ref.func $int32_cmp)
          (ref.null $compare)
          (ref.func $int32_hash)
@@ -117,12 +117,7 @@
          (i32.rotl (i32.and (local.get $i) (i32.const 0xFF00FF00))
                    (i32.const 8))))
 
-   (global $INT32_ERRMSG (ref $bytes)
-      (array.new_fixed $bytes 15 ;; "Int32.of_string"
-         (i32.const 73) (i32.const 110) (i32.const 116) (i32.const 51)
-         (i32.const 50) (i32.const 46) (i32.const 111) (i32.const 102)
-         (i32.const 95) (i32.const 115) (i32.const 116) (i32.const 114)
-         (i32.const 105) (i32.const 110) (i32.const 103)))
+   (@string $INT32_ERRMSG "Int32.of_string")
 
    (func (export "caml_int32_of_string") (param $v (ref eq)) (result (ref eq))
       (return_call $caml_copy_int32
@@ -137,7 +132,7 @@
 
    (global $nativeint_ops (export "nativeint_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $bytes 2 (i32.const 95) (i32.const 110)) ;; "_n"
+         (@string "_n")
          (ref.func $int32_cmp)
          (ref.null $compare)
          (ref.func $int32_hash)
@@ -153,15 +148,12 @@
          (struct.get $int32 1 (ref.cast (ref $int32) (local.get $v))))
       (tuple.make 2 (i32.const 4) (i32.const 8)))
 
-   (data $integer_too_large "input_value: native integer value too large")
+   (@string $integer_too_large "input_value: native integer value too large")
 
    (func $nativeint_deserialize
       (param $s (ref eq)) (result (ref eq)) (result i32)
       (if (i32.ne (call $caml_deserialize_uint_1 (local.get $s)) (i32.const 1))
-         (then
-            (call $caml_failwith
-               (array.new_data $bytes $integer_too_large
-                  (i32.const 0) (i32.const 43)))))
+         (then (call $caml_failwith (global.get $integer_too_large))))
       (tuple.make 2
          (struct.new $int32 (global.get $nativeint_ops)
             (call $caml_deserialize_int_4 (local.get $s)))
@@ -171,12 +163,7 @@
       (param $i i32) (result (ref eq))
       (struct.new $int32 (global.get $nativeint_ops) (local.get $i)))
 
-   (global $NATIVEINT_ERRMSG (ref $bytes)
-      (array.new_fixed $bytes 16 ;; "Nativeint.of_string"
-         (i32.const 78) (i32.const 97) (i32.const 116) (i32.const 105)
-         (i32.const 118) (i32.const 101) (i32.const 46) (i32.const 111)
-         (i32.const 102) (i32.const 95) (i32.const 115) (i32.const 116)
-         (i32.const 114) (i32.const 105) (i32.const 110) (i32.const 103)))
+   (@string $NATIVEINT_ERRMSG "Nativeint.of_string")
 
    (func (export "caml_nativeint_of_string")
       (param $v (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/int64.wat b/runtime/wasm/int64.wat
index a873f39ff6..e419dcb229 100644
--- a/runtime/wasm/int64.wat
+++ b/runtime/wasm/int64.wat
@@ -57,7 +57,7 @@
 
    (global $int64_ops (export "int64_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $bytes 2 (i32.const 95) (i32.const 106)) ;; "_j"
+         (@string "_j")
          (ref.func $int64_cmp)
          (ref.null $compare)
          (ref.func $int64_hash)
@@ -128,12 +128,7 @@
       (ref.i31 (i32.sub (i64.gt_s (local.get $i1) (local.get $i2))
                         (i64.lt_s (local.get $i1) (local.get $i2)))))
 
-   (global $INT64_ERRMSG (ref $bytes)
-      (array.new_fixed $bytes 15 ;; "Int64.of_string"
-         (i32.const 73) (i32.const 110) (i32.const 116) (i32.const 54)
-         (i32.const 52) (i32.const 46) (i32.const 111) (i32.const 102)
-         (i32.const 95) (i32.const 115) (i32.const 116) (i32.const 114)
-         (i32.const 105) (i32.const 110) (i32.const 103)))
+   (@string $INT64_ERRMSG "Int64.of_string")
 
    ;; Parse a sequence of digits into an i64 as dicted by $base,
    ;; $signedness and $sign. The sequence is read in $s starting from $i.
@@ -142,7 +137,7 @@
    ;; package "integers".
    (func $caml_i64_of_digits (export "caml_i64_of_digits")
       (param $base i32) (param $signedness i32) (param $sign i32)
-      (param $s (ref $bytes)) (param $i i32) (param $errmsg (ref $bytes))
+      (param $s (ref $bytes)) (param $i i32) (param $errmsg (ref eq))
       (result i64)
       (local $len i32) (local $d i32) (local $c i32)
       (local $res i64) (local $threshold i64)
@@ -209,8 +204,6 @@
                                   (local.get $i)
                                   (global.get $INT64_ERRMSG))))
 
-   (data $caml_int64_create_lo_mi_hi "caml_int64_create_lo_mi_hi")
-
    (func $format_int64_default (param $d i64) (result (ref eq))
       (local $s (ref $bytes))
       (local $negative i32) (local $i i32) (local $n i64)
diff --git a/runtime/wasm/ints.wat b/runtime/wasm/ints.wat
index 4ec4e3b0f6..1b256e0a9a 100644
--- a/runtime/wasm/ints.wat
+++ b/runtime/wasm/ints.wat
@@ -94,7 +94,7 @@
       (return (i32.const -1)))
 
    (func $parse_int (export "parse_int")
-      (param $v (ref eq)) (param $nbits i32) (param $errmsg (ref $bytes))
+      (param $v (ref eq)) (param $nbits i32) (param $errmsg (ref eq))
       (result i32)
       (local $s (ref $bytes))
       (local $i i32) (local $len i32) (local $d i32) (local $c i32)
@@ -157,12 +157,7 @@
          (then (local.set $res (i32.sub (i32.const 0) (local.get $res)))))
       (local.get $res))
 
-   (global $INT_ERRMSG (ref $bytes)
-      (array.new_fixed $bytes 13 ;; "int.of_string"
-         (i32.const 105) (i32.const 110) (i32.const 116) (i32.const 95)
-         (i32.const 111) (i32.const 102) (i32.const 95) (i32.const 115)
-         (i32.const 116) (i32.const 114) (i32.const 105) (i32.const 110)
-         (i32.const 103)))
+   (@string $INT_ERRMSG "int_of_string")
 
    (func (export "caml_int_of_string")
       (param $v (ref eq)) (result (ref eq))
@@ -222,7 +217,7 @@
                (i32.const 45)))) ;; '-'
       (local.get $s))
 
-   (data $format_error "format_int: bad format")
+   (@string $format_error "format_int: bad format")
 
    (func $parse_int_format (export "parse_int_format")
       (param $s (ref $bytes)) (result i32 i32 i32 i32 i32)
@@ -283,9 +278,7 @@
             (else
                (br $bad_format)))))))))))
             (br $return))
-         (call $caml_invalid_argument
-            (array.new_data $bytes $format_error
-               (i32.const 0) (i32.const 22))))
+         (call $caml_invalid_argument (global.get $format_error)))
       (tuple.make 5
          (local.get $sign_style)
          (local.get $alternate)
diff --git a/runtime/wasm/io.wat b/runtime/wasm/io.wat
index fc07d84d02..f28c5e2202 100644
--- a/runtime/wasm/io.wat
+++ b/runtime/wasm/io.wat
@@ -25,6 +25,31 @@
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jslib" "caml_list_of_js_array"
       (func $caml_list_of_js_array (param (ref eq)) (result (ref eq))))
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "fd_close"
+      (func $fd_close (param i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_write"
+      (func $fd_write (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_read"
+      (func $fd_read (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_seek"
+      (func $fd_seek (param i32 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_open"
+      (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "blit_memory_to_substring"
+      (func $blit_memory_to_substring (param i32 (ref $bytes) i32 i32)))
+   (import "wasi_memory" "blit_substring_to_memory"
+      (func $blit_substring_to_memory (param i32 (ref $bytes) i32 i32)))
+   (import "sys" "caml_handle_sys_error"
+      (func $caml_handle_sys_error (param (ref eq) i32)))
+   (import "fs" "caml_sys_resolve_path"
+      (func $caml_sys_resolve_path (param (ref eq)) (result i32 i32 i32)))
+)
+(@else
    (import "bindings" "open"
       (func $open (param anyref) (param i32) (param i32) (result i32)))
    (import "bindings" "close" (func $close (param i32)))
@@ -65,6 +90,11 @@
       (func $ta_blit_to_bytes
          (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
          (param i32)))
+   (import "sys" "caml_handle_sys_error"
+      (func $caml_handle_sys_error (param externref)))
+   (import "fail" "javascript_exception"
+      (tag $javascript_exception (param externref)))
+))
    (import "bindings" "ta_subarray"
       (func $ta_subarray
          (param (ref extern)) (param i32) (param i32) (result (ref extern))))
@@ -80,13 +110,129 @@
       (func $caml_copy_int64 (param i64) (result (ref eq))))
    (import "int64" "Int64_val"
       (func $Int64_val (param (ref eq)) (result i64)))
-   (import "fail" "javascript_exception"
-      (tag $javascript_exception (param externref)))
-   (import "sys" "caml_handle_sys_error"
-      (func $caml_handle_sys_error (param externref)))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
 
+(@if wasi
+(@then
+   (func $ta_new (param $sz i32) (result (ref extern))
+      (extern.convert_any (array.new $bytes (i32.const 0) (local.get $sz))))
+
+   (func $ta_copy
+      (param $buf (ref extern))
+      (param $dst i32) (param $src i32) (param $end i32)
+      (local $b (ref $bytes))
+      (local.set $b
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf))))
+      (array.copy $bytes $bytes
+         (local.get $b) (local.get $dst)
+         (local.get $b) (local.get $src)
+         (i32.sub (local.get $end) (local.get $src))))
+
+   (func $ta_set_ui8 (param $buf (ref extern)) (param $i i32) (param $c i32)
+      (array.set $bytes
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $i) (local.get $c)))
+
+   (func $ta_get_ui8 (param $buf (ref extern)) (param $i i32) (result i32)
+      (array.get_u $bytes
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $i)))
+
+   (func $ta_blit_from_bytes
+      (param $s (ref $bytes)) (param $i i32) (param $buf (ref extern))
+      (param $j i32) (param $l i32)
+      (array.copy $bytes $bytes
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $j)
+         (local.get $s) (local.get $i)
+         (local.get $l)))
+
+   (func $ta_blit_to_bytes
+      (param $buf (ref extern)) (param $i i32) (param $s (ref $bytes))
+      (param $j i32) (param $l i32)
+      (array.copy $bytes $bytes
+         (local.get $s) (local.get $j)
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf)))
+         (local.get $i)
+         (local.get $l)))
+
+   (type $data
+      (struct
+         (field $array (ref array))
+         (field $offset i32)
+         (field $len i32)))
+
+   (func $ta_blit_from_buffer
+      (param $buf (ref extern)) (param $i i32)
+      (param $ta (ref extern)) (param $j i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $ta))))
+      (call $ta_blit_to_bytes
+         (local.get $buf)
+         (local.get $i)
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data)))
+         (i32.add (struct.get $data $offset (local.get $data)) (local.get $j))
+         (local.get $len)))
+
+   (func $ta_blit_to_buffer
+      (param $ta (ref extern)) (param $i i32)
+      (param $buf (ref extern)) (param $j i32)
+      (param $len i32)
+      (local $data (ref $data))
+      (local.set $data
+         (ref.cast (ref $data) (any.convert_extern (local.get $ta))))
+      (call $ta_blit_from_bytes
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data)))
+         (i32.add (struct.get $data $offset (local.get $data)) (local.get $i))
+         (local.get $buf)
+         (local.get $j)
+         (local.get $len)))
+
+   (global $caml_stdout
+      (mut (ref eq)) (ref.i31 (i32.const 0)))
+
+   (func $register_channel (param $ch (ref eq))
+      (if (i32.eq
+             (struct.get $channel $fd (ref.cast (ref $channel) (local.get $ch)))
+             (i32.const 1))
+         (then
+            (global.set $caml_stdout (local.get $ch)))))
+
+   (func $unregister_channel (param (ref eq)))
+   (func $map_new (result (ref extern))
+      (extern.convert_any (ref.i31 (i32.const 0))))
+   (func $map_get (param (ref extern)) (param i32) (result (ref $fd_offset))
+      (struct.new $fd_offset (i64.const 0) (i32.const 0)))
+   (func $map_set (param (ref extern)) (param i32) (param (ref $fd_offset)))
+   (func $map_delete (param (ref extern)) (param i32))
+
+   (func $file_size (param $fd i32) (result i64)
+      (local $cur i64) (local $end i64) (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (block $error
+         (local.set $res
+            (call $fd_seek
+               (local.get $fd) (i64.const 0) (i32.const 1) (local.get $buffer)))
+         (br_if $error (local.get $res))
+         (local.set $cur (i64.load (local.get $buffer)))
+         (local.set $res
+            (call $fd_seek
+               (local.get $fd) (i64.const 0) (i32.const 2) (local.get $buffer)))
+         (br_if $error (local.get $res))
+         (local.set $end (i64.load (local.get $buffer)))
+         (local.set $res
+            (call $fd_seek
+               (local.get $fd) (local.get $cur) (i32.const 0)
+               (local.get $buffer)))
+         (br_if $error (local.get $res))
+         (return (local.get $end)))
+      (call $caml_handle_sys_error (ref.i31 (i32.const 0)) (local.get $res))
+      (i64.const 0))
+)
+(@else
    (import "bindings" "map_new" (func $map_new (result (ref extern))))
    (import "bindings" "map_get"
       (func $map_get
@@ -97,6 +243,26 @@
    (import "bindings" "map_delete"
       (func $map_delete (param (ref extern)) (param i32)))
 
+   (func $ta_blit_from_buffer
+      (param $buf (ref extern)) (param $i i32)
+      (param $ta (ref extern)) (param $j i32)
+      (param $len i32)
+      (call $ta_set
+         (local.get $ta)
+         (call $ta_subarray (local.get $buf) (local.get $i)
+            (i32.add (local.get $i) (local.get $len)))
+         (local.get $j)))
+
+   (func $ta_blit_to_buffer
+      (param $ta (ref extern)) (param $i i32)
+      (param $buf (ref extern)) (param $j i32)
+      (param $len i32)
+      (call $ta_set (local.get $buf)
+         (call $ta_subarray (local.get $ta) (local.get $i)
+            (i32.add (local.get $i) (local.get $len)))
+         (local.get $j)))
+))
+
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
    (type $offset_array (array (mut i64)))
@@ -129,9 +295,7 @@
 
    (global $channel_ops (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $bytes 5 ;; "_chan"
-            (i32.const 95) (i32.const 99) (i32.const 104) (i32.const 97)
-            (i32.const 110))
+         (@string "_chan")
          (ref.func $custom_compare_id)
          (ref.null $compare)
          (ref.func $custom_hash_id)
@@ -180,7 +344,7 @@
    (func $release_fd_offset (export "release_fd_offset") (param $fd i32)
       (call $map_delete (call $get_fd_offsets) (local.get $fd)))
 
-   (data $bad_file_descriptor "Bad file descriptor")
+   (@string $bad_file_descriptor "Bad file descriptor")
 
    (func $get_fd_offset_unchecked (export "get_fd_offset_unchecked")
       (param $fd i32) (result (ref null $fd_offset))
@@ -190,15 +354,29 @@
       (local $res (ref null $fd_offset))
       (local.set $res (call $get_fd_offset_unchecked (local.get $fd)))
       (if (ref.is_null (local.get $res))
-         (then
-            (call $caml_raise_sys_error
-               (array.new_data $bytes $bad_file_descriptor
-                  (i32.const 0) (i32.const 19)))))
+         (then (call $caml_raise_sys_error (global.get $bad_file_descriptor))))
       (ref.as_non_null (local.get $res)))
 
    (global $IO_BUFFER_SIZE (export "IO_BUFFER_SIZE") i32 (i32.const 65536))
 
-   (type $open_flags (array i8))
+   (type $open_flags (array i16))
+
+(@if wasi
+(@then
+   ;;      1 O_RDONLY
+   ;;      2 O_WRONLY
+   ;;   0x10 O_CREAT
+   ;;   0x40 O_EXCL
+   ;;   0x80 O_TRUNC
+   ;;  0x100 O_APPEND
+   ;;  0x400 O_NONBLOCK
+   (global $sys_open_flags (ref $open_flags)
+      (array.new_fixed $open_flags 9
+         (i32.const 1) (i32.const 2) (i32.const 0x102) (i32.const 0x10)
+         (i32.const 0x80) (i32.const 0x40) (i32.const 0) (i32.const 0)
+         (i32.const 0x400)))
+)
+(@else
    ;;   1 O_RDONLY
    ;;   2 O_WRONLY
    ;;   4 O_RDWR
@@ -211,6 +389,7 @@
       (array.new_fixed $open_flags 9
          (i32.const 1) (i32.const 2) (i32.const 10) (i32.const 16) (i32.const 32)
          (i32.const 64) (i32.const 0) (i32.const 0) (i32.const 128)))
+))
 
    (func $convert_flag_list (export "convert_flag_list")
       (param $tbl (ref $open_flags)) (param $vflags (ref eq)) (result i32)
@@ -232,6 +411,41 @@
             (br $loop))))
       (local.get $flags))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_open")
+      (param $vpath (ref eq)) (param $vflags (ref eq)) (param $perm (ref eq))
+      (result (ref eq))
+      (local $fd i32) (local $flags i32) (local $offset i64)
+      (local $path (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $path (call $caml_sys_resolve_path (local.get $vpath)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $flags
+         (call $convert_flag_list
+            (global.get $sys_open_flags) (local.get $vflags)))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $path))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $path))
+            (tuple.extract 3 2 (local.get $path))
+            (i32.and (i32.shr_u (local.get $flags) (i32.const 4))
+              (i32.const 0xF))
+            (select (i64.const 0x860007c) (i64.const 0x820003e)
+               (i32.and (local.get $flags) (i32.const 2)))
+            (i64.const 0)
+            (i32.shr_u (local.get $flags) (i32.const 8))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $path)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error (local.get $vpath) (local.get $res))))
+      (local.set $fd (i32.load (local.get $buffer)))
+      (call $initialize_fd_offset (local.get $fd) (local.get $offset))
+      (ref.i31 (local.get $fd)))
+)
+(@else
    (func (export "caml_sys_open")
       (param $path (ref eq)) (param $vflags (ref eq)) (param $perm (ref eq))
       (result (ref eq))
@@ -248,14 +462,30 @@
                   (local.get $flags)
                   (i31.get_u (ref.cast (ref i31) (local.get $perm)))))
             (if (i32.and (local.get $flags) (i32.const 4)) ;; O_APPEND
-               (then (local.set $offset (call $file_size (local.get $fd))))))
+               (then (local.set $offset (call $file_size (local.get $fd)))))
+         )
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (call $initialize_fd_offset (local.get $fd) (local.get $offset))
       (ref.i31 (local.get $fd)))
+))
 
+(@if wasi
+(@then
    (func (export "caml_sys_close") (param (ref eq)) (result (ref eq))
-      (local $fd i32)
+      (local $fd i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get 0))))
+      (call $release_fd_offset (local.get $fd))
+      (local.set $res (call $fd_close (local.get $fd)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "caml_sys_close") (param (ref eq)) (result (ref eq))
+      (local $fd i32) (local $res i32)
       (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get 0))))
       (call $release_fd_offset (local.get $fd))
       (try
@@ -264,14 +494,40 @@
          (catch $javascript_exception
             (call $caml_handle_sys_error (pop externref))))
       (ref.i31 (i32.const 0)))
+))
 
    (func (export "caml_ml_set_channel_name")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $push_channel (param $l (ref eq)) (param $ch (ref eq)) (result (ref eq))
+      (local $c (ref $channel))
+      (block $continue
+         (br_if $continue (i32.eqz (ref.test (ref $channel) (local.get $ch))))
+         (local.set $c (ref.cast (ref $channel) (local.get $ch)))
+         (br_if $continue
+            (i32.eq (struct.get $channel $fd (local.get $c)) (i32.const -1)))
+         (local.set $l
+            (array.new_fixed $block 3
+               (ref.i31 (i32.const 0)) (local.get $ch) (local.get $l))))
+      (local.get $l))
+))
+
+(@if wasi
+(@then
+   (func (export "caml_ml_out_channels_list")
+      (param (ref eq)) (result (ref eq))
+      (call $push_channel
+         (call $push_channel (ref.i31 (i32.const 0)) (global.get $caml_stdout))
+         (global.get $caml_stderr)))
+)
+(@else
    (func (export "caml_ml_out_channels_list")
       (param (ref eq)) (result (ref eq))
       (return_call $caml_list_of_js_array (call $wrap (call $channel_list))))
+))
 
    (func (export "caml_ml_open_descriptor_in")
       (param $fd (ref eq)) (result (ref eq))
@@ -315,10 +571,19 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 1)))
 
+   (func (export "caml_channel_descriptor")
+      (param $ch (ref eq)) (result (ref eq))
+      (local $fd i32)
+      (local.set $fd
+         (struct.get $channel $fd (ref.cast (ref $channel) (local.get $ch))))
+      (if (i32.eq (local.get $fd) (i32.const -1))
+         (then (call $caml_raise_sys_error (global.get $bad_file_descriptor))))
+      (ref.i31 (local.get $fd)))
+
    (func (export "caml_ml_close_channel")
       (param (ref eq)) (result (ref eq))
       (local $ch (ref $channel))
-      (local $fd i32)
+      (local $fd i32) (local $res i32)
       (local.set $ch (ref.cast (ref $channel) (local.get 0)))
       ;; output channels: any output will trigger a flush since the
       ;; buffer is non-empty (curr > 0) and full (curr = size)
@@ -333,14 +598,56 @@
             (struct.set $channel $fd (local.get $ch) (i32.const -1))
             (call $unregister_channel (local.get $ch))
             (call $release_fd_offset (local.get $fd))
+(@if wasi
+(@then
+            (local.set $res (call $fd_close (local.get $fd)))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (ref.i31 (i32.const 0)) (local.get $res))))
+)
+(@else
             (try
                (do
                   (call $close (local.get $fd)))
                (catch $javascript_exception
                   ;; ignore exception
-                  (drop (pop externref))))))
+                  (drop (pop externref))))
+))
+      ))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $read
+      (param $fd i32) (param $buf (ref extern)) (param $pos i32) (param $n i32)
+      (result i32)
+      (local $buffer i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nread i32)
+      (local $s (ref $bytes)) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $nread (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $n))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $res
+         (call $fd_read
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nread)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (local.set $n (i32.load (local.get $nread)))
+      (local.set $s
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf))))
+      (call $blit_memory_to_substring
+         (local.get $buffer) (local.get $s) (local.get $pos) (local.get $n))
+      (local.get $n))
+))
+
    (func $caml_do_read
       (param $ch (ref $channel)) (param $pos i32) (param $len i32) (result i32)
       (local $fd i32)
@@ -348,6 +655,16 @@
       (local $offset i64)
       (local $n i32)
       (local.set $fd (struct.get $channel $fd (local.get $ch)))
+(@if wasi
+(@then
+      (local.set $n
+         (call $read
+            (local.get $fd)
+            (struct.get $channel $buffer (local.get $ch))
+            (local.get $pos)
+            (local.get $len)))
+)
+(@else
       (local.set $fd_offset (call $get_fd_offset (local.get $fd)))
       (local.set $offset (struct.get $fd_offset $offset (local.get $fd_offset)))
       (try
@@ -374,6 +691,7 @@
       (struct.set $fd_offset $offset
          (local.get $fd_offset)
          (i64.add (local.get $offset) (i64.extend_i32_u (local.get $n))))
+))
       (local.get $n))
 
    (func $caml_refill (param $ch (ref $channel)) (result i32)
@@ -446,12 +764,12 @@
          (then
             (if (i32.gt_u (local.get $len) (local.get $avail))
                (then (local.set $len (local.get $avail))))
-            (call $ta_set (local.get $d)
-               (call $ta_subarray (struct.get $channel $buffer (local.get $ch))
-                  (struct.get $channel $curr (local.get $ch))
-                  (i32.add (struct.get $channel $curr (local.get $ch))
-                     (local.get $len)))
-               (local.get $pos))
+            (call $ta_blit_from_buffer
+               (struct.get $channel $buffer (local.get $ch))
+               (struct.get $channel $curr (local.get $ch))
+               (local.get $d)
+               (local.get $pos)
+               (local.get $len))
             (struct.set $channel $curr (local.get $ch)
                (i32.add (struct.get $channel $curr (local.get $ch))
                   (local.get $len)))
@@ -462,10 +780,12 @@
       (struct.set $channel $max (local.get $ch) (local.get $nread))
       (if (i32.gt_u (local.get $len) (local.get $nread))
          (then (local.set $len (local.get $nread))))
-      (call $ta_set (local.get $d)
-         (call $ta_subarray (struct.get $channel $buffer (local.get $ch))
-            (i32.const 0) (local.get $len))
-         (local.get $pos))
+      (call $ta_blit_from_buffer
+         (struct.get $channel $buffer (local.get $ch))
+         (i32.const 0)
+         (local.get $d)
+         (local.get $pos)
+         (local.get $len))
       (struct.set $channel $curr (local.get $ch) (local.get $len))
       (local.get $len))
 
@@ -561,9 +881,7 @@
       (ref.i31
          (i32.sub
             (i32.wrap_i64
-               (struct.get $fd_offset $offset
-                  (call $get_fd_offset
-                     (struct.get $channel $fd (local.get $ch)))))
+               (call $caml_ml_get_channel_offset (local.get $ch)))
             (i32.sub
               (struct.get $channel $max (local.get $ch))
               (struct.get $channel $curr (local.get $ch))))))
@@ -574,9 +892,7 @@
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_copy_int64
          (i64.sub
-            (struct.get $fd_offset $offset
-               (call $get_fd_offset
-                  (struct.get $channel $fd (local.get $ch))))
+            (call $caml_ml_get_channel_offset (local.get $ch))
             (i64.extend_i32_s
                (i32.sub
                   (struct.get $channel $max (local.get $ch))
@@ -588,10 +904,7 @@
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (ref.i31
          (i32.add
-            (i32.wrap_i64
-               (struct.get $fd_offset $offset
-                  (call $get_fd_offset
-                     (struct.get $channel $fd (local.get $ch)))))
+            (i32.wrap_i64 (call $caml_ml_get_channel_offset (local.get $ch)))
             (struct.get $channel $curr (local.get $ch)))))
 
    (func (export "caml_ml_pos_out_64")
@@ -600,11 +913,40 @@
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_copy_int64
          (i64.add
-            (struct.get $fd_offset $offset
-               (call $get_fd_offset
-                  (struct.get $channel $fd (local.get $ch))))
+            (call $caml_ml_get_channel_offset (local.get $ch))
             (i64.extend_i32_s (struct.get $channel $curr (local.get $ch))))))
 
+(;ZZZ
+   (func $check_valid_offset
+      (param $fd i32) (param $offset i64) (param $prev_offset i64)
+      (if (i32.or (i32.lt_s (local.get $offset (i64.const 0)))
+             (i32.gt_s (local.get $offset)
+                (call $file_size (local.get $fd)))))
+         (then
+            ;;;
+         )))
+;)
+
+(@if wasi
+(@then
+   (func $caml_seek_in
+      (param $ch (ref $channel)) (param $dest i64) (result (ref eq))
+      (local $fd i32) (local $buffer i32) (local $res i32)
+      (local.set $fd (struct.get $channel $fd (local.get $ch)))
+      (local.set $buffer (call $get_buffer))
+      ;; ZZZ store current offset in channel do avoid some syscalls?
+      (local.set $res
+         (call $fd_seek
+            (local.get $fd) (local.get $dest) (i32.const 0) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (struct.set $channel $curr (local.get $ch) (i32.const 0))
+      (struct.set $channel $max (local.get $ch) (i32.const 0))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func $caml_seek_in
       (param $ch (ref $channel)) (param $dest i64) (result (ref eq))
       (local $fd i32) (local $offset i64)
@@ -636,6 +978,7 @@
             (struct.set $channel $curr (local.get $ch) (i32.const 0))
             (struct.set $channel $max (local.get $ch) (i32.const 0))))
       (ref.i31 (i32.const 0)))
+))
 
    (func (export "caml_ml_seek_in")
       (param $ch (ref eq)) (param $dest (ref eq)) (result (ref eq))
@@ -652,8 +995,25 @@
       (param $vch (ref eq)) (param $voffset (ref eq)) (result (ref eq))
       (local $ch (ref $channel))
       (local $fd_offset (ref $fd_offset))
+      (local $buffer i32) (local $res i32)
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_flush (local.get $ch))
+(@if wasi
+(@then
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (struct.get $channel $fd (local.get $ch))
+            (i64.extend_i32_s
+               (i31.get_s (ref.cast (ref i31) (local.get $voffset))))
+            (i32.const 0)
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+)
+(@else
       ;; ZZZ Check for error
       (local.set $fd_offset
          (call $get_fd_offset (struct.get $channel $fd (local.get $ch))))
@@ -661,20 +1021,38 @@
          (i64.extend_i32_s
             (i31.get_s (ref.cast (ref i31) (local.get $voffset)))))
       (struct.set $fd_offset $seeked (local.get $fd_offset) (i32.const 1))
+))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ml_seek_out_64")
       (param $vch (ref eq)) (param $voffset (ref eq)) (result (ref eq))
       (local $ch (ref $channel))
       (local $fd_offset (ref $fd_offset))
+      (local $buffer i32) (local $res i32)
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
       (call $caml_flush (local.get $ch))
+(@if wasi
+(@then
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (struct.get $channel $fd (local.get $ch))
+            (call $Int64_val (local.get $voffset))
+            (i32.const 0)
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+)
+(@else
       ;; ZZZ Check for error
       (local.set $fd_offset
          (call $get_fd_offset (struct.get $channel $fd (local.get $ch))))
       (struct.set $fd_offset $offset (local.get $fd_offset)
          (call $Int64_val (local.get $voffset)))
       (struct.set $fd_offset $seeked (local.get $fd_offset) (i32.const 1))
+))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ml_input_scan_line")
@@ -751,6 +1129,36 @@
          (then (call $caml_flush (local.get $ch))))
       (ref.i31 (i32.const 0)))
 
+(@if wasi
+(@then
+   (func $write
+      (param $fd i32) (param $buf (ref extern)) (param $pos i32) (param $n i32)
+      (result i32)
+      (local $buffer i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $s (ref $bytes)) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $n))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $s
+         (ref.cast (ref $bytes) (any.convert_extern (local.get $buf))))
+      (call $blit_substring_to_memory
+         (local.get $buffer) (local.get $s) (local.get $pos) (local.get $n))
+      (local.set $res
+         (call $fd_write
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nwritten)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (i32.load (local.get $nwritten)))
+))
+
    (func $caml_flush_partial (param $ch (ref $channel)) (result i32)
       (local $towrite i32) (local $written i32) (local $fd i32)
       (local $fd_offset (ref $fd_offset))
@@ -760,6 +1168,16 @@
          (then
             (local.set $buf (struct.get $channel $buffer (local.get $ch)))
             (local.set $fd (struct.get $channel $fd (local.get $ch)))
+(@if wasi
+(@then
+            (local.set $written
+               (call $write
+                  (local.get $fd)
+                  (local.get $buf)
+                  (i32.const 0)
+                  (local.get $towrite)))
+)
+(@else
             (local.set $fd_offset (call $get_fd_offset (local.get $fd)))
             (local.set $offset
                (struct.get $fd_offset $offset (local.get $fd_offset)))
@@ -789,6 +1207,7 @@
                (i64.add
                   (local.get $offset)
                   (i64.extend_i32_u (local.get $written))))
+))
             (if (i32.gt_u (local.get $towrite) (local.get $written))
                (then
                   (call $ta_copy (local.get $buf)
@@ -829,10 +1248,10 @@
       (if (i32.ge_u (local.get $len) (local.get $free))
          (then (local.set $len (local.get $free))))
       (local.set $buf (struct.get $channel $buffer (local.get $ch)))
-      (call $ta_set (local.get $buf)
-         (call $ta_subarray (local.get $d)
-            (local.get $pos) (i32.add (local.get $pos) (local.get $len)))
-         (local.get $curr))
+      (call $ta_blit_to_buffer
+         (local.get $d) (local.get $pos)
+         (local.get $buf) (local.get $curr)
+         (local.get $len))
       (struct.set $channel $curr (local.get $ch)
          (i32.add (local.get $curr) (local.get $len)))
       (if (i32.ge_u (local.get $len) (local.get $free))
@@ -959,11 +1378,31 @@
       (struct.set $channel $fd
          (ref.cast (ref $channel) (local.get 0)) (local.get 1)))
 
-   (func (export "caml_ml_get_channel_offset") (param $ch (ref eq)) (result i64)
+(@if wasi
+(@then
+   (func $caml_ml_get_channel_offset (export "caml_ml_get_channel_offset")
+      (param $ch (ref eq)) (result i64)
+      (local $fd i32) (local $buffer i32) (local $res i32)
+      (local.set $fd
+         (struct.get $channel $fd (ref.cast (ref $channel) (local.get $ch))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (local.get $fd) (i64.const 0) (i32.const 1) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (i64.load (local.get $buffer)))
+)
+(@else
+   (func $caml_ml_get_channel_offset (export "caml_ml_get_channel_offset")
+      (param $ch (ref eq)) (result i64)
       (struct.get $fd_offset $offset
          (call $get_fd_offset
             (struct.get $channel $fd
                (ref.cast (ref $channel) (local.get $ch))))))
+))
 
    (func (export "caml_ml_output_bigarray")
       (param $ch (ref eq)) (param $a (ref eq)) (param $vpos (ref eq))
diff --git a/runtime/wasm/jslib.wat b/runtime/wasm/jslib.wat
index befb48306b..1d6038c75d 100644
--- a/runtime/wasm/jslib.wat
+++ b/runtime/wasm/jslib.wat
@@ -16,6 +16,8 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if (not wasi)
+(@then
    (import "bindings" "identity" (func $to_float (param anyref) (result f64)))
    (import "bindings" "identity" (func $from_float (param f64) (result anyref)))
    (import "bindings" "identity" (func $to_bool (param anyref) (result i32)))
@@ -75,6 +77,8 @@
       (func $wrap_fun_arguments (param anyref) (result anyref)))
    (import "fail" "caml_failwith_tag"
       (func $caml_failwith_tag (result (ref eq))))
+   (import "fail" "javascript_exception"
+      (tag $javascript_exception (param externref)))
    (import "stdlib" "caml_named_value"
       (func $caml_named_value (param (ref eq)) (result (ref null eq))))
    (import "obj" "caml_callback_1"
@@ -627,12 +631,9 @@
                (br $loop))))
       (local.get $l))
 
-   (global $jsError (ref $bytes)
-      (array.new_fixed $bytes 7 ;; 'jsError'
-         (i32.const 106) (i32.const 115) (i32.const 69) (i32.const 114)
-         (i32.const 114) (i32.const 111) (i32.const 114)))
+   (@string $jsError "jsError")
 
-   (data $toString "toString")
+   (@string $toString "toString")
 
    (func (export "caml_wrap_exception") (param externref) (result (ref eq))
       (local $exn anyref)
@@ -651,9 +652,7 @@
                (call $meth_call
                   (local.get $exn)
                   (call $unwrap
-                     (call $caml_jsstring_of_bytes
-                        (array.new_data $bytes $toString
-                           (i32.const 0) (i32.const 8))))
+                     (call $caml_jsstring_of_bytes (global.get $toString)))
                   (any.convert_extern (call $new_array (i32.const 0))))))))
 
    (func (export "caml_js_error_option_of_exception")
@@ -671,6 +670,11 @@
                         (array.get $block (local.get $exn) (i32.const 2))))))))
       (ref.i31 (i32.const 0)))
 
+   (func (export "caml_throw_js_exception")
+      (param $exn (ref eq)) (result (ref eq))
+      (throw $javascript_exception
+         (extern.convert_any (call $unwrap (local.get $exn)))))
+
    (func (export "caml_js_error_of_exception")
       (param (ref eq)) (result (ref eq))
       (local $exn (ref $block))
@@ -684,4 +688,5 @@
                   (return
                      (array.get $block (local.get $exn) (i32.const 2)))))))
       (call $wrap (ref.null any)))
+))
 )
diff --git a/runtime/wasm/jslib_js_of_ocaml.wat b/runtime/wasm/jslib_js_of_ocaml.wat
index ee6a58ece1..ec69833df0 100644
--- a/runtime/wasm/jslib_js_of_ocaml.wat
+++ b/runtime/wasm/jslib_js_of_ocaml.wat
@@ -16,6 +16,8 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if (not wasi)
+(@then
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "caml_js_global"
@@ -27,34 +29,34 @@
    (import "jslib" "caml_js_from_array"
       (func $caml_js_from_array (param (ref eq)) (result (ref eq))))
    (import "js" "caml_js_html_escape"
-      (func $caml_js_html_escape (param anyref) (result anyref)))
+      (func $caml_js_html_escape_js (param anyref) (result anyref)))
    (import "js" "caml_js_html_entities"
-      (func $caml_js_html_entities (param anyref) (result anyref)))
+      (func $caml_js_html_entities_js (param anyref) (result anyref)))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
 
    (func (export "caml_js_html_escape") (param (ref eq)) (result (ref eq))
       (return_call $wrap
-         (call $caml_js_html_escape (call $unwrap (local.get 0)))))
+         (call $caml_js_html_escape_js (call $unwrap (local.get 0)))))
 
    (func (export "caml_js_html_entities") (param (ref eq)) (result (ref eq))
       (return_call $wrap
-         (call $caml_js_html_entities (call $unwrap (local.get 0)))))
+         (call $caml_js_html_entities_js (call $unwrap (local.get 0)))))
 
-   (data $console "console")
+   (@string $console "console")
 
    (func (export "caml_js_get_console") (param (ref eq)) (result (ref eq))
       (return_call $caml_js_get (call $caml_js_global (ref.i31 (i32.const 0)))
-         (array.new_data $bytes $console (i32.const 0) (i32.const 7))))
+         (global.get $console)))
 
-   (data $XMLHttpRequest "XMLHttpRequest")
+   (@string $XMLHttpRequest "XMLHttpRequest")
 
    (func (export "caml_xmlhttprequest_create") (param (ref eq)) (result (ref eq))
       (return_call $caml_js_new
          (call $caml_js_get
             (call $caml_js_global (ref.i31 (i32.const 0)))
-            (array.new_data $bytes $XMLHttpRequest
-               (i32.const 0) (i32.const 14)))
+            (global.get $XMLHttpRequest))
          (array.new_fixed $block 1 (ref.i31 (i32.const 0)))))
+))
 )
diff --git a/runtime/wasm/jsstring.wat b/runtime/wasm/jsstring.wat
index de0780d990..c769ea514a 100644
--- a/runtime/wasm/jsstring.wat
+++ b/runtime/wasm/jsstring.wat
@@ -16,6 +16,8 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if (not wasi)
+(@then
    (import "wasm:js-string" "compare"
       (func $compare_strings (param externref externref) (result i32)))
    (import "wasm:js-string" "test"
@@ -257,4 +259,5 @@
       (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
       (call $read_from_buffer (local.get $s) (i32.const 0) (local.get $len))
       (global.set $stack (struct.new $stack (local.get $s) (global.get $stack))))
+))
 )
diff --git a/runtime/wasm/lexing.wat b/runtime/wasm/lexing.wat
index 5ff59fdd0b..5016d8a379 100644
--- a/runtime/wasm/lexing.wat
+++ b/runtime/wasm/lexing.wat
@@ -52,7 +52,7 @@
    (global $lex_check_code i32 (i32.const 10))
    (global $lex_code i32 (i32.const 11))
 
-   (data $lexing_empty_token "lexing: empty token")
+   (@string $lexing_empty_token "lexing: empty token")
 
    (func (export "caml_lex_engine")
       (param $vtbl (ref eq)) (param $start_state (ref eq))
@@ -172,10 +172,7 @@
                   (array.get $block (local.get $lexbuf)
                      (global.get $lex_last_action)))
                (if (ref.eq (local.get $action) (ref.i31 (i32.const -1)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $bytes $lexing_empty_token
-                           (i32.const 0) (i32.const 19)))))
+                  (then (call $caml_failwith (global.get $lexing_empty_token))))
                (return (local.get $action))))
          (if (i32.eq (local.get $c) (i32.const 256))
             (then
@@ -363,10 +360,7 @@
                   (array.get $block (local.get $lexbuf)
                      (global.get $lex_last_action)))
                (if (ref.eq (local.get $action) (ref.i31 (i32.const -1)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $bytes $lexing_empty_token
-                           (i32.const 0) (i32.const 19)))))
+                  (then (call $caml_failwith (global.get $lexing_empty_token))))
                (return (local.get $action))))
          (local.set $base_code
             (call $get (local.get $lex_base_code) (local.get $pstate)))
diff --git a/runtime/wasm/libc.c b/runtime/wasm/libc.c
new file mode 100644
index 0000000000..3b0c44bd04
--- /dev/null
+++ b/runtime/wasm/libc.c
@@ -0,0 +1,175 @@
+/*
+Primitives implemented by the WASI libc. Use 'dune build @recompile-libc'
+to update libc.wasm.
+
+clang -O2  --target=wasm32-wasi --sysroot=/path/to/wasi-libc/sysroot -nodefaultlibs -lc libc.c -o libc.wasm
+*/
+
+#include <stdio.h>
+#include <math.h>
+#include <stdlib.h>
+#include <time.h>
+#include <string.h>
+
+__attribute__((export_name("cos")))
+double libc_cos (double x) {
+  return cos(x);
+}
+
+__attribute__((export_name("sin")))
+double libc_sin (double x) {
+  return sin(x);
+}
+
+__attribute__((export_name("tan")))
+double libc_tan (double x) {
+  return tan(x);
+}
+
+__attribute__((export_name("acos")))
+double libc_acos (double x) {
+  return acos(x);
+}
+
+__attribute__((export_name("asin")))
+double libc_asin (double x) {
+  return asin(x);
+}
+
+__attribute__((export_name("atan")))
+double libc_atan (double x) {
+  return atan(x);
+}
+
+__attribute__((export_name("cosh")))
+double libc_cosh (double x) {
+  return cosh(x);
+}
+
+__attribute__((export_name("sinh")))
+double libc_sinh (double x) {
+  return sinh(x);
+}
+
+__attribute__((export_name("tanh")))
+double libc_tanh (double x) {
+  return tanh(x);
+}
+
+__attribute__((export_name("acosh")))
+double libc_acosh (double x) {
+  return acosh(x);
+}
+
+__attribute__((export_name("asinh")))
+double libc_asinh (double x) {
+  return asinh(x);
+}
+
+__attribute__((export_name("atanh")))
+double libc_atanh (double x) {
+  return atanh(x);
+}
+
+__attribute__((export_name("cbrt")))
+double libc_cbrt (double x) {
+  return cbrt(x);
+}
+
+__attribute__((export_name("exp")))
+double libc_exp (double x) {
+  return exp(x);
+}
+
+__attribute__((export_name("expm1")))
+double libc_expm1 (double x) {
+  return expm1(x);
+}
+
+__attribute__((export_name("log")))
+double libc_log (double x) {
+  return log(x);
+}
+
+__attribute__((export_name("log1p")))
+double libc_log1p (double x) {
+  return log1p(x);
+}
+
+__attribute__((export_name("log2")))
+double libc_log2 (double x) {
+  return log2(x);
+}
+
+__attribute__((export_name("log10")))
+double libc_log10 (double x) {
+  return log10(x);
+}
+
+__attribute__((export_name("atan2")))
+double libc_atan2 (double x, double y) {
+  return atan2(x, y);
+}
+
+__attribute__((export_name("hypot")))
+double libc_hypot (double x, double y) {
+  return hypot(x, y);
+}
+
+__attribute__((export_name("pow")))
+double libc_pow (double x, double y) {
+  return pow(x, y);
+}
+
+__attribute__((export_name("fmod")))
+double libc_fmod (double x, double y) {
+  return fmod(x, y);
+}
+
+__attribute__((export_name("strtod")))
+double libc_strtod (const char * buf, char ** end) {
+  return strtod(buf, end);
+}
+
+__attribute__((export_name("format_float")))
+int format_float (char * buf, size_t len, const char * fmt, double f) {
+  return snprintf(buf, len, fmt, f);
+}
+
+__attribute__((export_name("malloc")))
+void * libc_malloc (size_t len) {
+  return malloc(len);
+}
+
+__attribute__((export_name("free")))
+void libc_free (void * ptr) {
+  return free(ptr);
+}
+
+__attribute__((export_name("strlen")))
+size_t libc_strlen (const char * s) {
+  return strlen(s);
+}
+
+
+__attribute__((export_name("gmtime")))
+struct tm * libc_gmtime (const time_t * timep) {
+  return gmtime(timep);
+}
+
+__attribute__((export_name("localtime")))
+struct tm * libc_localtime (const time_t * timep) {
+  return localtime(timep);
+}
+
+__attribute__((export_name("mktime")))
+time_t libc_mktime(struct tm *tm) {
+  return mktime(tm);
+}
+
+__attribute__((import_module("OCaml"), import_name("_initialize")))
+void start(void);
+
+int main () {
+  start();
+}
diff --git a/runtime/wasm/libc.wasm b/runtime/wasm/libc.wasm
new file mode 100644
index 0000000000..5e3f34061d
Binary files /dev/null and b/runtime/wasm/libc.wasm differ
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 9b68a38eb7..eae45781cc 100644
--- a/runtime/wasm/marshal.wat
+++ b/runtime/wasm/marshal.wat
@@ -30,11 +30,6 @@
       (func $caml_is_closure (param (ref eq)) (result i32)))
    (import "effect" "caml_is_continuation"
       (func $caml_is_continuation (param (ref eq)) (result i32)))
-   (import "bindings" "map_new" (func $map_new (result (ref any))))
-   (import "bindings" "map_get"
-      (func $map_get (param (ref any)) (param (ref eq)) (result i31ref)))
-   (import "bindings" "map_set"
-      (func $map_set (param (ref any)) (param (ref eq)) (param (ref i31))))
    (import "io" "caml_really_putblock"
       (func $caml_really_putblock
          (param (ref eq)) (param (ref $bytes)) (param i32) (param i32)))
@@ -49,17 +44,78 @@
    (import "custom" "caml_find_custom_operations"
       (func $caml_find_custom_operations
          (param (ref $bytes)) (result (ref null $custom_operations))))
-   (import "version-dependent" "caml_marshal_header_size"
-      (global $caml_marshal_header_size i32))
-
-   (global $input_val_from_string (ref $bytes)
-      (array.new_fixed $bytes 21
-         (i32.const 105) (i32.const 110) (i32.const 112) (i32.const 117)
-         (i32.const 116) (i32.const 95) (i32.const 118) (i32.const 97)
-         (i32.const 108) (i32.const 95) (i32.const 102) (i32.const 114)
-         (i32.const 111) (i32.const 109) (i32.const 95) (i32.const 115)
-         (i32.const 116) (i32.const 114) (i32.const 105) (i32.const 110)
-         (i32.const 103)))
+   (type $block (array (mut (ref eq))))
+(@if wasi
+(@then
+   (type $map
+      (struct
+         (field $size (mut i32))
+         (field $keys (mut (ref $block)))
+         (field $values (mut (ref $block)))))
+   (func $map_new (result (ref any))
+      (struct.new $map
+         (i32.const 0)
+         (array.new $block (ref.i31 (i32.const 0)) (i32.const 2))
+         (array.new $block (ref.i31 (i32.const 0)) (i32.const 2))))
+   (func $map_get (param $map (ref any)) (param $k (ref eq)) (result i31ref)
+      (local $m (ref $map)) (local $keys (ref $block))
+      (local $i i32) (local $size i32)
+      (local.set $m (ref.cast (ref $map) (local.get $map)))
+      (local.set $size (struct.get $map $size (local.get $m)))
+      (local.set $keys (struct.get $map $keys (local.get $m)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $size))
+            (then
+               (if (ref.eq (array.get $block (local.get $keys) (local.get $i))
+                      (local.get $k))
+                  (then
+                     (return
+                        (ref.cast (ref i31)
+                           (array.get $block
+                              (struct.get $map $values (local.get $m))
+                              (local.get $i))))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (ref.null i31))
+   (func $map_set (param $map (ref any)) (param $k (ref eq)) (param $v (ref i31))
+      (local $m (ref $map)) (local $i i32) (local $size i32)
+      (local $keys (ref $block)) (local $a (ref $block))
+      (local.set $m (ref.cast (ref $map) (local.get $map)))
+      (local.set $i (struct.get $map $size (local.get $m)))
+      (local.set $keys (struct.get $map $keys (local.get $m)))
+      (if (i32.eq (local.get $i) (array.len (local.get $keys)))
+         (then
+            (local.set $size (i32.shl (local.get $i) (i32.const 1)))
+            (local.set $a
+               (array.new $block (ref.i31 (i32.const 0)) (local.get $size)))
+            (array.copy $block $block
+               (local.get $a) (i32.const 0)
+               (local.get $keys) (i32.const 0)
+               (local.get $i))
+            (struct.set $map $keys (local.get $m) (local.get $a))
+            (local.set $keys (local.get $a))
+            (local.set $a
+               (array.new $block (ref.i31 (i32.const 0)) (local.get $size)))
+            (array.copy $block $block
+               (local.get $a) (i32.const 0)
+               (struct.get $map $values (local.get $m)) (i32.const 0)
+               (local.get $i))
+            (struct.set $map $values (local.get $m) (local.get $a))))
+      (array.set $block (local.get $keys) (local.get $i) (local.get $k))
+      (array.set $block (struct.get $map $values (local.get $m))
+         (local.get $i) (local.get $v))
+      (struct.set $map $size (local.get $m)
+         (i32.add (local.get $i) (i32.const 1))))
+)
+(@else
+   (import "bindings" "map_new" (func $map_new (result (ref any))))
+   (import "bindings" "map_get"
+      (func $map_get (param (ref any)) (param (ref eq)) (result i31ref)))
+   (import "bindings" "map_set"
+      (func $map_set (param (ref any)) (param (ref eq)) (param (ref i31))))
+))
+
+   (@string $input_val_from_string "input_value_from_string")
 
    (export "caml_input_value_from_string" (func $caml_input_value_from_bytes))
    (func $caml_input_value_from_bytes (export "caml_input_value_from_bytes")
@@ -83,13 +139,9 @@
             (call $bad_length (global.get $input_val_from_string))))
       (return_call $intern_rec (local.get $s) (local.get $h)))
 
-   (data $truncated_obj "input_value: truncated object")
+   (@string $truncated_obj "input_value: truncated object")
 
-   (global $input_value (ref $bytes)
-      (array.new_fixed $bytes 11
-         (i32.const 105) (i32.const 110) (i32.const 112) (i32.const 117)
-         (i32.const 116) (i32.const 95) (i32.const 118) (i32.const 97)
-         (i32.const 108) (i32.const 117) (i32.const 101)))
+   (@string $input_value "input_value")
 
    (func (export "caml_input_value") (param $ch (ref eq)) (result (ref eq))
       ;; ZZZ check binary channel?
@@ -103,10 +155,7 @@
       (if (i32.eqz (local.get $r))
          (then (call $caml_raise_end_of_file)))
       (if (i32.lt_u (local.get $r) (i32.const 20))
-         (then
-            (call $caml_failwith
-               (array.new_data $bytes $truncated_obj
-                  (i32.const 0) (i32.const 29)))))
+         (then (call $caml_failwith (global.get $truncated_obj))))
       (local.set $s
          (call $get_intern_state (local.get $header) (i32.const 0)))
       (local.set $h
@@ -117,14 +166,10 @@
              (call $caml_really_getblock (local.get $ch)
                 (local.get $buf) (i32.const 0) (local.get $len))
              (local.get $len))
-         (then
-            (call $caml_failwith
-               (array.new_data $bytes $truncated_obj
-                  (i32.const 0) (i32.const 29)))))
+         (then (call $caml_failwith (global.get $truncated_obj))))
       (local.set $s (call $get_intern_state (local.get $buf) (i32.const 0)))
       (return_call $intern_rec (local.get $s) (local.get $h)))
 
-   (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
@@ -395,13 +440,13 @@
          (field $pos (mut i32))
          (field $next (ref null $stack_item))))
 
-   (data $integer_too_large "input_value: integer too large")
-   (data $code_pointer "input_value: code pointer")
-   (data $ill_formed "input_value: ill-formed message")
+   (@string $integer_too_large "input_value: integer too large")
+   (@string $code_pointer "input_value: code pointer")
+   (@string $ill_formed "input_value: ill-formed message")
 
-   (data $unknown_custom "input_value: unknown custom block identifier")
-   (data $expected_size "input_value: expected a fixed-size custom block")
-   (data $incorrect_size
+   (@string $unknown_custom "input_value: unknown custom block identifier")
+   (@string $expected_size "input_value: expected a fixed-size custom block")
+   (@string $incorrect_size
       "input_value: incorrect length of serialized custom block")
 
    (func $intern_custom
@@ -438,17 +483,10 @@
                   (i32.ne (tuple.extract 2 1 (local.get $r))
                      (local.get $expected_size))
                   (i32.ne (local.get $code) (global.get $CODE_CUSTOM)))
-               (then
-                  (call $caml_failwith
-                     (array.new_data $bytes $incorrect_size
-                        (i32.const 0) (i32.const 56)))))
+               (then (call $caml_failwith (global.get $incorrect_size))))
             (return (tuple.extract 2 0 (local.get $r))))
-         (call $caml_failwith
-            (array.new_data $bytes $expected_size
-               (i32.const 0) (i32.const 47))))
-      (call $caml_failwith
-         (array.new_data $bytes $unknown_custom
-            (i32.const 0) (i32.const 44)))
+         (call $caml_failwith (global.get $expected_size)))
+      (call $caml_failwith (global.get $unknown_custom))
       (ref.i31 (i32.const 0)))
 
    (func $intern_rec
@@ -544,8 +582,7 @@
                                             (local.get $code)))
                                         ;; default
                                         (call $caml_failwith
-                                           (array.new_data $bytes $ill_formed
-                                              (i32.const 0) (i32.const 31)))
+                                           (global.get $ill_formed))
                                         (br $done))
                                        ;; CUSTOM
                                        (local.set $v
@@ -556,8 +593,7 @@
                                        (br $done))
                                       ;; CODEPOINTER
                                       (call $caml_failwith
-                                        (array.new_data $bytes $code_pointer
-                                           (i32.const 0) (i32.const 25)))
+                                        (global.get $code_pointer))
                                       (br $done))
                                      ;; DOUBLE_ARRAY32
                                      (local.set $len
@@ -601,8 +637,7 @@
                              (br $read_shared))
                             ;; INT64
                             (call $caml_failwith
-                               (array.new_data $bytes $integer_too_large
-                                  (i32.const 0) (i32.const 30)))
+                               (global.get $integer_too_large))
                             (br $done))
                            ;; INT32
                            (local.set $v (ref.i31 (call $read32 (local.get $s))))
@@ -674,26 +709,23 @@
         (br $loop)))
       (array.get $block (local.get $res) (i32.const 0)))
 
-   (data $too_large ": object too large to be read back on a 32-bit platform")
+   (@string $too_large ": object too large to be read back on a 32-bit platform")
 
-   (func $too_large (param $prim (ref $bytes))
+   (func $too_large (param $prim (ref eq))
       (call $caml_failwith
-         (call $caml_string_concat (local.get $prim)
-            (array.new_data $bytes $too_large (i32.const 0) (i32.const 55)))))
+         (call $caml_string_concat (local.get $prim) (global.get $too_large))))
 
-   (data $bad_object ": bad object")
+   (@string $bad_object ": bad object")
 
-   (func $bad_object (param $prim (ref $bytes))
+   (func $bad_object (param $prim (ref eq))
       (call $caml_failwith
-         (call $caml_string_concat (local.get $prim)
-            (array.new_data $bytes $bad_object (i32.const 0) (i32.const 12)))))
+         (call $caml_string_concat (local.get $prim) (global.get $bad_object))))
 
-   (data $bad_length ": bad length")
+   (@string $bad_length ": bad length")
 
-   (func $bad_length (param $prim (ref $bytes))
+   (func $bad_length (param $prim (ref eq))
       (call $caml_failwith
-         (call $caml_string_concat (local.get $prim)
-            (array.new_data $bytes $bad_length (i32.const 0) (i32.const 12)))))
+         (call $caml_string_concat (local.get $prim) (global.get $bad_length))))
 
    (type $marshal_header
       (struct
@@ -701,7 +733,7 @@
          (field $num_objects i32)))
 
    (func $parse_header
-      (param $s (ref $intern_state)) (param $prim (ref $bytes))
+      (param $s (ref $intern_state)) (param $prim (ref eq))
       (result (ref $marshal_header))
       (local $magic i32)
       (local $data_len i32) (local $num_objects i32) (local $whsize i32)
@@ -720,7 +752,17 @@
          (local.get $data_len)
          (local.get $num_objects)))
 
-   (data $marshal_data_size "Marshal.data_size")
+   (@string $marshal_data_size "Marshal.data_size")
+
+(@if (>= ocaml_version (5 1 0))
+(@then
+   (global $caml_marshal_header_size (export "caml_marshal_header_size") i32
+      (i32.const 16))
+)
+(@else
+   (global $caml_marshal_header_size (export "caml_marshal_header_size") i32
+      (i32.const 20))
+))
 
    (func (export "caml_marshal_data_size")
       (param $buf (ref eq)) (param $ofs (ref eq)) (result (ref eq))
@@ -732,15 +774,9 @@
             (i31.get_u (ref.cast (ref i31) (local.get $ofs)))))
       (local.set $magic (call $read32 (local.get $s)))
       (if (i32.eq (local.get $magic) (global.get $Intext_magic_number_big))
-         (then
-            (call $too_large
-               (array.new_data $bytes $marshal_data_size
-                  (i32.const 0) (i32.const 17)))))
+         (then (call $too_large (global.get $marshal_data_size))))
       (if (i32.ne (local.get $magic) (global.get $Intext_magic_number_small))
-         (then
-            (call $bad_object
-               (array.new_data $bytes $marshal_data_size
-                  (i32.const 0) (i32.const 17)))))
+         (then (call $bad_object (global.get $marshal_data_size))))
       (ref.i31
          (i32.add
             (i32.sub (i32.const 20)
@@ -799,7 +835,7 @@
          (local.get $output)
          (local.get $output)))
 
-   (data $buffer_overflow "Marshal.to_buffer: buffer overflow")
+   (@string $buffer_overflow "Marshal.to_buffer: buffer overflow")
 
    (global $SIZE_EXTERN_OUTPUT_BLOCK i32 (i32.const 8100))
 
@@ -816,10 +852,7 @@
                (i32.add (local.get $pos) (local.get $required)))
             (return (local.get $pos))))
       (if (struct.get $extern_state $user_provided_output (local.get $s))
-         (then
-            (call $caml_failwith
-               (array.new_data $bytes $buffer_overflow
-                  (i32.const 0) (i32.const 34)))))
+         (then (call $caml_failwith (global.get $buffer_overflow))))
       (local.set $last (struct.get $extern_state $output_last (local.get $s)))
       (struct.set $output_block $end (local.get $last)
          (struct.get $extern_state $pos (local.get $s)))
@@ -1073,7 +1106,7 @@
                (global.get $CODE_DOUBLE_ARRAY32_LITTLE) (local.get $nfloats))))
       (call $writefloats (local.get $s) (local.get $v)))
 
-   (data $incorrect_sizes "output_value: incorrect fixed sizes specified by ")
+   (@string $incorrect_sizes "output_value: incorrect fixed sizes specified by ")
 
    (func $extern_custom
       (param $s (ref $extern_state)) (param $v (ref $custom)) (result i32 i32)
@@ -1109,8 +1142,7 @@
                 (then
                    (call $caml_failwith
                       (call $caml_string_concat
-                         (array.new_data $bytes $incorrect_sizes
-                            (i32.const 0) (i32.const 49))
+                         (global.get $incorrect_sizes)
                          (struct.get $custom_operations $id
                             (local.get $ops))))))
             (return (local.get $r)))
@@ -1130,15 +1162,14 @@
          (call $store32 (local.get $buf) (i32.add (local.get $pos) (i32.const 8))
             (tuple.extract 2 1 (local.get $r)))
          (return (local.get $r)))
-      (call $caml_invalid_argument
-         (array.new_data $bytes $cust_value (i32.const 0) (i32.const 37)))
+      (call $caml_invalid_argument (global.get $cust_value))
       (return (tuple.make 2 (i32.const 0) (i32.const 0))))
 
-   (data $func_value "output_value: functional value")
-   (data $cont_value "output_value: continuation value")
-   (data $js_value "output_value: abstract value (JavaScript value)")
-   (data $abstract_value "output_value: abstract value")
-   (data $cust_value "output_value: abstract value (Custom)")
+   (@string $func_value "output_value: functional value")
+   (@string $cont_value "output_value: continuation value")
+   (@string $js_value "output_value: abstract value (JavaScript value)")
+   (@string $abstract_value "output_value: abstract value")
+   (@string $cust_value "output_value: abstract value (Custom)")
 
    (func $extern_rec (param $s (ref $extern_state)) (param $v (ref eq))
       (local $sp (ref null $stack_item))
@@ -1246,24 +1277,12 @@
                         (i32.const 3)))
                (br $next_item)))
             (if (call $caml_is_closure (local.get $v))
-               (then
-                  (call $caml_invalid_argument
-                     (array.new_data $bytes $func_value
-                        (i32.const 0) (i32.const 30)))))
+               (then (call $caml_invalid_argument (global.get $func_value))))
             (if (call $caml_is_continuation (local.get $v))
-               (then
-                  (call $caml_invalid_argument
-                     (array.new_data $bytes $cont_value
-                        (i32.const 0) (i32.const 32)))))
+               (then (call $caml_invalid_argument (global.get $cont_value))))
             (if (ref.test (ref $js) (local.get $v))
-               (then
-                  (call $caml_invalid_argument
-                     (array.new_data $bytes $js_value
-                        (i32.const 0) (i32.const 47)))))
-            (call $caml_invalid_argument
-               (array.new_data $bytes $abstract_value
-                  (i32.const 0) (i32.const 28)))
-         )
+               (then (call $caml_invalid_argument (global.get $js_value))))
+            (call $caml_invalid_argument (global.get $abstract_value)))
          ;; next_item
          (block $done
             (local.set $item (br_on_null $done (local.get $sp)))
diff --git a/runtime/wasm/obj.wat b/runtime/wasm/obj.wat
index e735b4f452..8e44ecd376 100644
--- a/runtime/wasm/obj.wat
+++ b/runtime/wasm/obj.wat
@@ -341,21 +341,18 @@
          (local.get $v))
       (ref.i31 (i32.const 0)))
 
-   (data $not_implemented "Obj.add_offset is not supported")
+   (@string $not_implemented "Obj.add_offset is not supported")
 
    (func (export "caml_obj_add_offset")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (call $caml_failwith
-         (array.new_data $bytes $not_implemented (i32.const 0) (i32.const 31)))
+      (call $caml_failwith (global.get $not_implemented))
       (ref.i31 (i32.const 0)))
 
-   (data $truncate_not_implemented "Obj.truncate is not supported")
+   (@string $truncate_not_implemented "Obj.truncate is not supported")
 
    (func (export "caml_obj_truncate")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (call $caml_failwith
-         (array.new_data $bytes $truncate_not_implemented
-            (i32.const 0) (i32.const 29)))
+      (call $caml_failwith (global.get $truncate_not_implemented))
       (ref.i31 (i32.const 0)))
 
    (global $method_cache (mut (ref $int_array))
diff --git a/runtime/wasm/parsing.wat b/runtime/wasm/parsing.wat
index 8567734727..686f411145 100644
--- a/runtime/wasm/parsing.wat
+++ b/runtime/wasm/parsing.wat
@@ -111,7 +111,8 @@
                (br $loop))))
       (i32.sub (local.get $i) (local.get $p)))
 
-   (data $unknown_token "<unknown token>")
+   (@string $unknown_token "<unknown token>")
+
    (func $token_name
       (param $vnames (ref eq)) (param $number i32) (result (ref eq))
       (local $names (ref $bytes)) (local $i i32) (local $len i32)
@@ -119,10 +120,7 @@
       (local.set $names (ref.cast (ref $bytes) (local.get $vnames)))
       (loop $loop
          (if (i32.eqz (array.get_u $bytes (local.get $names) (local.get $i)))
-            (then
-               (return
-                  (array.new_data $bytes $unknown_token
-                     (i32.const 0) (i32.const 15)))))
+            (then (return (global.get $unknown_token))))
          (if (i32.ne (local.get $number) (i32.const 0))
             (then
                (local.set $i
@@ -149,18 +147,16 @@
    (func $output_nl
       (drop
          (call $caml_ml_output (global.get $caml_stderr)
-            (array.new_fixed $bytes 1 (i32.const 10))
+            (@string "\n")
             (ref.i31 (i32.const 0)) (ref.i31 (i32.const 1))))
       (drop (call $caml_ml_flush (global.get $caml_stderr))))
 
    (func $output_int (param i32)
       (call $output
-         (call $caml_format_int
-            (array.new_fixed $bytes 2 (i32.const 37) (i32.const 100))
-            (ref.i31 (local.get 0)))))
+         (call $caml_format_int (@string "%d") (ref.i31 (local.get 0)))))
 
-   (data $State "State ")
-   (data $read_token ": read token ")
+   (@string $State "State ")
+   (@string $read_token ": read token ")
 
    (func $print_token
       (param $tables (ref $block)) (param $state i32) (param $tok (ref eq))
@@ -168,11 +164,9 @@
       (local $v (ref eq))
       (if (ref.test (ref i31) (local.get $tok))
          (then
-            (call $output
-               (array.new_data $bytes $State (i32.const 0) (i32.const 6)))
+            (call $output (global.get $State))
             (call $output_int (local.get $state))
-            (call $output
-               (array.new_data $bytes $read_token (i32.const 0) (i32.const 13)))
+            (call $output (global.get $read_token))
             (call $output
                (call $token_name
                   (array.get $block (local.get $tables)
@@ -180,11 +174,9 @@
                   (i31.get_u (ref.cast (ref i31) (local.get $tok)))))
             (call $output_nl))
          (else
-            (call $output
-               (array.new_data $bytes $State (i32.const 0) (i32.const 6)))
+            (call $output (global.get $State))
             (call $output_int (local.get $state))
-            (call $output
-               (array.new_data $bytes $read_token (i32.const 0) (i32.const 13)))
+            (call $output (global.get $read_token))
             (local.set $b (ref.cast (ref $block) (local.get $tok)))
             (call $output
                (call $token_name
@@ -193,7 +185,7 @@
                   (i31.get_u
                      (ref.cast (ref i31)
                         (array.get $block (local.get $b) (i32.const 0))))))
-            (call $output (array.new_fixed $bytes 1 (i32.const 40))) ;; "("
+            (call $output (@string "("))
             (local.set $v (array.get $block (local.get $b) (i32.const 1)))
             (if (ref.test (ref i31) (local.get $v))
                (then
@@ -204,22 +196,18 @@
             (else (if (ref.test (ref $float) (local.get $v))
                (then
                   (call $output
-                     (call $caml_format_float
-                        (array.new_fixed $bytes 2
-                           (i32.const 37) (i32.const 103))
-                        (local.get $v))))
+                     (call $caml_format_float (@string "%g") (local.get $v))))
             (else
-               (call $output
-                  (array.new_fixed $bytes 1 (i32.const 95))))))))) ;; '_'
-            (call $output (array.new_fixed $bytes 1 (i32.const 41))) ;; ")"
+               (call $output (@string "_"))))))))
+            (call $output (@string ")"))
             (call $output_nl))))
 
-   (data $recovering_in_state "Recovering in state ")
-   (data $discarding_state "Discarding state ")
-   (data $no_more_states_to_discard "No more states to discard")
-   (data $discarding_last_token_read "Discarding last token read")
-   (data $shift_to_state ": shift to state ")
-   (data $reduce_by_rule ": reduce by rule ")
+   (@string $recovering_in_state "Recovering in state ")
+   (@string $discarding_state "Discarding state ")
+   (@string $no_more_states_to_discard "No more states to discard")
+   (@string $discarding_last_token_read "Discarding last token read")
+   (@string $shift_to_state ": shift to state ")
+   (@string $reduce_by_rule ": reduce by rule ")
 
    (func (export "caml_parse_engine")
       (param $vtables (ref eq)) (param $venv (ref eq)) (param $vcmd (ref eq))
@@ -456,10 +444,8 @@
                                          (if (global.get $caml_parser_trace)
                                             (then
                                                (call $output
-                                                  (array.new_data $bytes
-                                                     $recovering_in_state
-                                                     (i32.const 0)
-                                                     (i32.const 20)))
+                                                  (global.get
+                                                     $recovering_in_state))
                                                (call $output_int
                                                   (local.get $state1))
                                                (call $output_nl)))
@@ -469,8 +455,7 @@
                        (if (global.get $caml_parser_trace)
                           (then
                              (call $output
-                                (array.new_data $bytes $discarding_state
-                                   (i32.const 0) (i32.const 17)))
+                                (global.get $discarding_state))
                              (call $output_int (local.get $state1))
                              (call $output_nl)))
                        (if (i32.le_s (local.get $sp)
@@ -482,9 +467,7 @@
                              (if (global.get $caml_parser_trace)
                                 (then
                                    (call $output
-                                      (array.new_data $bytes
-                                          $no_more_states_to_discard
-                                          (i32.const 0) (i32.const 25)))
+                                      (global.get $no_more_states_to_discard))
                                    (call $output_nl)))
                              (return (ref.i31 (global.get $RAISE_PARSE_ERROR)))))
                        (local.set $sp (i32.sub (local.get $sp) (i32.const 1)))
@@ -499,8 +482,7 @@
                      (if (global.get $caml_parser_trace)
                         (then
                            (call $output
-                              (array.new_data $bytes $discarding_last_token_read
-                                 (i32.const 0) (i32.const 26)))
+                              (global.get $discarding_last_token_read))
                            (call $output_nl)))
                      (array.set $block (local.get $env)
                         (global.get $env_curr_char)
@@ -518,13 +500,9 @@
              ;; shift_recover:
              (if (global.get $caml_parser_trace)
                 (then
-                   (call $output
-                      (array.new_data $bytes $State
-                         (i32.const 0) (i32.const 6)))
+                   (call $output (global.get $State))
                    (call $output_int (local.get $state))
-                   (call $output
-                      (array.new_data $bytes $shift_to_state
-                         (i32.const 0) (i32.const 17)))
+                   (call $output (global.get $shift_to_state))
                    (call $output_int
                       (call $get (local.get $tbl_table) (local.get $n2)))
                    (call $output_nl)))
@@ -568,12 +546,9 @@
            ;; reduce:
            (if (global.get $caml_parser_trace)
               (then
-                 (call $output
-                    (array.new_data $bytes $State (i32.const 0) (i32.const 6)))
+                 (call $output (global.get $State))
                  (call $output_int (local.get $state))
-                 (call $output
-                    (array.new_data $bytes $reduce_by_rule
-                       (i32.const 0) (i32.const 17)))
+                 (call $output (global.get $reduce_by_rule))
                  (call $output_int (local.get $n))
                  (call $output_nl)))
            (local.set $m (call $get (local.get $tbl_len) (local.get $n)))
diff --git a/runtime/wasm/printexc.wat b/runtime/wasm/printexc.wat
index 0f93596955..b30bc45b6a 100644
--- a/runtime/wasm/printexc.wat
+++ b/runtime/wasm/printexc.wat
@@ -117,8 +117,7 @@
                        (then
                            (call $add_string (local.get $buf)
                               (call $caml_format_int
-                                  (array.new_fixed $bytes 2
-                                     (i32.const 37) (i32.const 100)) ;; %d
+                                  (@string "%d")
                                   (ref.cast (ref i31) (local.get $v)))))
                     (else (if (ref.test (ref $bytes) (local.get $v))
                        (then
diff --git a/runtime/wasm/prng.wat b/runtime/wasm/prng.wat
index 4918eaa0bf..08e242056e 100644
--- a/runtime/wasm/prng.wat
+++ b/runtime/wasm/prng.wat
@@ -16,10 +16,20 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+
+(@if wasi
+(@then
+   (import "bigarray" "ta_get_i32"
+      (func $ta_get_i32 (param (ref extern)) (param i32) (result i32)))
+   (import "bigarray" "ta_set_i32"
+      (func $ta_set_i32 (param (ref extern)) (param i32) (param i32)))
+)
+(@else
    (import "bindings" "ta_get_i32"
       (func $ta_get_i32 (param (ref extern)) (param i32) (result i32)))
    (import "bindings" "ta_set_i32"
       (func $ta_set_i32 (param (ref extern)) (param i32) (param i32)))
+))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
 
diff --git a/runtime/wasm/runtime-wasi.js b/runtime/wasm/runtime-wasi.js
new file mode 100644
index 0000000000..b5f4d38d6a
--- /dev/null
+++ b/runtime/wasm/runtime-wasi.js
@@ -0,0 +1,84 @@
+// Wasm_of_ocaml runtime support
+// http://www.ocsigen.org/js_of_ocaml/
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, with linking exception;
+// either version 2.1 of the License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+(js) => async (args) => {
+  // biome-ignore lint/suspicious/noRedundantUseStrict:
+  "use strict";
+
+  const emitWarning = process.emitWarning;
+  process.emitWarning = function (...args) {
+    if (args[1] !== "ExperimentalWarning") emitWarning(...args);
+  };
+
+  const { link, src, generated } = args;
+
+  const { argv, env } = require("node:process");
+  const { WASI } = require("node:wasi");
+  const wasi = new WASI({
+    version: "preview1",
+    args: argv.slice(1),
+    env,
+    preopens: { ".": ".", "/tmp": "/tmp" },
+    returnOnExit: false,
+  });
+  const imports = wasi.getImportObject();
+  function loadRelative(src) {
+    const path = require("node:path");
+    const f = path.join(path.dirname(require.main.filename), src);
+    return require("node:fs/promises").readFile(f);
+  }
+  async function instantiateModule(code) {
+    return WebAssembly.instantiate(await code, imports);
+  }
+  async function instantiateFromDir() {
+    imports.env = {};
+    imports.OCaml = {};
+    const deps = [];
+    async function loadModule(module, isRuntime) {
+      const sync = module[1].constructor !== Array;
+      async function instantiate() {
+        const code = loadRelative(src + "/" + module[0] + ".wasm");
+        await Promise.all(sync ? deps : module[1].map((i) => deps[i]));
+        const wasmModule = await instantiateModule(code);
+        Object.assign(
+          isRuntime ? imports.env : imports.OCaml,
+          wasmModule.instance.exports,
+        );
+      }
+      const promise = instantiate();
+      deps.push(promise);
+      return promise;
+    }
+    async function loadModules(lst) {
+      for (const module of lst) {
+        await loadModule(module);
+      }
+    }
+    await loadModule(link[0], 1);
+    if (link.length > 1) {
+      await loadModule(link[1]);
+      const workers = new Array(20)
+        .fill(link.slice(2).values())
+        .map(loadModules);
+      await Promise.all(workers);
+    }
+    return { instance: { exports: Object.assign(imports.env, imports.OCaml) } };
+  }
+  const wasmModule = await instantiateFromDir();
+
+  wasi.start(wasmModule.instance);
+};
diff --git a/runtime/wasm/runtime.js b/runtime/wasm/runtime.js
index 1de4458c2c..087cc66a1c 100644
--- a/runtime/wasm/runtime.js
+++ b/runtime/wasm/runtime.js
@@ -70,6 +70,10 @@
 
   const fs_cst = fs?.constants;
 
+  const access_flags = fs
+    ? [fs_cst.R_OK, fs_cst.W_OK, fs_cst.X_OK, fs_cst.F_OK]
+    : [];
+
   const open_flags = fs
     ? [
         fs_cst.O_RDONLY,
@@ -374,6 +378,15 @@
       return pad ? " " + s : s;
     },
     gettimeofday: () => new Date().getTime() / 1000,
+    times: () => {
+      if (globalThis.process?.cpuUsage) {
+        var t = globalThis.process.cpuUsage();
+        return caml_alloc_times(t.user / 1e6, t.system / 1e6);
+      } else {
+        var t = performance.now() / 1000;
+        return call_alloc_times(t, t);
+      }
+    },
     gmtime: (t) => {
       var d = new Date(t * 1000);
       var d_num = d.getTime();
@@ -417,6 +430,11 @@
     mktime: (year, month, day, h, m, s) =>
       new Date(year, month, day, h, m, s).getTime(),
     random_seed: () => crypto.getRandomValues(new Int32Array(12)),
+    access: (p, flags) =>
+      fs.accessSync(
+        p,
+        access_flags.reduce((f, v, i) => (flags & (1 << i) ? f | v : f), 0),
+      ),
     open: (p, flags, perm) =>
       fs.openSync(
         p,
@@ -432,6 +450,7 @@
           ),
           l),
     read: (fd, b, o, l, p) => fs.readSync(fd, b, o, l, p),
+    fsync: (fd) => fs.fsyncSync(fd),
     file_size: (fd) => fs.fstatSync(fd, { bigint: true }).size,
     register_channel,
     unregister_channel,
@@ -454,13 +473,25 @@
     chdir: (x) => process.chdir(x),
     mkdir: (p, m) => fs.mkdirSync(p, m),
     rmdir: (p) => fs.rmdirSync(p),
+    link: (d, s) => fs.linkSync(d, s),
+    symlink: (t, p, kind) => fs.symlinkSync(t, p, [null, "file", "dir"][kind]),
+    readlink: (p) => fs.readlinkSync(p),
     unlink: (p) => fs.unlinkSync(p),
-    readdir: (p) => fs.readdirSync(p),
+    read_dir: (p) => fs.readdirSync(p),
+    opendir: (p) => fs.opendirSync(p),
+    readdir: (d) => {
+      var n = d.readSync()?.name;
+      return n === undefined ? null : n;
+    },
+    closedir: (d) => d.closeSync(),
     stat: (p, l) => alloc_stat(fs.statSync(p), l),
     lstat: (p, l) => alloc_stat(fs.lstatSync(p), l),
     fstat: (fd, l) => alloc_stat(fs.fstatSync(fd), l),
+    chmod: (p, perms) => fs.chmodSync(p, perms),
+    fchmod: (p, perms) => fs.fchmodSync(p, perms),
     file_exists: (p) => +fs.existsSync(p),
     is_directory: (p) => +fs.lstatSync(p).isDirectory(),
+    is_file: (p) => +fs.lstatSync(p).isFile(),
     utimes: (p, a, m) => fs.utimesSync(p, a, m),
     truncate: (p, l) => fs.truncateSync(p, l),
     ftruncate: (fd, l) => fs.ftruncateSync(fd, l),
@@ -583,6 +614,7 @@
 
   var {
     caml_callback,
+    caml_alloc_times,
     caml_alloc_tm,
     caml_alloc_stat,
     caml_start_fiber,
diff --git a/runtime/wasm/runtime_events.wat b/runtime/wasm/runtime_events.wat
index be15719f6c..5d207d0820 100644
--- a/runtime/wasm/runtime_events.wat
+++ b/runtime/wasm/runtime_events.wat
@@ -33,6 +33,18 @@
          (local.get $evtag)
          (local.get $evtype)))
 
+(@if (>= ocaml_version (5 2 0))
+(@then
+   (func (export "caml_runtime_events_user_write")
+      (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "caml_runtime_events_user_write")
+      (param (ref eq)) (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+))
+
    (func (export "caml_runtime_events_user_resolve")
       (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
diff --git a/runtime/wasm/stdlib.wat b/runtime/wasm/stdlib.wat
index 3fbaae82a0..3686074745 100644
--- a/runtime/wasm/stdlib.wat
+++ b/runtime/wasm/stdlib.wat
@@ -31,7 +31,6 @@
    (import "obj" "caml_callback_2"
       (func $caml_callback_2
          (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "bindings" "write" (func $write (param i32) (param anyref)))
    (import "string" "caml_string_concat"
       (func $caml_string_concat
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
@@ -39,9 +38,26 @@
       (func $caml_format_exception (param (ref eq)) (result (ref eq))))
    (import "sys" "ocaml_exit" (tag $ocaml_exit))
    (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "proc_exit" (func $exit (param i32)))
+   (import "wasi_snapshot_preview1" "fd_write"
+      (func $fd_write (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "release_memory"
+      (func $release_memory (param i32 i32)))
+   (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
+   (import "libc" "memory" (memory 2))
+)
+(@else
+   (import "bindings" "write" (func $write (param i32) (param anyref)))
+   (import "bindings" "exit" (func $exit (param i32)))
+   (import "bindings" "throw" (func $throw (param externref)))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
-   (import "bindings" "exit" (func $exit (param i32)))
+))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -181,12 +197,14 @@
 
    (type $func (func (result (ref eq))))
 
-   (data $fatal_error "Fatal error: exception ")
-   (data $handle_uncaught_exception "Printexc.handle_uncaught_exception")
-   (data $do_at_exit "Pervasives.do_at_exit")
+   (@string $fatal_error "Fatal error: exception ")
+   (@string $handle_uncaught_exception "Printexc.handle_uncaught_exception")
+   (@string $do_at_exit "Pervasives.do_at_exit")
 
    (global $uncaught_exception (mut externref) (ref.null extern))
 
+(@if (not wasi)
+(@then
    (func $reraise_exception (result (ref eq))
       (throw $javascript_exception (global.get $uncaught_exception))
       (ref.i31 (i32.const 0)))
@@ -194,11 +212,30 @@
    (func (export "caml_handle_uncaught_exception") (param $exn externref)
       (global.set $uncaught_exception (local.get $exn))
       (call $caml_main (ref.func $reraise_exception)))
+))
+
+   (type $wrapper_func (func (param (ref $func))))
+   (global $caml_main_wrapper (export "caml_main_wrapper")
+      (mut (ref null $wrapper_func))
+      (ref.null $wrapper_func))
 
    (func $caml_main (export "caml_main") (param $start (ref func))
       (local $exn (ref eq))
+      (local $msg (ref eq))
+(@if wasi
+(@then
+      (local $buffer i32) (local $i i32) (local $len i32)
+      (local $buf i32) (local $remaining i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $res i32)
+))
       (try
          (do
+            (block $fallback
+               (call_ref $wrapper_func
+                  (ref.cast (ref $func) (local.get $start))
+                  (br_on_null $fallback (global.get $caml_main_wrapper)))
+               (return))
             (drop (call_ref $func (ref.cast (ref $func) (local.get $start)))))
          (catch $ocaml_exit)
          (catch $ocaml_exception
@@ -211,9 +248,7 @@
                            (call $caml_callback_2
                               (br_on_null $not_registered
                                  (call $caml_named_value
-                                     (array.new_data $bytes
-                                        $handle_uncaught_exception
-                                        (i32.const 0) (i32.const 34))))
+                                     (global.get $handle_uncaught_exception)))
                               (local.get $exn)
                               (ref.i31 (i32.const 0)))))
                      (catch $ocaml_exit
@@ -223,19 +258,50 @@
                   (drop
                      (call $caml_callback_1
                         (br_on_null $null
-                           (call $caml_named_value
-                              (array.new_data $bytes $do_at_exit
-                                 (i32.const 0) (i32.const 21))))
+                           (call $caml_named_value (global.get $do_at_exit)))
                         (ref.i31 (i32.const 0)))))
+               (local.set $msg
+                  (call $caml_string_concat
+                     (global.get $fatal_error)
+                     (call $caml_string_concat
+                        (call $caml_format_exception (local.get $exn))
+                        (@string "\n"))))
+(@if wasi
+(@then
+               (local.set $len
+                  (array.len (ref.cast (ref $bytes) (local.get $msg))))
+               (local.set $buffer (call $get_buffer))
+               (local.set $nwritten (local.get $buffer))
+               (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+               (local.set $iovs_len (i32.const 1))
+               (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+               (local.set $buf
+                  (call $write_string_to_memory
+                     (local.get $buf) (global.get $IO_BUFFER_SIZE)
+                     (local.get $msg)))
+               (local.set $remaining (local.get $buf))
+               (loop $write
+                  (i32.store (local.get $iovs) (local.get $remaining))
+                  (i32.store offset=4 (local.get $iovs) (local.get $len))
+                  (local.set $res
+                     (call $fd_write
+                        (i32.const 2) (local.get $iovs) (local.get $iovs_len)
+                        (local.get $nwritten)))
+                  (if (i32.eqz (local.get $res))
+                     (then
+                        (local.set $len
+                           (i32.sub (local.get $len)
+                              (i32.load (local.get $nwritten))))
+                        (local.set $remaining
+                           (i32.add (local.get $remaining)
+                              (i32.load (local.get $nwritten))))
+                        (br_if $write (local.get $len)))))
+               (call $release_memory (local.get $buffer) (local.get $buf))
+)
+(@else
                (call $write (i32.const 2)
                   (call $unwrap
-                     (call $caml_jsstring_of_string
-                        (call $caml_string_concat
-                           (array.new_data $bytes $fatal_error
-                              (i32.const 0) (i32.const 23))
-                           (call $caml_string_concat
-                              (call $caml_format_exception (local.get $exn))
-                              (array.new_fixed $bytes 1
-                                 (i32.const 10)))))))) ;; `\n`
-            (call $exit (i32.const 2)))))
-)
+                     (call $caml_jsstring_of_string (local.get $msg))))
+))
+            )
+            (call $exit (i32.const 2))))))
diff --git a/runtime/wasm/str.wat b/runtime/wasm/str.wat
index 5890d6b2e0..47b92bd5da 100644
--- a/runtime/wasm/str.wat
+++ b/runtime/wasm/str.wat
@@ -507,7 +507,7 @@
       ;; reject
       (ref.i31 (i32.const 0)))
 
-   (data $search_forward "Str.search_forward")
+   (@string $search_forward "Str.search_forward")
 
    (func (export "re_search_forward")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
@@ -520,10 +520,7 @@
       (local.set $pos (i31.get_s (ref.cast (ref i31) (local.get $vpos))))
       (local.set $len (array.len (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $search_forward
-                  (i32.const 0) (i32.const 18)))))
+         (then (call $caml_invalid_argument (global.get $search_forward))))
       (loop $loop
          (local.set $res
             (call $re_match
@@ -535,7 +532,7 @@
          (br_if $loop (i32.le_u (local.get $pos) (local.get $len))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $search_backward "Str.search_backward")
+   (@string $search_backward "Str.search_backward")
 
    (func (export "re_search_backward")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
@@ -549,9 +546,7 @@
       (local.set $len (array.len (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
          (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $search_backward
-                  (i32.const 0) (i32.const 19)))))
+            (call $caml_invalid_argument (global.get $search_backward))))
       (loop $loop
          (local.set $res
             (call $re_match
@@ -563,7 +558,7 @@
          (br_if $loop (i32.ge_s (local.get $pos) (i32.const 0))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $string_match "Str.string_match")
+   (@string $string_match "Str.string_match")
 
    (func (export "re_string_match")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
@@ -576,10 +571,7 @@
       (local.set $pos (i31.get_s (ref.cast (ref i31) (local.get $vpos))))
       (local.set $len (array.len (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $string_match
-                  (i32.const 0) (i32.const 16)))))
+         (then (call $caml_invalid_argument (global.get $string_match))))
       (local.set $res
          (call $re_match
             (local.get $re) (local.get $s) (local.get $pos) (i32.const 0)))
@@ -588,7 +580,7 @@
             (return (local.get $res))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $string_partial_match "Str.string_partial_match")
+   (@string $string_partial_match "Str.string_partial_match")
 
    (func (export "re_partial_match")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
@@ -602,9 +594,7 @@
       (local.set $len (array.len (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
          (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $string_partial_match
-                  (i32.const 0) (i32.const 24)))))
+            (call $caml_invalid_argument (global.get $string_partial_match))))
       (local.set $res
          (call $re_match
             (local.get $re) (local.get $s) (local.get $pos) (i32.const 1)))
@@ -613,8 +603,8 @@
             (return (local.get $res))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $illegal_backslash "Str.replace: illegal backslash sequence")
-   (data $unmatched_group "Str.replace: reference to unmatched group")
+   (@string $illegal_backslash "Str.replace: illegal backslash sequence")
+   (@string $unmatched_group "Str.replace: reference to unmatched group")
 
    (func (export "re_replacement_text")
       (param $vrepl (ref eq)) (param $vgroups (ref eq)) (param $vorig (ref eq))
@@ -640,10 +630,7 @@
                      (local.set $len (i32.add (local.get $len) (i32.const 1)))
                      (br $loop)))
                (if (i32.eq (local.get $i) (local.get $l))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $bytes $illegal_backslash
-                           (i32.const 0) (i32.const 39)))))
+                  (then (call $caml_failwith (global.get $illegal_backslash))))
                (local.set $c
                   (array.get_u $bytes (local.get $repl) (local.get $i)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
@@ -659,10 +646,7 @@
                (local.set $c (i32.shl (local.get $c) (i32.const 1)))
                (if (i32.gt_u (i32.add (local.get $c) (i32.const 1))
                       (array.len (local.get $groups)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $bytes $unmatched_group
-                           (i32.const 0) (i32.const 41)))))
+                  (then (call $caml_failwith (global.get $unmatched_group))))
                (local.set $start
                   (i31.get_s
                      (ref.cast (ref i31)
@@ -674,10 +658,7 @@
                         (array.get $block (local.get $groups)
                            (i32.add (local.get $c) (i32.const 2))))))
                (if (i32.eq (local.get $start) (i32.const -1))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $bytes $unmatched_group
-                           (i32.const 0) (i32.const 41)))))
+                  (then (call $caml_failwith (global.get $unmatched_group))))
                (local.set $len
                    (i32.add (local.get $len)
                       (i32.sub (local.get $end) (local.get $start))))
@@ -718,10 +699,7 @@
                (local.set $c (i32.shl (local.get $c) (i32.const 1)))
                (if (i32.gt_u (i32.add (local.get $c) (i32.const 1))
                       (array.len (local.get $groups)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $bytes $unmatched_group
-                           (i32.const 0) (i32.const 41)))))
+                  (then (call $caml_failwith (global.get $unmatched_group))))
                (local.set $start
                   (i31.get_s
                      (ref.cast (ref i31)
diff --git a/runtime/wasm/string.wat b/runtime/wasm/string.wat
index 10b8f89cb3..66183061b4 100644
--- a/runtime/wasm/string.wat
+++ b/runtime/wasm/string.wat
@@ -119,17 +119,14 @@
       (param $v (ref eq)) (result (ref eq))
       (local.get $v))
 
-   (data $Bytes_create "Bytes.create")
+   (@string $Bytes_create "Bytes.create")
 
    (func (export "caml_create_bytes")
       (param $len (ref eq)) (result (ref eq))
       (local $l i32)
       (local.set $l (i31.get_s (ref.cast (ref i31) (local.get $len))))
       (if (i32.lt_s (local.get $l) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $Bytes_create
-                               (i32.const 0) (i32.const 12)))))
+         (then (call $caml_invalid_argument (global.get $Bytes_create))))
       (array.new $bytes (i32.const 0) (local.get $l)))
 
    (export "caml_blit_bytes" (func $caml_blit_string))
diff --git a/runtime/wasm/sync.wat b/runtime/wasm/sync.wat
index 7d18263301..1b498d4a93 100644
--- a/runtime/wasm/sync.wat
+++ b/runtime/wasm/sync.wat
@@ -53,9 +53,7 @@
 
    (global $mutex_ops (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $bytes 6 ;; "_mutex"
-            (i32.const 95) (i32.const 109) (i32.const 117) (i32.const 116)
-            (i32.const 101) (i32.const 120))
+         (@string "_mutex")
          (ref.func $custom_compare_id)
          (ref.null $compare)
          (ref.func $custom_hash_id)
@@ -75,16 +73,13 @@
       (struct.new $mutex
          (global.get $mutex_ops) (call $custom_next_id) (i32.const 0)))
 
-   (data $lock_failure "Mutex.lock: mutex already locked. Cannot wait.")
+   (@string $lock_failure "Mutex.lock: mutex already locked. Cannot wait.")
 
    (func (export "caml_ml_mutex_lock") (param (ref eq)) (result (ref eq))
       (local $t (ref $mutex))
       (local.set $t (ref.cast (ref $mutex) (local.get 0)))
       (if (struct.get $mutex $state (local.get $t))
-         (then
-            (call $caml_failwith
-               (array.new_data $bytes $lock_failure
-                  (i32.const 0) (i32.const 46)))))
+         (then (call $caml_failwith (global.get $lock_failure))))
       (struct.set $mutex $state (local.get $t) (i32.const 1))
       (ref.i31 (i32.const 0)))
 
@@ -106,13 +101,11 @@
    (func (export "caml_ml_condition_new") (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
-   (data $condition_failure "Condition.wait: cannot wait")
+   (@string $condition_failure "Condition.wait: cannot wait")
 
    (func (export "caml_ml_condition_wait")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (call $caml_failwith
-         (array.new_data $bytes $condition_failure
-            (i32.const 0) (i32.const 27)))
+      (call $caml_failwith (global.get $condition_failure))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ml_condition_signal") (param (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/sys.wat b/runtime/wasm/sys.wat
index 6cfa7817e1..89e3e3e83e 100644
--- a/runtime/wasm/sys.wat
+++ b/runtime/wasm/sys.wat
@@ -16,11 +16,40 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-   (import "bindings" "ta_length"
-      (func $ta_length (param (ref extern)) (result i32)))
-   (import "bindings" "ta_get_i32"
-      (func $ta_get_i32 (param (ref extern)) (param i32) (result i32)))
-   (import "bindings" "random_seed" (func $random_seed (result (ref extern))))
+   (import "fail" "caml_invalid_argument"
+      (func $caml_invalid_argument (param (ref eq))))
+   (import "fail" "caml_raise_sys_error"
+      (func $caml_raise_sys_error (param (ref eq))))
+   (import "fail" "caml_raise_not_found" (func $caml_raise_not_found))
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "random_get"
+      (func $random_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "clock_time_get"
+      (func $clock_time_get (param i32 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "args_get"
+      (func $args_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "args_sizes_get"
+      (func $args_sizes_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "environ_get"
+      (func $environ_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "environ_sizes_get"
+      (func $environ_sizes_get (param i32 i32) (result i32)))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "libc" "strlen" (func $strlen (param i32) (result i32)))
+   (import "wasi_memory" "checked_malloc"
+      (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+   (import "wasi_errors" "error_messages" (global $error_messages (ref $block)))
+   (import "string" "caml_string_concat"
+      (func $caml_string_concat
+         (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "wasi_snapshot_preview1" "proc_exit" (func $exit (param i32)))
+)
+(@else
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "caml_jsstring_of_string"
@@ -32,23 +61,26 @@
    (import "jslib" "caml_js_meth_call"
       (func $caml_js_meth_call
          (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "fail" "caml_raise_sys_error"
-      (func $caml_raise_sys_error (param (ref eq))))
-   (import "fail" "caml_raise_not_found" (func $caml_raise_not_found))
+   (import "bindings" "ta_length"
+      (func $ta_length (param (ref extern)) (result i32)))
+   (import "bindings" "ta_get_i32"
+      (func $ta_get_i32 (param (ref extern)) (param i32) (result i32)))
+   (import "bindings" "random_seed" (func $random_seed (result (ref extern))))
    (import "bindings" "argv" (func $argv (result (ref extern))))
    (import "bindings" "on_windows" (global $on_windows i32))
+   (import "bindings" "isatty"
+      (func $isatty (param (ref eq)) (result (ref eq))))
    (import "bindings" "system" (func $system (param anyref) (result (ref eq))))
    (import "bindings" "getenv" (func $getenv (param anyref) (result anyref)))
    (import "bindings" "time" (func $time (result f64)))
-   (import "bindings" "array_length"
-      (func $array_length (param (ref extern)) (result i32)))
-   (import "bindings" "array_get"
-      (func $array_get (param (ref extern)) (param i32) (result anyref)))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
    (import "jsstring" "jsstring_test"
       (func $jsstring_test (param anyref) (result i32)))
-   (import "bindings" "exit" (func $exit (param (ref eq))))
+   (import "bindings" "exit" (func $exit (param i32)))
+))
+   (import "io" "caml_channel_descriptor"
+      (func $caml_channel_descriptor (param (ref eq)) (result (ref eq))))
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
@@ -58,12 +90,100 @@
 
    (func (export "caml_sys_exit") (export "unix_exit") (export "caml_unix_exit")
       (param $code (ref eq)) (result (ref eq))
-      (call $exit (local.get $code))
+      (call $exit (i31.get_s (ref.cast (ref i31) (local.get $code))))
       ;; Fallback: try to exit through an exception
       (throw $ocaml_exit))
 
-   (export "caml_sys_unsafe_getenv" (func $caml_sys_getenv))
-   (func $caml_sys_getenv (export "caml_sys_getenv")
+(@if wasi
+(@then
+   (global $environment (mut i32) (i32.const 0))
+   (global $environment_count (mut i32) (i32.const 0))
+   (global $environment_data (mut i32) (i32.const 0))
+
+   (func $initialize_env
+      (local $buffer i32) (local $res i32) (local $env i32) (local $data i32)
+      (if (i32.eqz (global.get $environment))
+         (then
+            (local.set $buffer (call $get_buffer))
+            (local.set $res
+               (call $environ_sizes_get
+                  (local.get $buffer)
+                  (i32.add (local.get $buffer) (i32.const 4))))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (ref.i31 (i32.const 0)) (local.get $res))))
+            (local.set $env
+               (call $checked_malloc
+                  (i32.shl (i32.load (local.get $buffer)) (i32.const 2))))
+            (local.set $data
+               (call $checked_malloc (i32.load offset=4 (local.get $buffer))))
+            (local.set $res
+               (call $environ_get (local.get $env) (local.get $data)))
+            (if (local.get $res)
+               (then
+                  (call $caml_handle_sys_error
+                     (ref.i31 (i32.const 0)) (local.get $res))))
+            (global.set $environment (local.get $env))
+            (global.set $environment_data (local.get $data))
+            (global.set $environment_count (i32.load (local.get $buffer))))))
+
+   (func (export "caml_sys_getenv") (export "caml_sys_unsafe_getenv")
+      (param $name (ref eq)) (result (ref eq))
+      (local $var (ref $bytes)) (local $i i32) (local $j i32)
+      (local $len i32) (local $s i32) (local $c i32)
+      (call $initialize_env)
+      (local.set $var (ref.cast (ref $bytes) (local.get $name)))
+      (local.set $len (array.len (local.get $var)))
+      (block $not_found
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (local.get $len))
+               (then
+                  (br_if $not_found
+                     (i32.eq (i32.const 61) ;; '='
+                         (array.get_u $bytes (local.get $var) (local.get $i))))
+                  (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop))))
+         (local.set $i (i32.const 0))
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (global.get $environment_count))
+               (then
+                  (local.set $s
+                     (i32.load
+                        (i32.add (global.get $environment)
+                           (i32.shl (local.get $i) (i32.const 2)))))
+                  (local.set $j (i32.const 0))
+                  (block $next
+                     (loop $scan
+                        (if (i32.lt_u (local.get $j) (local.get $len))
+                           (then
+                              (local.set $c
+                                 (i32.load8_u
+                                    (i32.add (local.get $s) (local.get $j))))
+                              (br_if $next (i32.eqz (local.get $c)))
+                              (br_if $next
+                                  (i32.ne (local.get $c)
+                                     (array.get $bytes
+                                        (local.get $var) (local.get $j))))
+                              (local.set $j
+                                 (i32.add (local.get $j) (i32.const 1)))
+                              (br $scan))))
+                     (br_if $next
+                        (i32.ne (i32.const 61) ;; '='
+                           (i32.load8_u
+                              (i32.add (local.get $s) (local.get $j)))))
+                     (local.set $s
+                        (i32.add (local.get $s)
+                           (i32.add (local.get $j) (i32.const 1))))
+                     (return_call $blit_memory_to_string
+                        (local.get $s) (call $strlen (local.get $s))))
+                  (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop)))))
+      (call $caml_raise_not_found)
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "caml_sys_getenv") (export "caml_sys_unsafe_getenv")
       (param (ref eq)) (result (ref eq))
       (local $res anyref)
       (local.set $res
@@ -73,7 +193,65 @@
          (then
             (call $caml_raise_not_found)))
       (return_call $caml_string_of_jsstring (call $wrap (local.get $res))))
+))
 
+(@if wasi
+(@then
+   (global $argv (mut (ref null $block)) (ref.null $block))
+
+   (func $caml_sys_argv (export "caml_sys_argv")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $argc i32) (local $argv i32) (local $argv_buf i32)
+      (local $args (ref $block)) (local $arg i32) (local $i i32)
+      (block $init
+         (return (br_on_null $init (global.get $argv))))
+      (block $error
+         (local.set $buffer (call $get_buffer))
+         (local.set $res
+            (call $args_sizes_get
+               (local.get $buffer)
+               (i32.add (local.get $buffer) (i32.const 4))))
+         (br_if $error (local.get $res))
+         (local.set $argc (i32.load (local.get $buffer)))
+         (local.set $argv
+            (call $checked_malloc (i32.shl (local.get $argc) (i32.const 2))))
+         (local.set $argv_buf
+            (call $checked_malloc (i32.load offset=4 (local.get $buffer))))
+         (local.set $res
+            (call $args_get (local.get $argv) (local.get $argv_buf)))
+         (br_if $error (local.get $res))
+         (local.set $args
+            (array.new $block (ref.i31 (i32.const 0))
+               (i32.add (local.get $argc) (i32.const 1))))
+         (loop $loop
+            (if (i32.lt_u (local.get $i) (local.get $argc))
+               (then
+                  (local.set $arg
+                     (i32.load
+                        (i32.add (local.get $argv)
+                           (i32.shl (local.get $i) (i32.const 2)))))
+                  (array.set $block (local.get $args)
+                     (i32.add (local.get $i) (i32.const 1))
+                     (call $blit_memory_to_string
+                        (local.get $arg) (call $strlen (local.get $arg))))
+                  (local.set $i
+                     (i32.add (local.get $i) (i32.const 1)))
+                  (br $loop))))
+         (global.set $argv (local.get $args))
+         (call $free (local.get $argv))
+         (call $free (local.get $argv_buf))
+         (return (local.get $args)))
+      (call $caml_handle_sys_error (ref.i31 (i32.const 0)) (local.get $res))
+      (array.new_fixed $block 0))
+
+   (func (export "caml_sys_executable_name")
+      (param (ref eq)) (result (ref eq))
+      (array.get $block
+         (ref.cast (ref $block) (call $caml_sys_argv (ref.i31 (i32.const 0))))
+         (i32.const 1)))
+)
+(@else
    (func (export "caml_sys_argv") (param (ref eq)) (result (ref eq))
       ;; ZZZ
       (call $caml_js_to_string_array (call $argv)))
@@ -83,24 +261,83 @@
       (array.get $block
          (ref.cast (ref $block) (call $caml_js_to_string_array (call $argv)))
          (i32.const 1)))
+))
 
-   (export "caml_sys_time_include_children" (func $caml_sys_time))
-   (func $caml_sys_time (export "caml_sys_time")
+(@if wasi
+(@then
+   (func (export "caml_sys_time") (export "caml_sys_time_include_children")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get (i32.const 2) (i64.const 1) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (struct.new $float
+         (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+            (f64.const 1e-9))))
+)
+(@else
+   (func (export "caml_sys_time") (export "caml_sys_time_include_children")
       (param (ref eq)) (result (ref eq))
       (struct.new $float (f64.mul (call $time) (f64.const 0.001))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_system_command")
+      (param (ref eq)) (result (ref eq))
+      (call $caml_invalid_argument (@string "Sys.command not implemented"))
+      (return (ref.i31 (i32.const 0))))
+)
+(@else
    (func (export "caml_sys_system_command")
       (param (ref eq)) (result (ref eq))
-      ;; ZZZ
       (try
          (do
             (return
                (call $system
                   (call $unwrap (call $caml_jsstring_of_string (local.get 0))))))
          (catch $javascript_exception
-            (call $caml_handle_sys_error (pop externref))
-            (return (ref.i31 (i32.const 0))))))
+            (call $caml_handle_sys_error (pop externref))))
+      (return (ref.i31 (i32.const 0))))
+))
 
+(@if wasi
+(@then
+   (func (export "caml_sys_random_seed")
+      (param (ref eq)) (result (ref eq))
+      (local $r (ref extern))
+      (local $a (ref $block))
+      (local $i i32) (local $n i32)
+      (local $buffer i32) (local $res i32)
+      (local.set $n (i32.const 12))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res (call $random_get (local.get $buffer) (i32.const 96)))
+      (if (local.get $res)
+         (then
+            (call $caml_handle_sys_error
+               (ref.i31 (i32.const 0)) (local.get $res))))
+      (local.set $a
+         (array.new $block (ref.i31 (i32.const 0))
+            (i32.add (local.get $n) (i32.const 1))))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $n))
+            (then
+               (array.set $block
+                  (local.get $a) (i32.add (local.get $i) (i32.const 1))
+                  (ref.i31
+                     (i32.load
+                        (i32.add
+                           (local.get $buffer
+                             (i32.shl (local.get $i) (i32.const 2)))))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (local.get $a))
+)
+(@else
    (func (export "caml_sys_random_seed")
       (param (ref eq)) (result (ref eq))
       (local $r (ref extern))
@@ -111,7 +348,6 @@
       (local.set $a
          (array.new $block (ref.i31 (i32.const 0))
             (i32.add (local.get $n) (i32.const 1))))
-      (local.set $i (i32.const 0))
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $n))
             (then
@@ -121,6 +357,7 @@
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop))))
       (local.get $a))
+))
 
    (func (export "caml_sys_const_bigendian")
       (param (ref eq)) (result (ref eq))
@@ -138,6 +375,11 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0xfffffff)))
 
+(@if wasi
+(@then
+   (global $on_windows i32 (i32.const 0))
+))
+
    (func (export "caml_sys_const_ostype_unix")
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.eqz (global.get $on_windows))))
@@ -150,29 +392,34 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
-   (data $Unix "Unix")
-   (data $Win32 "Win32")
+   (@string $Unix "Unix")
+   (@string $Win32 "Win32")
 
    (func (export "caml_sys_get_config")
       (param (ref eq)) (result (ref eq))
       (array.new_fixed $block 4 (ref.i31 (i32.const 0))
-         (if (result (ref eq)) (global.get $on_windows)
-            (then
-               (array.new_data $bytes $Win32 (i32.const 0) (i32.const 5)))
-            (else
-               (array.new_data $bytes $Unix (i32.const 0) (i32.const 4))))
+         (select (result (ref eq)) (global.get $Win32) (global.get $Unix)
+            (global.get $on_windows))
          (ref.i31 (i32.const 32))
          (ref.i31 (i32.const 0))))
 
+(@if wasi
+(@then
    (func (export "caml_sys_isatty")
-      (param (ref eq)) (result (ref eq))
+      (param $ch (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "caml_sys_isatty")
+      (param $ch (ref eq)) (result (ref eq))
+      (return_call $isatty (call $caml_channel_descriptor (local.get $ch))))
+))
 
    (func (export "caml_runtime_variant") (param (ref eq)) (result (ref eq))
-      (array.new_fixed $bytes 0))
+      (@string ""))
 
    (func (export "caml_runtime_parameters") (param (ref eq)) (result (ref eq))
-      (array.new_fixed $bytes 0))
+      (@string ""))
 
    (func (export "caml_install_signal_handler")
       (param (ref eq) (ref eq)) (result (ref eq))
@@ -190,14 +437,37 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (global.get $caml_runtime_warnings)))
 
-   (data $toString "toString")
+   (@string $toString "toString")
 
+(@if wasi
+(@then
+   (func $caml_handle_sys_error (export "caml_handle_sys_error")
+      (param $arg (ref eq)) (param $errno i32)
+      (local $msg (ref eq))
+      (local.set $msg
+         (if (result (ref eq)) (i32.gt_u (local.get $errno)
+                (array.len (global.get $error_messages)))
+            (then
+               (@string "unknown system error"))
+            (else
+               (array.get $block (global.get $error_messages)
+                  (local.get $errno)))))
+      (if (ref.test (ref $bytes) (local.get $arg))
+         (then
+            (local.set $msg
+               (call $caml_string_concat (local.get $arg)
+                  (call $caml_string_concat (@string ": ") (local.get $msg))))))
+      (call $caml_raise_sys_error (local.get $msg))
+   )
+)
+(@else
    (func $caml_handle_sys_error (export "caml_handle_sys_error")
       (param $exn externref)
       (call $caml_raise_sys_error
          (call $caml_string_of_jsstring
             (call $caml_js_meth_call
                (call $wrap (any.convert_extern (local.get $exn)))
-               (array.new_data $bytes $toString (i32.const 0) (i32.const 8))
+               (global.get $toString)
                (array.new_fixed $block 1 (ref.i31 (i32.const 0)))))))
+))
 )
diff --git a/runtime/wasm/unix.wat b/runtime/wasm/unix.wat
index 0e5fbf7535..65878ed423 100644
--- a/runtime/wasm/unix.wat
+++ b/runtime/wasm/unix.wat
@@ -16,7 +16,75 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
+(@if wasi
+(@then
+   (import "wasi_snapshot_preview1" "clock_time_get"
+      (func $clock_time_get (param i32 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_filestat_get"
+      (func $path_filestat_get (param i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_rename"
+      (func $path_rename (param i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_link"
+      (func $path_link (param i32 i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_symlink"
+      (func $path_symlink (param i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_readlink"
+      (func $path_readlink (param i32 i32 i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_create_directory"
+      (func $path_create_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_unlink_file"
+      (func $path_unlink_file (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_remove_directory"
+      (func $path_remove_directory (param i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_open"
+      (func $path_open (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "path_filestat_set_times"
+      (func $path_filestat_set_times
+         (param i32 i32 i32 i32 i64 i64 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_filestat_get"
+      (func $fd_filestat_get (param i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_filestat_set_size"
+      (func $fd_filestat_set_size (param i32 i64) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_write"
+      (func $fd_write (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_read"
+      (func $fd_read (param i32 i32 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_seek"
+      (func $fd_seek (param i32 i64 i32 i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_sync"
+      (func $fd_sync (param i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_close"
+      (func $fd_close (param i32) (result i32)))
+   (import "wasi_snapshot_preview1" "fd_readdir"
+      (func $fd_readddir (param i32 i32 i32 i64 i32) (result i32)))
+   (import "libc" "memory" (memory 2))
+   (import "libc" "free" (func $free (param i32)))
+   (import "libc" "gmtime" (func $gmtime (param i32) (result i32)))
+   (import "libc" "localtime" (func $localtime (param i32) (result i32)))
+   (import "libc" "mktime" (func $mktime (param i32) (result i64)))
+   (import "wasi_memory" "checked_malloc"
+      (func $checked_malloc (param i32) (result i32)))
+   (import "wasi_memory" "get_buffer" (func $get_buffer (result i32)))
+   (import "wasi_memory" "write_string_to_memory"
+      (func $write_string_to_memory (param i32 i32 (ref eq)) (result i32)))
+   (import "wasi_memory" "blit_memory_to_string"
+      (func $blit_memory_to_string (param i32 i32) (result (ref $bytes))))
+   (import "wasi_memory" "blit_memory_to_substring"
+      (func $blit_memory_to_substring (param i32 (ref $bytes) i32 i32)))
+   (import "wasi_memory" "blit_substring_to_memory"
+      (func $blit_substring_to_memory (param i32 (ref $bytes) i32 i32)))
+   (import "fs" "wasi_resolve_path"
+      (func $wasi_resolve_path (param (ref eq)) (result i32 i32 i32)))
+   (import "fs" "wasi_chdir" (func $wasi_chdir (param (ref eq))))
+   (import "wasi_errors" "error_messages" (global $error_messages (ref $block)))
+   (import "ints" "caml_format_int"
+      (func $caml_format_int (param (ref eq) (ref eq)) (result (ref eq))))
+   (import "string" "caml_string_concat"
+      (func $caml_string_concat (param (ref eq) (ref eq)) (result (ref eq))))
+)
+(@else
    (import "bindings" "gettimeofday" (func $gettimeofday (result f64)))
+   (import "bindings" "times" (func $times (result (ref eq))))
    (import "bindings" "gmtime" (func $gmtime (param f64) (result (ref eq))))
    (import "bindings" "localtime"
       (func $localtime (param f64) (result (ref eq))))
@@ -29,12 +97,26 @@
    (import "bindings" "lstat" (func $lstat (param anyref i32) (result (ref eq))))
    (import "bindings" "fstat"
       (func $fstat (param (ref eq) i32) (result (ref eq))))
+   (import "bindings" "chmod" (func $chmod (param anyref (ref eq))))
+   (import "bindings" "fchmod" (func $fchmod (param (ref eq) (ref eq))))
    (import "bindings" "rename" (func $rename (param anyref) (param anyref)))
+   (import "bindings" "getcwd" (func $getcwd (result anyref)))
+   (import "bindings" "chdir" (func $chdir (param anyref)))
+   (import "bindings" "mkdir" (func $mkdir (param anyref) (param i32)))
+   (import "bindings" "opendir" (func $opendir (param anyref) (result anyref)))
+   (import "bindings" "readdir" (func $readdir (param anyref) (result anyref)))
+   (import "bindings" "closedir" (func $closedir (param anyref)))
+   (import "bindings" "rmdir" (func $rmdir (param anyref)))
+   (import "bindings" "link" (func $link (param anyref anyref)))
+   (import "bindings" "symlink" (func $symlink (param anyref anyref i32)))
+   (import "bindings" "readlink" (func $readlink (param anyref) (result anyref)))
+   (import "bindings" "unlink" (func $unlink (param anyref)))
    (import "bindings" "truncate" (func $truncate (param anyref (ref eq))))
    (import "bindings" "truncate" (func $truncate_64 (param anyref f64)))
    (import "bindings" "ftruncate" (func $ftruncate (param (ref eq) (ref eq))))
    (import "bindings" "ftruncate" (func $ftruncate_64 (param (ref eq) f64)))
    (import "bindings" "file_size" (func $file_size (param i32) (result i64)))
+   (import "bindings" "access" (func $access (param anyref) (param i32)))
    (import "bindings" "open"
       (func $open (param anyref) (param i32) (param i32) (result i32)))
    (import "bindings" "write"
@@ -45,10 +127,13 @@
       (func $read (param i32 (ref extern) i32 i32 i64) (result i32)))
    (import "bindings" "read"
       (func $read' (param i32 (ref extern) i32 i32 nullexternref) (result i32)))
+   (import "bindings" "fsync" (func $fsync (param (ref eq))))
    (import "bindings" "close" (func $close (param (ref eq))))
    (import "bindings" "isatty"
       (func $isatty (param (ref eq)) (result (ref eq))))
    (import "js" "unix_error" (global $unix_error_js (ref any)))
+   (import "js" "caml_strerror"
+      (func $caml_strerror (param i32) (result (ref any))))
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "caml_js_meth_call"
@@ -62,15 +147,18 @@
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jsstring" "jsstring_test"
       (func $jsstring_test (param anyref) (result i32)))
+))
    (import "stdlib" "caml_named_value"
       (func $caml_named_value (param (ref eq)) (result (ref null eq))))
    (import "fail" "ocaml_exception" (tag $ocaml_exception (param (ref eq))))
    (import "fail" "javascript_exception"
       (tag $javascript_exception (param externref)))
+   (import "fail" "caml_raise_end_of_file" (func $caml_raise_end_of_file))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param (ref eq))))
+   (import "fail" "caml_raise_not_found" (func $caml_raise_not_found))
    (import "io" "convert_flag_list"
-      (func $convert_flag_list (param (ref $open_flags) (ref eq)) (result i32)))
+      (func $convert_flag_list (param (ref $flags) (ref eq)) (result i32)))
    (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
    (import "io" "initialize_fd_offset"
       (func $initialize_fd_offset (param i32 i64)))
@@ -100,6 +188,7 @@
    (type $bytes (array (mut i8)))
    (type $block (array (mut (ref eq))))
    (type $float (struct (field f64)))
+   (type $float_array (array (mut f64)))
    (type $js (struct (field anyref)))
 
    (type $fd_offset
@@ -107,9 +196,9 @@
 
    (global $unix_error_exn (mut (ref eq)) (ref.i31 (i32.const 0)))
 
-   (data $unix_error "Unix.Unix_error")
+   (@string $unix_error_str "Unix.Unix_error")
 
-   (data $unix_error_not_initialized
+   (@string $unix_error_not_initialized
       "Exception Unix.Unix_error not initialized, please link unix.cma")
 
    (func $get_unix_error_exn (result (ref eq))
@@ -117,20 +206,113 @@
       (if (ref.test (ref i31) (global.get $unix_error_exn))
          (then
             (local.set $unix_error_exn
-               (call $caml_named_value
-                  (array.new_data $bytes $unix_error
-                     (i32.const 0) (i32.const 15))))
+               (call $caml_named_value (global.get $unix_error_str)))
             (if (ref.is_null (local.get $unix_error_exn))
                (then
                   (call $caml_invalid_argument
-                      (array.new_data $bytes $unix_error_not_initialized
-                         (i32.const 0) (i32.const 63)))))
+                      (global.get $unix_error_not_initialized))))
             (global.set $unix_error_exn
                (ref.as_non_null (local.get $unix_error_exn)))))
       (global.get $unix_error_exn))
 
-   (global $no_arg (ref eq) (array.new_fixed $bytes 0))
+   (@string $no_arg "")
+
+(@if wasi
+(@then
+   (func $unix_resolve_path (export "unix_resolve_path")
+      (param $cmd (ref eq)) (param $path (ref eq)) (result i32 i32 i32)
+      (local $res (tuple i32 i32 i32))
+      (local.set $res (call $wasi_resolve_path (local.get $path)))
+      (if (i32.lt_s (tuple.extract 3 0 (local.get $res)) (i32.const 0))
+         (then
+            (call $caml_unix_error
+               (i32.const 44) ;; ENOENT
+               (local.get $cmd) (local.get $path))))
+      (local.get $res))
+
+   (type $constr_table (array i8))
+   (global $error_codes (ref $constr_table)
+      (array.new_fixed $constr_table 77
+         (i32.const -1)
+         (i32.const  0) (i32.const  1) (i32.const 50) (i32.const 51)
+         (i32.const 49) (i32.const  2) (i32.const 39) (i32.const  3)
+         (i32.const -1) (i32.const  4) (i32.const -1) (i32.const  5)
+         (i32.const 55) (i32.const 63) (i32.const 56) (i32.const  6)
+         (i32.const 41) (i32.const  7) (i32.const -1) (i32.const  8)
+         (i32.const  9) (i32.const 10) (i32.const 65) (i32.const -1)
+         (i32.const -1) (i32.const 38) (i32.const 11) (i32.const 12)
+         (i32.const 13) (i32.const 58) (i32.const 14) (i32.const 66)
+         (i32.const 15) (i32.const 16) (i32.const 42) (i32.const -1)
+         (i32.const 17) (i32.const 52) (i32.const 54) (i32.const 53)
+         (i32.const 18) (i32.const 57) (i32.const 19) (i32.const 20)
+         (i32.const 21) (i32.const 22) (i32.const -1) (i32.const 23)
+         (i32.const -1) (i32.const 44) (i32.const 24) (i32.const 25)
+         (i32.const 59) (i32.const 26) (i32.const 27) (i32.const -1)
+         (i32.const 40) (i32.const 47) (i32.const 28) (i32.const 29)
+         (i32.const 67) (i32.const -1) (i32.const 30) (i32.const 31)
+         (i32.const -1) (i32.const 45) (i32.const 43) (i32.const 32)
+         (i32.const 33) (i32.const 34) (i32.const 35) (i32.const -1)
+         (i32.const 62) (i32.const -1) (i32.const 36) (i32.const -1)))
+
+   (func $caml_unix_error_of_code (param $errcode i32) (result (ref eq))
+      (local $err i32)
+      (if (i32.le_u (local.get $errcode) (i32.const 76))
+         (then
+            (local.set $err
+               (array.get_s $constr_table (global.get $error_codes)
+                  (local.get $errcode)))
+            (if (i32.ne (local.get $err) (i32.const -1))
+               (then
+                  (return (ref.i31 (local.get $err)))))))
+      (array.new_fixed $block 2
+         (ref.i31 (i32.const 0)) (ref.i31 (local.get $errcode))))
+
+   (func $caml_unix_error
+      (param $errcode i32) (param $cmd_name (ref eq)) (param $cmd_arg (ref eq))
+      (throw $ocaml_exception
+         (array.new_fixed $block 5
+            (ref.i31 (i32.const 0))
+            (call $get_unix_error_exn)
+            (call $caml_unix_error_of_code (local.get $errcode))
+            (local.get $cmd_name)
+            (local.get $cmd_arg))))
 
+   (func (export "unix_error_message") (export "caml_unix_error_message")
+      (param $err (ref eq)) (result (ref eq))
+      (local $errcode i32) (local $i i32) (local $n i32)
+      (if (ref.test (ref i31) (local.get $err))
+         (then
+            (local.set $n (i31.get_u (ref.cast (ref i31) (local.get $err))))
+            (loop $loop
+               (if (i32.lt_u (local.get $errcode)
+                      (array.len (global.get $error_codes)))
+                  (then
+                     (if (i32.ne (local.get $n)
+                            (array.get $constr_table (global.get $error_codes)
+                               (local.get $errcode)))
+                        (then
+                           (local.set $errcode
+                              (i32.add (local.get $errcode) (i32.const 1)))
+                           (br $loop))))
+                  (else
+                     (local.set $errcode (i32.const -1))))))
+         (else
+            (local.set $errcode
+               (i31.get_u
+                  (ref.cast (ref i31)
+                     (array.get $block
+                        (ref.cast (ref $block) (local.get $err))
+                        (i32.const 1)))))))
+      (if (i32.gt_u (local.get $errcode)
+             (array.len (global.get $error_messages)))
+         (then
+            (return_call $caml_string_concat
+               (@string "Unknown error ")
+               (call $caml_format_int (@string "%d")
+                  (ref.i31 (local.get $errcode))))))
+      (array.get $block (global.get $error_messages) (local.get $errcode)))
+)
+(@else
    (global $unix_error (ref eq) (struct.new $js (global.get $unix_error_js)))
 
    (func $ensure_string (param $s (ref eq)) (result (ref eq))
@@ -146,11 +328,11 @@
          (return (call $caml_string_of_jsstring (local.get $s)))))
       (return (global.get $no_arg)))
 
-   (data $code "code")
-   (data $errno "errno")
-   (data $indexOf "indexOf")
-   (data $syscall "syscall")
-   (data $path "path")
+   (@string $code "code")
+   (@string $errno "errno")
+   (@string $indexOf "indexOf")
+   (@string $syscall "syscall")
+   (@string $path "path")
 
    (func $caml_unix_error (param $exception externref) (param $cmd eqref)
       (local $exn (ref eq))
@@ -158,19 +340,16 @@
       (local $errno (ref eq))
       (local $variant (ref eq))
       (local.set $exn (call $wrap (any.convert_extern (local.get $exception))))
-      (local.set $code
-         (call $caml_js_get (local.get $exn)
-            (array.new_data $bytes $code (i32.const 0) (i32.const 4))))
+      (local.set $code (call $caml_js_get (local.get $exn) (global.get $code)))
       (local.set $variant
          (call $caml_js_meth_call (global.get $unix_error)
-            (array.new_data $bytes $indexOf (i32.const 0) (i32.const 7))
+            (global.get $indexOf)
             (array.new_fixed $block 2 (ref.i31 (i32.const 0))
                (local.get $code))))
       (if (ref.eq (local.get $variant) (ref.i31 (i32.const -1)))
          (then
             (local.set $errno
-               (call $caml_js_get (local.get $exn)
-                  (array.new_data $bytes $errno (i32.const 0) (i32.const 4))))
+               (call $caml_js_get (local.get $exn) (global.get $errno)))
             (local.set $errno
                (ref.i31
                   (if (result i32) (ref.test (ref i31) (local.get $errno))
@@ -191,19 +370,100 @@
                (then
                   (call $ensure_string
                      (call $caml_js_get (local.get $exn)
-                        (array.new_data $bytes $syscall
-                           (i32.const 0) (i32.const 7)))))
+                        (global.get $syscall))))
                (else
                   (ref.as_non_null (local.get $cmd))))
             (call $ensure_string
-               (call $caml_js_get (local.get $exn)
-                  (array.new_data $bytes $path (i32.const 0) (i32.const 4)))))))
+               (call $caml_js_get (local.get $exn) (global.get $path))))))
+
+   (func (export "unix_error_message") (export "caml_unix_error_message")
+      (param $err (ref eq)) (result (ref eq))
+      (local $errno i32)
+      (local.set $errno
+         (if (result i32) (ref.test (ref i31) (local.get $err))
+            (then
+               (i31.get_u (ref.cast (ref i31) (local.get $err))))
+            (else
+               (i32.sub (i32.const 0)
+                  (i31.get_u
+                     (ref.cast (ref i31)
+                        (array.get $block
+                           (ref.cast (ref $block) (local.get $err))
+                           (i32.const 1))))))))
+      (return_call $caml_string_of_jsstring
+         (call $wrap (call $caml_strerror (local.get $errno)))))
+))
 
-   (export "caml_unix_gettimeofday" (func $unix_gettimeofday))
-   (func $unix_gettimeofday (export "unix_gettimeofday")
+(@if wasi
+(@then
+   (func (export "unix_gettimeofday") (export "caml_unix_gettimeofday")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get
+            (i32.const 0) (i64.const 1000) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "gettimeofday") (global.get $no_arg))))
+      (struct.new $float
+         (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+            (f64.const 1e-9))))
+)
+(@else
+   (func (export "unix_gettimeofday") (export "caml_unix_gettimeofday")
       (param (ref eq)) (result (ref eq))
       (struct.new $float (call $gettimeofday)))
+))
+
+(@if wasi
+(@then
+   (func (export "unix_times") (export "caml_unix_times")
+      (param (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get
+            (i32.const 2) (i64.const 1) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res)
+               (@string "time")
+               (global.get $no_arg))))
+      (array.new_fixed $float_array 4
+         (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+            (f64.const 1e-9))
+         (f64.const 0) (f64.const 0) (f64.const 0)))
+)
+(@else
+   (func (export "caml_alloc_times")
+      (param $u f64) (param $s f64) (result (ref eq))
+      (array.new_fixed $float_array 4
+         (local.get $u) (local.get $s) (f64.const 0) (f64.const 0)))
+
+   (func (export "unix_times") (export "caml_unix_times")
+      (param (ref eq)) (result (ref eq))
+      (return_call $times))
+))
 
+(@if wasi
+(@then
+  (func $alloc_tm (param $tm i32) (result (ref eq))
+      (array.new_fixed $block 10 (ref.i31 (i32.const 0))
+         (ref.i31 (i32.load (local.get $tm)))
+         (ref.i31 (i32.load offset=4 (local.get $tm)))
+         (ref.i31 (i32.load offset=8 (local.get $tm)))
+         (ref.i31 (i32.load offset=12 (local.get $tm)))
+         (ref.i31 (i32.load offset=16 (local.get $tm)))
+         (ref.i31 (i32.load offset=20 (local.get $tm)))
+         (ref.i31 (i32.load offset=24 (local.get $tm)))
+         (ref.i31 (i32.load offset=28 (local.get $tm)))
+         (ref.i31 (select (i32.const 1) (i32.const 0)
+            (i32.load offset=32 (local.get $tm))))))
+)
+(@else
    (func (export "caml_alloc_tm")
       (param $sec i32) (param $min i32) (param $hour i32) (param $mday i32)
       (param $mon i32) (param $year i32) (param $wday i32) (param $yday i32)
@@ -218,24 +478,133 @@
          (ref.i31 (local.get $wday))
          (ref.i31 (local.get $yday))
          (ref.i31 (local.get $isdst))))
+))
 
-   (export "caml_unix_gmtime" (func $unix_gmtime))
-   (func $unix_gmtime (export "unix_gmtime") (param (ref eq)) (result (ref eq))
+(@if wasi
+(@then
+   (func (export "caml_unix_gmtime") (export "unix_gmtime")
+      (param $t (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $tm i32)
+      (local.set $buffer (call $get_buffer))
+      (i64.store (local.get $buffer)
+         (i64.trunc_sat_f64_s
+            (struct.get $float 0 (ref.cast (ref $float) (local.get $t)))))
+      (local.set $tm (call $gmtime (local.get $buffer)))
+      (if (i32.eqz (local.get $tm))
+         (then
+            (call $caml_unix_error (i32.const 28) (; EINVAL ;)
+               (@string "gmtime") (global.get $no_arg))))
+      (return_call $alloc_tm (local.get $tm)))
+)
+(@else
+   (func (export "caml_unix_gmtime") (export "unix_gmtime")
+      (param (ref eq)) (result (ref eq))
       (call $gmtime
          (struct.get $float 0 (ref.cast (ref $float) (local.get 0)))))
+))
 
-   (export "caml_unix_localtime" (func $unix_localtime))
-   (func $unix_localtime (export "unix_localtime")
+(@if wasi
+(@then
+   (func (export "caml_unix_localtime") (export "unix_localtime")
+      (param $t (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $tm i32)
+      (local.set $buffer (call $get_buffer))
+      (i64.store (local.get $buffer)
+         (i64.trunc_sat_f64_s
+            (struct.get $float 0 (ref.cast (ref $float) (local.get $t)))))
+      (local.set $tm (call $localtime (local.get $buffer)))
+      (if (i32.eqz (local.get $tm))
+         (then
+            (call $caml_unix_error (i32.const 28) (; EINVAL ;)
+               (@string "localtime") (global.get $no_arg))))
+      (return_call $alloc_tm (local.get $tm)))
+)
+(@else
+   (func (export "caml_unix_localtime") (export "unix_localtime")
       (param (ref eq)) (result (ref eq))
       (call $localtime
          (struct.get $float 0 (ref.cast (ref $float) (local.get 0)))))
+))
 
-   (export "caml_unix_time" (func $unix_time))
-   (func $unix_time (export "unix_time") (param (ref eq)) (result (ref eq))
+(@if wasi
+(@then
+   (func (export "caml_unix_time") (export "unix_time") (param (ref eq))
+      (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $clock_time_get
+            (i32.const 0) (i64.const 1000) (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "time") (global.get $no_arg))))
+      (struct.new $float
+         (f64.floor
+            (f64.mul (f64.convert_i64_u (i64.load (local.get $buffer)))
+               (f64.const 1e-9)))))
+)
+(@else
+   (func (export "caml_unix_time") (export "unix_time") (param (ref eq))
+      (result (ref eq))
       (struct.new $float (f64.floor (call $gettimeofday))))
+))
 
-   (export "caml_unix_mktime" (func $unix_mktime))
-   (func $unix_mktime (export "unix_mktime") (param (ref eq)) (result (ref eq))
+(@if wasi
+(@then
+   (func (export "caml_unix_mktime") (export "unix_mktime")
+      (param $v (ref eq)) (result (ref eq))
+      (local $t (ref $block)) (local $tm i32) (local $time i64)
+      (local.set $t (ref.cast (ref $block) (local.get $v)))
+      (local.set $tm (call $get_buffer))
+      (i32.store (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 1)))))
+      (i32.store offset=4 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 2)))))
+      (i32.store offset=8 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 3)))))
+      (i32.store offset=12 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 4)))))
+      (i32.store offset=16 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 5)))))
+      (i32.store offset=20 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 6)))))
+      (i32.store offset=24 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 7)))))
+      (i32.store offset=28 (local.get $tm)
+         (i31.get_s
+            (ref.cast (ref i31)
+               (array.get $block (local.get $t) (i32.const 8)))))
+      (i32.store offset=32 (local.get $tm)
+         (i32.const -1))
+      (local.set $time (call $mktime (local.get $tm)))
+      (if (i64.eq (local.get $time) (i64.const -1))
+         (then
+            (call $caml_unix_error
+               (i32.const 68) (; ERANGE ;)
+               (@string "mktime") (global.get $no_arg))))
+      (array.new_fixed $block 3
+         (ref.i31 (i32.const 0))
+         (struct.new $float (f64.convert_i64_s (local.get $time)))
+         (call $alloc_tm (local.get $tm))))
+)
+(@else
+   (func (export "caml_unix_mktime") (export "unix_mktime")
+      (param (ref eq)) (result (ref eq))
       (local $tm (ref $block)) (local $t f64)
       (local.set $tm (ref.cast (ref $block) (local.get 0)))
       (local.set $t
@@ -265,7 +634,53 @@
       (array.new_fixed $block 3 (ref.i31 (i32.const 0))
          (struct.new $float (local.get $t))
          (call $localtime (local.get $t))))
+))
+
+(@if wasi
+(@then
+   (@string $utimes "utimes")
 
+   (func (export "unix_utimes") (export "caml_unix_utimes")
+      (param $path (ref eq)) (param $atime (ref eq)) (param $mtime (ref eq))
+      (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $atim i64) (local $mtim i64)
+      (local $set_to_now i32) (local $res i32)
+      (local $at f64) (local $mt f64)
+      (local.set $p
+         (call $unix_resolve_path (global.get $utimes) (local.get $path)))
+      (local.set $at
+         (struct.get $float 0 (ref.cast (ref $float) (local.get $atime))))
+      (local.set $mt
+         (struct.get $float 0 (ref.cast (ref $float) (local.get $mtime))))
+      (local.set $set_to_now
+         (i32.and (f64.eq (local.get $at) (f64.const 0))
+            (f64.eq (local.get $mt) (f64.const 0))))
+      (if (i32.eqz (local.get $set_to_now))
+         (then
+            (local.set $atim
+               (i64.trunc_sat_f64_s
+                  (f64.mul (local.get $at) (f64.const 1e9))))
+            (local.set $mtim
+               (i64.trunc_sat_f64_s
+                  (f64.mul (local.get $mt) (f64.const 1e9))))))
+      (local.set $res
+         (call $path_filestat_set_times
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 0)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $atim)
+            (local.get $mtim)
+            (i32.shl (i32.const 5) (local.get $set_to_now))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $utimes) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_utimes") (export "caml_unix_utimes")
       (param $path (ref eq)) (param $atime (ref eq)) (param $mtime (ref eq))
       (result (ref eq))
@@ -287,6 +702,48 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (global $file_kinds (ref $constr_table)
+      (array.new_fixed $constr_table 8
+         (i32.const 3)
+         (i32.const 3)
+         (i32.const 2)
+         (i32.const 1)
+         (i32.const 0)
+         (i32.const 6)
+         (i32.const 6)
+         (i32.const 4)))
+
+   (func $alloc_stat (param $large i32) (param $p i32) (result (ref eq))
+      (array.new_fixed $block 13 (ref.i31 (i32.const 0))
+         (ref.i31 (i32.wrap_i64 (i64.load (local.get $p))))
+         (ref.i31 (i32.wrap_i64 (i64.load offset=8 (local.get $p))))
+         (ref.i31
+            (array.get $constr_table
+               (global.get $file_kinds) (i32.load8_u offset=16 (local.get $p))))
+         (ref.i31 (i32.const 384 (;0600;)))
+         (ref.i31 (i32.wrap_i64 (i64.load offset=24 (local.get $p))))
+         (ref.i31 (i32.const 1))
+         (ref.i31 (i32.const 1))
+         (ref.i31 (i32.wrap_i64 (i64.load (local.get $p))))
+         (if (result (ref eq)) (local.get $large)
+            (then
+               (call $caml_copy_int64 (i64.load offset=32 (local.get $p))))
+            (else
+               (ref.i31 (i32.wrap_i64 (i64.load offset=32 (local.get $p))))))
+         (struct.new $float
+            (f64.mul (f64.const 1e-9)
+               (f64.convert_i64_s (i64.load offset=40 (local.get $p)))))
+         (struct.new $float
+            (f64.mul (f64.const 1e-9)
+               (f64.convert_i64_s (i64.load offset=48 (local.get $p)))))
+         (struct.new $float
+            (f64.mul (f64.const 1e-9)
+               (f64.convert_i64_s (i64.load offset=56 (local.get $p)))))))
+))
 
    (func (export "caml_alloc_stat")
       (param $large i32)
@@ -312,6 +769,76 @@
          (struct.new $float (local.get $mtime))
          (struct.new $float (local.get $ctime))))
 
+(@if wasi
+(@then
+   (func $stat
+      (param $path (ref eq)) (param $large i32) (param $follow i32)
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (local.get $name) (local.get $path)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (local.get $follow) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (local.get $name) (local.get $path))))
+      (return_call $alloc_stat (local.get $large) (local.get $buffer)))
+
+   (@string $stat "stat")
+
+   (func (export "unix_stat") (export "caml_unix_stat")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 0) (i32.const 1) (global.get $stat)))
+
+   (func (export "unix_stat_64") (export "caml_unix_stat_64")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 1) (i32.const 1) (global.get $stat)))
+
+   (@string $lstat "lstat")
+
+   (func (export "unix_lstat") (export "caml_unix_lstat")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 0) (i32.const 0) (global.get $lstat)))
+
+   (func (export "unix_lstat_64") (export "caml_unix_lstat_64")
+      (param $path (ref eq)) (result (ref eq))
+      (return_call $stat
+         (local.get $path) (i32.const 1) (i32.const 0) (global.get $lstat)))
+
+   (func $fstat (param $fd (ref eq)) (param $large i32) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_filestat_get
+            (i31.get_u (ref.cast (ref i31) (local.get $fd)))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "fstat") (global.get $no_arg))))
+      (return_call $alloc_stat (local.get $large) (local.get $buffer)))
+
+   (func (export "unix_fstat") (export "caml_unix_fstat")
+      (param $fd (ref eq)) (result (ref eq))
+      (return_call $fstat (local.get $fd) (i32.const 0)))
+
+   (func (export "unix_fstat_64") (export "caml_unix_fstat_64")
+      (param $fd (ref eq)) (result (ref eq))
+      (return_call $fstat (local.get $fd) (i32.const 1)))
+)
+(@else
    (func (export "unix_stat") (export "caml_unix_stat")
       (param $path (ref eq)) (result (ref eq))
       (try (result (ref eq))
@@ -373,7 +900,76 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))
             (ref.i31 (i32.const 0)))))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_chmod") (export "caml_unix_chmod")
+      (param (ref eq) (ref eq)) (result (ref eq))
+      ;; no notion of permissions in WASI
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_chmod") (export "caml_unix_chmod")
+      (param $path (ref eq)) (param $perms (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $chmod
+               (call $unwrap (call $caml_jsstring_of_string (local.get $path)))
+               (local.get $perms)))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (func (export "unix_fchmod") (export "caml_unix_fchmod")
+      (param (ref eq) (ref eq)) (result (ref eq))
+      ;; no notion of permissions in WASI
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_fchmod") (export "caml_unix_fchmod")
+      (param $fd (ref eq)) (param $perms (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $fchmod (local.get $fd) (local.get $perms)))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $rename "rename")
+
+   (func (export "unix_rename") (export "caml_unix_rename")
+      (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
+      (local $op (tuple i32 i32 i32))
+      (local $np (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $op
+         (call $unix_resolve_path (global.get $rename)  (local.get $o)))
+      (local.set $np
+         (call $unix_resolve_path (global.get $rename) (local.get $n)))
+      (local.set $res
+         (call $path_rename
+            (tuple.extract 3 0 (local.get $op))
+            (tuple.extract 3 1 (local.get $op))
+            (tuple.extract 3 2 (local.get $op))
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (tuple.extract 3 1 (local.get $op)))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $rename) (local.get $o))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_rename") (export "caml_unix_rename")
       (param $o (ref eq)) (param $n (ref eq)) (result (ref eq))
       (try
@@ -384,7 +980,609 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $chdir "chdir")
+
+   (func (export "unix_chdir") (export "caml_unix_chdir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32) (local $kind i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $chdir) (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $chdir) (local.get $name))))
+      (local.set $kind (i32.load8_u offset=16 (local.get $buffer)))
+      (if (i32.ne (local.get $kind) (i32.const 3))
+         (then
+            (call $caml_unix_error (i32.const 54) ;; ENOTDIR
+               (global.get $chdir) (local.get $name))))
+      (call $wasi_chdir (local.get $name))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_getcwd") (export "caml_unix_getcwd")
+      (param (ref eq)) (result (ref eq))
+      (try (result (ref eq))
+         (do
+            (call $caml_string_of_jsstring (call $wrap (call $getcwd))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))
+            (ref.i31 (i32.const 0)))))
+
+   (func (export "unix_chdir") (export "caml_unix_chdir")
+      (param $name (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $chdir
+               (call $unwrap (call $caml_jsstring_of_string (local.get $name)))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $mkdir "mkdir")
+
+   (func (export "unix_mkdir") (export "caml_unix_mkdir")
+      (param $path (ref eq)) (param $perm (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $mkdir) (local.get $path)))
+      (local.set $res
+         (call $path_create_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $mkdir) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_mkdir") (export "caml_unix_mkdir")
+      (param $name (ref eq)) (param $perm (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $mkdir
+               (call $unwrap (call $caml_jsstring_of_string (local.get $name)))
+               (i31.get_u (ref.cast (ref i31) (local.get $perm)))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (type $directory
+      (struct
+         (field $fd i32)
+         (field $buffer (mut i32))
+         (field $size (mut i32))
+         (field $pos (mut i32))
+         (field $available (mut i32))
+         (field $cookie (mut i64))))
+
+   (@string $opendir "opendir")
+
+   (func $unix_opendir (export "unix_opendir") (export "caml_unix_opendir")
+      (param $name (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $opendir) (local.get $name)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (i32.const 2) ;; O_DIRECTORY
+            (i64.const 0x4000) ;; allow fd_readdir
+            (i64.const 0)
+            (i32.const 0)
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $opendir) (local.get $name))))
+      (struct.new $directory
+         (i32.load (local.get $buffer))
+         (call $checked_malloc (i32.const 512))
+         (i32.const 512)
+         (i32.const 0)
+         (i32.const 0)
+         (i64.const 0)))
+
+   (func $readdir_helper
+      (param $vdir (ref eq)) (result (ref eq))
+      (local $dir (ref $directory)) (local $buf i32) (local $res i32)
+      (local $buffer i32) (local $available i32) (local $left i32)
+      (local $namelen i32) (local $entry i32) (local $entry_size i32)
+      (local.set $dir (ref.cast (ref $directory) (local.get $vdir)))
+      (loop $loop
+         (block $refill
+            (local.set $left
+               (i32.sub (struct.get $directory $available (local.get $dir))
+                  (struct.get $directory $pos (local.get $dir))))
+            (br_if $refill (i32.lt_u (local.get $left) (i32.const 24)))
+            (local.set $entry
+               (i32.add (struct.get $directory $buffer (local.get $dir))
+                  (struct.get $directory $pos (local.get $dir))))
+            (local.set $namelen (i32.load offset=16 (local.get $entry)))
+            (local.set $entry_size (i32.add (local.get $namelen) (i32.const 24)))
+            (br_if $refill (i32.lt_u (local.get $left) (local.get $entry_size)))
+            (struct.set $directory $pos (local.get $dir)
+               (i32.add (struct.get $directory $pos (local.get $dir))
+                  (local.get $entry_size)))
+            (struct.set $directory $cookie (local.get $dir)
+               (i64.load (local.get $entry)))
+            (return_call $blit_memory_to_string
+                (i32.add (local.get $entry) (i32.const 24))
+                (local.get $namelen)))
+         ;; refill
+         (if (i32.lt_u (struct.get $directory $size (local.get $dir))
+                (local.get $entry_size))
+            (then
+               ;; the entry does not fit
+               (local.set $buf (call $checked_malloc (local.get $entry_size)))
+               (call $free (struct.get $directory $buffer (local.get $dir)))
+               (struct.set $directory $buffer (local.get $dir) (local.get $buf))
+               (struct.set $directory $size (local.get $dir)
+                  (local.get $entry_size))))
+         (block $done
+            (br_if $done
+               (i32.and
+                  (i32.ne (i32.const 0)
+                     (struct.get $directory $available (local.get $dir))
+                  (i32.lt_u (struct.get $directory $available (local.get $dir))
+                     (struct.get $directory $size (local.get $dir))))))
+            (local.set $buffer (call $get_buffer))
+            (local.set $res
+               (call $fd_readddir
+                  (struct.get $directory $fd (local.get $dir))
+                  (struct.get $directory $buffer (local.get $dir))
+                  (struct.get $directory $size (local.get $dir))
+                  (struct.get $directory $cookie (local.get $dir))
+                  (local.get $buffer)))
+            (if (local.get $res)
+               (then
+                  (call $caml_unix_error
+                     (local.get $res) (@string "readdir") (global.get $no_arg))))
+            (local.set $available (i32.load (local.get $buffer)))
+            (br_if $done (i32.eqz (local.get $available)))
+            (struct.set $directory $pos (local.get $dir) (i32.const 0))
+            (struct.set $directory $available (local.get $dir)
+               (local.get $available))
+            (br $loop)))
+      ;; done
+      (call $caml_raise_end_of_file)
+      (ref.i31 (i32.const 0)))
+
+   (func $unix_closedir (export "unix_closedir") (export "caml_unix_closedir")
+      (param $vdir (ref eq)) (result (ref eq))
+      (local $dir (ref $directory)) (local $buf i32) (local $res i32)
+      (local.set $dir (ref.cast (ref $directory) (local.get $vdir)))
+      (local.set $buf (struct.get $directory $buffer (local.get $dir)))
+      (block $error
+         (if (i32.eqz (local.get $buf))
+            (then
+               (local.set $res (i32.const 8)) ;; EBADF
+               (br $error)))
+         (call $free (local.get $buf))
+         (struct.set $directory $buffer (local.get $dir) (i32.const 0))
+         (local.set $res
+            (call $fd_close (struct.get $directory $fd (local.get $dir))))
+         (br_if $error (local.get $res))
+         (return (ref.i31 (i32.const 0))))
+      (call $caml_unix_error
+         (local.get $res) (@string "closedir") (global.get $no_arg))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "unix_rewinddir") (export "caml_unix_rewinddir")
+      (param $vdir (ref eq)) (result (ref eq))
+      (local $dir (ref $directory))
+      (local.set $dir (ref.cast (ref $directory) (local.get $vdir)))
+      (struct.set $directory $cookie (local.get $dir) (i64.const 0))
+      (struct.set $directory $pos (local.get $dir) (i32.const 0))
+      (struct.set $directory $available (local.get $dir) (i32.const 0))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func $unix_opendir (export "unix_opendir") (export "caml_unix_opendir")
+      (param $name (ref eq)) (result (ref eq))
+      (try (result (ref eq))
+         (do
+            (call $wrap
+               (call $opendir
+                  (call $unwrap
+                     (call $caml_jsstring_of_string (local.get $name))))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))
+            (ref.i31 (i32.const 0)))))
+
+   (func $throw_ebadf (param $cmd (ref eq))
+      (throw $ocaml_exception
+         (array.new_fixed $block 5
+            (ref.i31 (i32.const 0))
+            (call $get_unix_error_exn)
+            (ref.i31 (i32.const 3)) ;; EBADF
+            (local.get $cmd)
+            (global.get $no_arg))))
+
+   (func $readdir_helper (param $dir (ref eq)) (result (ref eq))
+      (block $end
+         (return
+            (try (result (ref eq))
+               (do
+                  (call $caml_string_of_jsstring
+                     (call $wrap
+                       (br_on_null $end
+                          (call $readdir (call $unwrap (local.get $dir)))))))
+               (catch $javascript_exception
+                  (drop (pop externref))
+                  (call $throw_ebadf (@string "readdir"))
+                  (ref.i31 (i32.const 0))))))
+      (call $caml_raise_end_of_file)
+      (ref.i31 (i32.const 0)))
+
+   (func $unix_closedir (export "unix_closedir") (export "caml_unix_closedir")
+      (export "win_findclose") (export "caml_unix_findclose")
+      (param $dir (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $closedir (call $unwrap (local.get $dir))))
+         (catch $javascript_exception
+            (drop (pop externref))
+            (call $throw_ebadf (@string "closedir"))))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "unix_rewinddir") (export "caml_unix_rewinddir")
+      (param (ref eq)) (result (ref eq))
+      (call $caml_invalid_argument
+         (@string "rewinddir not implemented"))
+      (ref.i31 (i32.const 0)))
+))
+
+   (func (export "unix_readdir") (export "caml_unix_readdir")
+      (param $dir (ref eq)) (result (ref eq))
+      (block $return (result (ref eq))
+         (br_on_non_null $return (call $readdir_helper (local.get $dir)))
+         (call $caml_raise_end_of_file)
+         (ref.i31 (i32.const 0))))
+
+   (func $win_find_next (export "win_findnext") (export "caml_unix_findnext")
+      (param $dir (ref eq)) (result (ref eq))
+      (block $return (result (ref eq))
+         (br_on_non_null $return (call $readdir_helper (local.get $dir)))
+         (drop (call $unix_closedir (local.get $dir)))
+         (call $caml_raise_end_of_file)
+         (ref.i31 (i32.const 0))))
+
+   (func (export "win_findfirst") (export "caml_unix_findfirst")
+      (param $vpath (ref eq)) (result (ref eq))
+      (local $dir (ref eq)) (local $p (ref $bytes)) (local $p' (ref $bytes))
+      (local $len i32)
+      (local.set $p (ref.cast (ref $bytes) (local.get $vpath)))
+      (local.set $len (i32.sub (array.len (local.get $p)) (i32.const 3)))
+      (local.set $p' (array.new $bytes (i32.const 0) (local.get $len)))
+      (array.copy $bytes $bytes
+         (local.get $p') (i32.const 0)
+         (local.get $p) (i32.const 0)
+         (local.get $len))
+      (local.set $dir (call $unix_opendir (local.get $p')))
+      (array.new_fixed $block 3 (ref.i31 (i32.const 0))
+         (call $win_find_next (local.get $dir))
+         (local.get $dir)))
+
+(@if wasi
+(@then
+   (@string $unlink "unlink")
+
+   (func (export "unix_unlink") (export "caml_unix_unlink")
+      (param $path (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $unlink) (local.get $path)))
+      (local.set $res
+         (call $path_unlink_file
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $unlink) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_unlink") (export "caml_unix_unlink")
+      (param $p (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $unlink
+               (call $unwrap (call $caml_jsstring_of_string (local.get $p)))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $rmdir "rmdir")
+
+   (func (export "unix_rmdir") (export "caml_unix_rmdir")
+      (param $path (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $rmdir) (local.get $path)))
+      (local.set $res
+         (call $path_remove_directory
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $rmdir) (local.get $path))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_rmdir") (export "caml_unix_rmdir")
+      (param $p (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $rmdir
+               (call $unwrap (call $caml_jsstring_of_string (local.get $p)))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $link "link")
+
+   (func (export "unix_link") (export "caml_unix_link")
+      (param $follow (ref eq)) (param $o (ref eq)) (param $n (ref eq))
+      (result (ref eq))
+      (local $op (tuple i32 i32 i32))
+      (local $np (tuple i32 i32 i32))
+      (local $flags i32)
+      (local $res i32)
+      (local.set $op (call $unix_resolve_path (global.get $link) (local.get $o)))
+      (local.set $np (call $unix_resolve_path (global.get $link) (local.get $n)))
+      (if (ref.test (ref $block) (local.get $follow))
+         (then
+            (local.set $flags
+               (i31.get_u
+                  (ref.cast (ref i31)
+                     (array.get $block
+                        (ref.cast (ref $block) (local.get $follow))
+                        (i32.const 1)))))))
+      (local.set $res
+         (call $path_link
+            (tuple.extract 3 0 (local.get $op))
+            (local.get $flags)
+            (tuple.extract 3 1 (local.get $op))
+            (tuple.extract 3 2 (local.get $op))
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (tuple.extract 3 1 (local.get $op)))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $link) (local.get $o))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (data $link "link")
+
+   (func (export "unix_link") (export "caml_unix_link")
+      (param $follow (ref eq)) (param $d (ref eq)) (param $s (ref eq))
+      (result (ref eq))
+      (if (ref.test (ref $block) (local.get $follow))
+         (then
+            (throw $ocaml_exception
+               (array.new_fixed $block 5
+                  (ref.i31 (i32.const 0))
+                  (call $get_unix_error_exn)
+                  (ref.i31 (i32.const 25)) ;; ENOSYS
+                  (array.new_data $bytes $link (i32.const 0) (i32.const 4))
+                  (global.get $no_arg)))))
+      (try
+         (do
+            (call $link
+               (call $unwrap (call $caml_jsstring_of_string (local.get $d)))
+               (call $unwrap (call $caml_jsstring_of_string (local.get $s)))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+   (func (export "unix_has_symlink") (export "caml_unix_has_symlink")
+      (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 1)))
+
+(@if wasi
+(@then
+   (@string $symlink "symlink")
+
+   (func (export "unix_symlink") (export "caml_unix_symlink")
+      (param $to_dir (ref eq)) (param $o (ref eq)) (param $n (ref eq))
+      (result (ref eq))
+      (local $path (ref $bytes))
+      (local $len i32)
+      (local $op i32)
+      (local $np (tuple i32 i32 i32))
+      (local $flags i32)
+      (local $res i32)
+      (local.set $path (ref.cast (ref $bytes) (local.get $o)))
+      (local.set $len (array.len (local.get $path)))
+      (local.set $op
+         (call $write_string_to_memory
+            (i32.const 0) (i32.const 0) (local.get $path)))
+      (local.set $np
+         (call $unix_resolve_path (global.get $symlink) (local.get $n)))
+      (local.set $res
+         (call $path_symlink
+            (local.get $op)
+            (local.get $len)
+            (tuple.extract 3 0 (local.get $np))
+            (tuple.extract 3 1 (local.get $np))
+            (tuple.extract 3 2 (local.get $np))))
+      (call $free (local.get $op))
+      (call $free (tuple.extract 3 1 (local.get $np)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $symlink) (local.get $o))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_symlink") (export "caml_unix_symlink")
+      (param $to_dir (ref eq)) (param $t (ref eq)) (param $p (ref eq))
+      (result (ref eq))
+      (local $kind i32)
+      (if (ref.test (ref $block) (local.get $to_dir))
+         (then
+            (local.set $kind
+               (i32.add (i32.const 1)
+                  (i31.get_s
+                     (ref.cast (ref i31)
+                        (array.get $block
+                           (ref.cast (ref $block) (local.get $to_dir))
+                           (i32.const 0))))))))
+      (try
+         (do
+            (call $symlink
+               (call $unwrap (call $caml_jsstring_of_string (local.get $t)))
+               (call $unwrap (call $caml_jsstring_of_string (local.get $p)))
+               (local.get $kind)))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $readlink "readlink")
+
+   (func (export "unix_readlink") (export "caml_unix_readlink")
+      (param $path (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $buffer i32) (local $buf i32) (local $res i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $readlink) (local.get $path)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $buf (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $res
+         (call $path_readlink
+            (tuple.extract 3 0 (local.get $p))
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buf)
+            (global.get $IO_BUFFER_SIZE)
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $readlink) (local.get $path))))
+      (return_call $blit_memory_to_string
+         (local.get $buf) (i32.load (local.get $buffer))))
+)
+(@else
+   (func (export "unix_readlink") (export "caml_unix_readlink")
+      (param $path (ref eq)) (result (ref eq))
+      (try
+         (do
+            (return_call $caml_string_of_jsstring
+               (call $wrap
+                  (call $readlink
+                     (call $unwrap
+                        (call $caml_jsstring_of_string (local.get $path)))))))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $truncate "truncate")
+
+   (func $truncate (param $path (ref eq)) (param $len i64) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $fd i32) (local $res i32) (local $buffer i32)
+      (block $error
+         (local.set $p
+            (call $unix_resolve_path (global.get $truncate) (local.get $path)))
+         (local.set $buffer (call $get_buffer))
+         (local.set $res
+            (call $path_open
+               (tuple.extract 3 0 (local.get $p))
+               (i32.const 1) ;; symlink_follow
+               (tuple.extract 3 1 (local.get $p))
+               (tuple.extract 3 2 (local.get $p))
+               (i32.const 0)
+               (i64.const 0x400000) ;; allow fd_filestat_set_size
+               (i64.const 0)
+               (i32.const 0)
+               (local.get $buffer)))
+         (call $free (tuple.extract 3 1 (local.get $p)))
+         (br_if $error (local.get $res))
+         (local.set $fd (i32.load (local.get $buffer)))
+         (local.set $res
+            (call $fd_filestat_set_size (local.get $fd) (local.get $len)))
+         (if (local.get $res)
+            (then
+               (drop (call $fd_close (local.get $fd)))
+               (br $error)))
+         (local.set $res (call $fd_close (local.get $fd)))
+         (br_if $error (local.get $res))
+         (return (ref.i31 (i32.const 0))))
+      (call $caml_unix_error
+         (local.get $res) (global.get $truncate) (local.get $path))
+      (return (ref.i31 (i32.const 0))))
 
+   (func (export "unix_truncate") (export "caml_unix_truncate")
+      (param $path (ref eq)) (param $len (ref eq))
+      (result (ref eq))
+      (return_call $truncate (local.get $path)
+         (i64.extend_i32_s
+            (i31.get_s (ref.cast (ref i31) (local.get $len))))))
+
+   (func (export "unix_truncate_64") (export "caml_unix_truncate_64")
+      (param $path (ref eq)) (param $len (ref eq))
+      (result (ref eq))
+      (return_call $truncate (local.get $path)
+         (call $Int64_val (local.get $len))))
+)
+(@else
    (func (export "unix_truncate") (export "caml_unix_truncate")
       (param $path (ref eq)) (param $len (ref eq))
       (result (ref eq))
@@ -410,7 +1608,33 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (func $ftruncate (param $vfd (ref eq)) (param $len i64) (result (ref eq))
+      (local $fd i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $res
+         (call $fd_filestat_set_size (local.get $fd) (local.get $len)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "ftruncate") (global.get $no_arg))))
+      (ref.i31 (i32.const 0)))
+
+   (func (export "unix_ftruncate") (export "caml_unix_ftruncate")
+      (param $fd (ref eq)) (param $len (ref eq)) (result (ref eq))
+      (return_call $ftruncate (local.get $fd)
+         (i64.extend_i32_s
+            (i31.get_s (ref.cast (ref i31) (local.get $len))))))
 
+   (func (export "unix_ftruncate_64") (export "caml_unix_ftruncate_64")
+      (param $fd (ref eq)) (param $len (ref eq)) (result (ref eq))
+      (return_call $ftruncate (local.get $fd)
+         (call $Int64_val (local.get $len))))
+)
+(@else
    (func (export "unix_ftruncate") (export "caml_unix_ftruncate")
       (param $fd (ref eq)) (param $vlen (ref eq))
       (result (ref eq))
@@ -459,8 +1683,116 @@
              (struct.set $fd_offset $offset (local.get $fd_offset)
                 (local.get $len))))
       (ref.i31 (i32.const 0)))
+))
+
+(@if wasi
+(@then
+   (@string $access "access")
+
+   ;; We can only check that the file exists
+   (func (export "unix_access") (export "caml_unix_access")
+      (param $path (ref eq)) (param $flags (ref eq)) (result (ref eq))
+      (local $p (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $p
+         (call $unix_resolve_path (global.get $access) (local.get $path)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $path_filestat_get
+            (tuple.extract 3 0 (local.get $p))
+            (i32.const 1)
+            (tuple.extract 3 1 (local.get $p))
+            (tuple.extract 3 2 (local.get $p))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $p)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $access) (local.get $path))))
+      (return (ref.i31 (i32.const 0))))
+)
+(@else
+   (global $access_flags (ref $flags)
+      (array.new_fixed $flags 4
+         (i32.const 1) (i32.const 2) (i32.const 4) (i32.const 8)))
+
+   (func (export "unix_access") (export "caml_unix_access")
+      (param $path (ref eq)) (param $vflags (ref eq)) (result (ref eq))
+      (local $flags i32)
+      (local.set $flags
+         (call $convert_flag_list
+            (global.get $access_flags) (local.get $vflags)))
+      (try
+         (do
+            (call $access
+               (call $unwrap (call $caml_jsstring_of_string (local.get $path)))
+               (local.get $flags)))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+   (type $flags (array i16))
+
+(@if wasi
+(@then
+   ;;    0x1 O_RDONLY
+   ;;    0x2 O_WRONLY
+   ;;    0x3 O_RDWR
+   ;;  0x400 O_NONBLOCK
+   ;;  0x100 O_APPEND
+   ;;   0x10 O_CREAT
+   ;;   0x80 O_TRUNC
+   ;;   0x40 O_EXCL
+   ;;      0 O_NOCTTY
+   ;;  0x200 O_DSYNC
+   ;; 0x1000 O_SYNC
+   ;;  0x800 O_RSYNC
+   (global $unix_open_flags (ref $flags)
+      (array.new_fixed $flags 15
+         (i32.const 1) (i32.const 2) (i32.const 3) (i32.const 0x400)
+         (i32.const 0x100) (i32.const 0x10) (i32.const 0x80) (i32.const 0x40)
+         (i32.const 0) (i32.const 0x200) (i32.const 0x1000) (i32.const 0x800)
+         (i32.const 0) (i32.const 0) (i32.const 0)))
+
+   (@string $open "open")
 
-   (type $open_flags (array i8))
+   (func (export "unix_open") (export "caml_unix_open")
+      (param $vpath (ref eq)) (param $vflags (ref eq)) (param $perm (ref eq))
+      (result (ref eq))
+      (local $flags i32) (local $offset i64)
+      (local $path (tuple i32 i32 i32))
+      (local $res i32) (local $buffer i32)
+      (local.set $path
+         (call $unix_resolve_path (global.get $open) (local.get $vpath)))
+      (local.set $buffer (call $get_buffer))
+      (local.set $flags
+         (call $convert_flag_list
+            (global.get $unix_open_flags) (local.get $vflags)))
+      (local.set $res
+         (call $path_open
+            (tuple.extract 3 0 (local.get $path))
+            (i32.const 1) ;; symlink_follow
+            (tuple.extract 3 1 (local.get $path))
+            (tuple.extract 3 2 (local.get $path))
+            (i32.and (i32.shr_u (local.get $flags) (i32.const 4))
+              (i32.const 0xF))
+            (select
+               (i64.const 0x860007e)
+               (select (i64.const 0x860007c) (i64.const 0x820003e)
+                  (i32.and (local.get $flags) (i32.const 2)))
+               (i32.eq (i32.and (local.get $flags) (i32.const 3)) (i32.const 3)))
+            (i64.const 0)
+            (i32.shr_u (local.get $flags) (i32.const 8))
+            (local.get $buffer)))
+      (call $free (tuple.extract 3 1 (local.get $path)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (global.get $open) (local.get $vpath))))
+      (ref.i31 (i32.load (local.get $buffer))))
+)
+(@else
    ;;    1 O_RDONLY
    ;;    2 O_WRONLY
    ;;    4 O_RDWR
@@ -472,8 +1804,8 @@
    ;;  256 O_NOCTTY
    ;;  512 O_DSYNC
    ;; 1024 O_SYNC
-   (global $unix_open_flags (ref $open_flags)
-      (array.new_fixed $open_flags 15
+   (global $unix_open_flags (ref $flags)
+      (array.new_fixed $flags 15
          (i32.const 1) (i32.const 2) (i32.const 4) (i32.const 128)
          (i32.const 8) (i32.const 16) (i32.const 32) (i32.const 64)
          (i32.const 256) (i32.const 512) (i32.const 1024) (i32.const 0)
@@ -500,6 +1832,7 @@
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (call $initialize_fd_offset (local.get $fd) (local.get $offset))
       (ref.i31 (local.get $fd)))
+))
 
    (global $io_buffer (mut externref) (ref.null extern))
 
@@ -515,6 +1848,217 @@
             (br_on_null $null (call $get_fd_offset_unchecked (local.get $fd)))))
       (struct.new $fd_offset (i64.const 0) (i32.const 0)))
 
+(@if wasi
+(@then
+   (func (export "unix_write") (export "caml_unix_write")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $fd i32) (local $s (ref $bytes))
+      (local $pos i32) (local $len i32) (local $numbytes i32)
+      (local $written i32) (local $n i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vbuf)))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (local.set $iovs_len (i32.const 1))
+      (loop $loop
+         (if (i32.gt_u (local.get $len) (i32.const 0))
+            (then
+               (local.set $numbytes
+                  (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+                     (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+               (call $blit_substring_to_memory
+                  (local.get $buffer) (local.get $s) (local.get $pos)
+                  (local.get $numbytes))
+               (i32.store offset=4 (local.get $iovs) (local.get $numbytes))
+               (local.set $res
+                  (call $fd_write
+                      (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+                      (local.get $nwritten)))
+               (if (local.get $res)
+                  (then
+                     (call $caml_unix_error (local.get $res) (@string "write")
+                        (global.get $no_arg))))
+               (local.set $n (i32.load (local.get $nwritten)))
+               (local.set $written (i32.add (local.get $written) (local.get $n)))
+               (local.set $pos (i32.add (local.get $pos) (local.get $n)))
+               (local.set $len (i32.sub (local.get $len) (local.get $n)))
+               (br $loop))))
+      (ref.i31 (local.get $n)))
+
+   (func (export "unix_single_write") (export "caml_unix_single_write")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nwritten i32)
+      (local $fd i32) (local $s (ref $bytes))
+      (local $pos i32) (local $len i32) (local $numbytes i32)
+      (local $written i32) (local $n i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vbuf)))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (if (i32.eqz (local.get $len))
+         (then (return (ref.i31 (i32.const 0)))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $numbytes
+         (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+            (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+      (call $blit_substring_to_memory
+         (local.get $buffer) (local.get $s) (local.get $pos)
+         (local.get $numbytes))
+      (i32.store offset=4 (local.get $iovs) (local.get $numbytes))
+      (local.set $res
+         (call $fd_write
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nwritten)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error (local.get $res) (@string "write")
+               (global.get $no_arg))))
+      (ref.i31 (i32.load (local.get $nwritten))))
+
+   (func (export "unix_read") (export "caml_unix_read")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $buffer i32) (local $res i32)
+      (local $iovs i32) (local $iovs_len i32) (local $nread i32)
+      (local $fd i32) (local $pos i32) (local $len i32) (local $n i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (if (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))
+         (then
+            (local.set $len (global.get $IO_BUFFER_SIZE))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nread (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $len))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $res
+         (call $fd_read
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nread)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error (local.get $res) (@string "read")
+               (global.get $no_arg))))
+      (local.set $n (i32.load (local.get $nread)))
+      (call $blit_memory_to_substring (local.get $buffer)
+         (ref.cast (ref $bytes) (local.get $vbuf))
+         (local.get $pos) (local.get $n))
+      (ref.i31 (local.get $n)))
+
+   (type $data
+      (struct
+         (field $array (ref array))
+         (field $offset i32)
+         (field $len i32)))
+
+   (func (export "unix_write_bigarray") (export "caml_unix_write_bigarray")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (param $vsingle (ref eq)) (result (ref eq))
+      (local $fd i32) (local $data (ref $data)) (local $buf (ref $bytes))
+      (local $pos i32) (local $len i32) (local $n i32) (local $written i32)
+      (local $buffer i32) (local $nwritten i32) (local $iovs i32)
+      (local $iovs_len i32) (local $numbytes i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $data
+         (ref.cast (ref $data)
+            (any.convert_extern (call $caml_ba_get_data (local.get $vbuf)))))
+      (local.set $buf
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $pos
+          (i32.add (i31.get_u (ref.cast (ref i31) (local.get $vpos)))
+             (struct.get $data $offset (local.get $data))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nwritten (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (local.set $iovs_len (i32.const 1))
+      (loop $loop
+         (if (i32.gt_u (local.get $len) (i32.const 0))
+            (then
+               (local.set $numbytes
+                  (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+                     (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+               (call $blit_substring_to_memory
+                  (local.get $buffer) (local.get $buf) (local.get $pos)
+                  (local.get $numbytes))
+               (i32.store offset=4 (local.get $iovs) (local.get $numbytes))
+               (local.set $res
+                  (call $fd_write
+                      (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+                      (local.get $nwritten)))
+               (if (local.get $res)
+                  (then
+                     (call $caml_unix_error (local.get $res) (@string "write")
+                        (global.get $no_arg))))
+               (local.set $n (i32.load (local.get $nwritten)))
+               (local.set $written (i32.add (local.get $written) (local.get $n)))
+               (local.set $pos (i32.add (local.get $pos) (local.get $n)))
+               (local.set $len (i32.sub (local.get $len) (local.get $n)))
+               (br_if $loop
+                  (ref.eq (local.get $vsingle) (ref.i31 (i32.const 0)))))))
+      (ref.i31 (local.get $written)))
+
+   (func (export "unix_read_bigarray") (export "caml_unix_read_bigarray")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $fd i32) (local $data (ref $data)) (local $buf (ref $bytes))
+      (local $pos i32) (local $len i32) (local $n i32)
+      (local $buffer i32) (local $nread i32) (local $iovs i32)
+      (local $iovs_len i32) (local $res i32)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $data
+         (ref.cast (ref $data)
+            (any.convert_extern (call $caml_ba_get_data (local.get $vbuf)))))
+      (local.set $buf
+         (ref.cast (ref $bytes) (struct.get $data $array (local.get $data))))
+      (local.set $pos
+          (i32.add (i31.get_u (ref.cast (ref i31) (local.get $vpos)))
+             (struct.get $data $offset (local.get $data))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (if (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))
+         (then
+            (local.set $len (global.get $IO_BUFFER_SIZE))))
+      (local.set $buffer (call $get_buffer))
+      (local.set $nread (local.get $buffer))
+      (local.set $iovs (i32.add (local.get $buffer) (i32.const 4)))
+      (local.set $buffer (i32.add (local.get $buffer) (i32.const 12)))
+      (i32.store (local.get $iovs) (local.get $buffer))
+      (i32.store offset=4 (local.get $iovs) (local.get $len))
+      (local.set $iovs_len (i32.const 1))
+      (local.set $res
+         (call $fd_read
+             (local.get $fd) (local.get $iovs) (local.get $iovs_len)
+             (local.get $nread)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error (local.get $res) (@string "read")
+               (global.get $no_arg))))
+      (local.set $n (i32.load (local.get $nread)))
+      (call $blit_memory_to_substring (local.get $buffer)
+         (local.get $buf) (local.get $pos) (local.get $n))
+      (ref.i31 (local.get $n)))
+)
+(@else
    (func (export "unix_write") (export "caml_unix_write")
       (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
       (param $vlen (ref eq)) (result (ref eq))
@@ -566,6 +2110,51 @@
                (br $loop))))
       (ref.i31 (local.get $n)))
 
+   (func (export "unix_single_write") (export "caml_unix_single_write")
+      (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $fd i32) (local $s (ref $bytes)) (local $buf (ref extern))
+      (local $pos i32) (local $len i32) (local $numbytes i32)
+      (local $written i32) (local $n i32)
+      (local $fd_offset (ref $fd_offset))
+      (local $offset i64)
+      (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vbuf)))
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (local.set $buf (call $get_io_buffer))
+      (local.set $fd_offset (call $get_fd_offset (local.get $fd)))
+      (local.set $offset
+         (struct.get $fd_offset $offset (local.get $fd_offset)))
+      (if (i32.gt_u (local.get $len) (i32.const 0))
+         (then
+            (local.set $numbytes
+               (select (global.get $IO_BUFFER_SIZE) (local.get $len)
+                  (i32.gt_u (local.get $len) (global.get $IO_BUFFER_SIZE))))
+            (call $ta_blit_from_bytes
+               (local.get $s) (local.get $pos)
+               (local.get $buf) (i32.const 0) (local.get $numbytes))
+            (try
+               (do
+                  (local.set $n
+                     (if (result i32)
+                         (struct.get $fd_offset $seeked (local.get $fd_offset))
+                        (then
+                           (call $write (local.get $fd) (local.get $buf)
+                              (i32.const 0) (local.get $numbytes)
+                              (local.get $offset)))
+                        (else
+                           (call $write' (local.get $fd) (local.get $buf)
+                              (i32.const 0) (local.get $numbytes)
+                              (ref.null extern))))))
+               (catch $javascript_exception
+                  (call $caml_unix_error (pop externref) (ref.null eq))))
+            (local.set $offset
+               (i64.add (local.get $offset) (i64.extend_i32_u (local.get $n))))
+            (struct.set $fd_offset $offset
+               (local.get $fd_offset) (local.get $offset))))
+      (ref.i31 (local.get $n)))
+
    (func (export "unix_read") (export "caml_unix_read")
       (param $vfd (ref eq)) (param $vbuf (ref eq)) (param $vpos (ref eq))
       (param $vlen (ref eq)) (result (ref eq))
@@ -596,7 +2185,7 @@
       (call $ta_blit_to_bytes
          (local.get $buf) (i32.const 0)
          (ref.cast (ref $bytes) (local.get $vbuf)) (local.get $pos)
-         (local.get $len))
+         (local.get $n))
       (ref.i31 (local.get $n)))
 
    (func (export "unix_write_bigarray") (export "caml_unix_write_bigarray")
@@ -669,27 +2258,48 @@
       (struct.set $fd_offset $offset (local.get $fd_offset)
          (i64.add (local.get $offset) (i64.extend_i32_s (local.get $n))))
       (ref.i31 (local.get $n)))
+))
 
-   (data $lseek "lseek")
+(@if wasi
+(@then
+   (func $lseek
+      (param $fd (ref eq)) (param $offset i64) (param $cmd (ref eq))
+      (result i64)
+      (local $res i32) (local $buffer i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_seek
+            (i31.get_u (ref.cast (ref i31) (local.get $fd)))
+            (local.get $offset)
+            (i31.get_u (ref.cast (ref i31) (local.get $cmd)))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "lseek") (global.get $no_arg))))
+      (i64.load (local.get $buffer)))
+)
+(@else
+   (@string $lseek "lseek")
 
    (func $lseek_exn (param $errno i32) (result (ref eq))
       (array.new_fixed $block 5
          (ref.i31 (i32.const 0))
          (call $get_unix_error_exn)
          (ref.i31 (local.get $errno))
-         (array.new_data $bytes $lseek (i32.const 0) (i32.const 5))
+         (global.get $lseek)
          (global.get $no_arg)))
 
    (func $lseek
       (param $vfd (ref eq)) (param $offset i64) (param $cmd (ref eq))
-      (result (ref eq))
+      (result i64)
       (local $fd i32) (local $fd_offset (ref $fd_offset))
       (local.set $fd (i31.get_u (ref.cast (ref i31) (local.get $vfd))))
       (local.set $fd_offset
          (block $non_null (result (ref $fd_offset))
             (br_on_non_null $non_null
                (call $get_fd_offset_unchecked (local.get $fd)))
-            (throw $ocaml_exception (call $lseek_exn (i32.const 9))))) ;; EBADF
+            (throw $ocaml_exception (call $lseek_exn (i32.const 3))))) ;; EBADF
       (if (ref.eq (local.get $cmd) (ref.i31 (i32.const 1)))
          (then
             (local.set $offset
@@ -706,36 +2316,88 @@
                 (call $lseek_exn (i32.const 12))))) ;; EINVAL
       (struct.set $fd_offset $offset (local.get $fd_offset) (local.get $offset))
       (struct.set $fd_offset $seeked (local.get $fd_offset) (i32.const 1))
-      (ref.i31 (i32.const 0)))
+      (local.get $offset))
+))
 
    (func (export "unix_lseek") (export "caml_unix_lseek")
       (param $fd (ref eq)) (param $ofs (ref eq)) (param $cmd (ref eq))
       (result (ref eq))
-      (return_call $lseek
-         (local.get $fd)
-         (i64.extend_i32_s (i31.get_s (ref.cast (ref i31) (local.get $ofs))))
-         (local.get $cmd)))
+      (ref.i31
+         (i32.wrap_i64
+            (call $lseek
+               (local.get $fd)
+               (i64.extend_i32_s
+                  (i31.get_s (ref.cast (ref i31) (local.get $ofs))))
+               (local.get $cmd)))))
 
    (func (export "unix_lseek_64") (export "caml_unix_lseek_64")
       (param $fd (ref eq)) (param $ofs (ref eq)) (param $cmd (ref eq))
       (result (ref eq))
-      (return_call $lseek
-         (local.get $fd)
-         (call $Int64_val (local.get $ofs))
-         (local.get $cmd)))
+      (return_call $caml_copy_int64
+         (call $lseek
+            (local.get $fd)
+            (call $Int64_val (local.get $ofs))
+            (local.get $cmd))))
 
-   (data $out_channel_of_descr "out_channel_of_descr")
-   (data $in_channel_of_descr "in_channel_of_descr")
+(@if wasi
+(@then
+   (func (export "unix_fsync") (export "caml_unix_fsync")
+      (param $fd (ref eq)) (result (ref eq))
+      (local $res i32)
+      (local.set $res
+         (call $fd_sync (i31.get_u (ref.cast (ref i31) (local.get $fd)))))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "fsync") (global.get $no_arg))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
+   (func (export "unix_fsync") (export "caml_unix_fsync")
+      (param $fd (ref eq)) (result (ref eq))
+      (try
+         (do
+            (call $fsync (local.get $fd)))
+         (catch $javascript_exception
+            (call $caml_unix_error (pop externref) (ref.null eq))))
+      (ref.i31 (i32.const 0)))
+))
+
+   (@string $out_channel_of_descr "out_channel_of_descr")
+   (@string $in_channel_of_descr "in_channel_of_descr")
 
    (func $channel_of_descr_name (param $out i32) (result (ref eq))
-      (if (result (ref eq)) (local.get $out)
-         (then
-            (array.new_data $bytes $out_channel_of_descr
-               (i32.const 0) (i32.const 20)))
-         (else
-            (array.new_data $bytes $in_channel_of_descr
-               (i32.const 0) (i32.const 19)))))
+      (select (result (ref eq))
+         (global.get $out_channel_of_descr)
+         (global.get $in_channel_of_descr)
+         (local.get $out)))
 
+(@if wasi
+(@then
+   (func $caml_unix_check_stream_semantics (param $fd (ref eq)) (param $out i32)
+      (local $s (ref $block)) (local $kind i32)
+      (local $buffer i32) (local $res i32) (local $file_type i32)
+      (local.set $buffer (call $get_buffer))
+      (local.set $res
+         (call $fd_filestat_get
+            (i31.get_u (ref.cast (ref i31) (local.get $fd)))
+            (local.get $buffer)))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res)
+               (call $channel_of_descr_name (local.get $out))
+               (global.get $no_arg))))
+      (local.set $kind (i32.load8_u offset=16 (local.get $buffer)))
+      (block $ok
+         (block $bad
+            (br_table $bad $bad $ok $bad $ok $bad $ok $bad (local.get $kind)))
+         (call $caml_unix_error
+            (i32.const 28) (; EINVAL ;)
+            (call $channel_of_descr_name (local.get $out))
+            (global.get $no_arg))))
+)
+(@else
    (func $caml_unix_check_stream_semantics (param $fd (ref eq)) (param $out i32)
       (local $s (ref $block)) (local $kind i32)
       (local.set $s
@@ -761,6 +2423,7 @@
                (ref.i31 (i32.const 12)) ;; EINVAL
                (call $channel_of_descr_name (local.get $out))
                (global.get $no_arg)))))
+))
 
    (func (export "unix_inchannel_of_filedescr")
       (export "win_inchannel_of_filedescr")
@@ -776,6 +2439,20 @@
       (call $caml_unix_check_stream_semantics (local.get $fd) (i32.const 1))
       (return_call $caml_ml_open_descriptor_out (local.get $fd)))
 
+(@if wasi
+(@then
+   (func (export "unix_close") (export "caml_unix_close")
+      (param $fd (ref eq)) (result (ref eq))
+      (local $res i32)
+      (local.set $res
+         (call $fd_close (i31.get_u (ref.cast (ref i31) (local.get $fd)))))
+      (if (local.get $res)
+         (then
+            (call $caml_unix_error
+               (local.get $res) (@string "close") (global.get $no_arg))))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (func (export "unix_close") (export "caml_unix_close")
       (param $fd (ref eq)) (result (ref eq))
       (call $release_fd_offset (i31.get_u (ref.cast (ref i31) (local.get $fd))))
@@ -785,19 +2462,43 @@
          (catch $javascript_exception
             (call $caml_unix_error (pop externref) (ref.null eq))))
       (ref.i31 (i32.const 0)))
+))
 
+(@if wasi
+(@then
+   (func (export "unix_isatty") (export "caml_unix_isatty")
+      (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 0)))
+)
+(@else
    (export "unix_isatty" (func $isatty))
    (export "caml_unix_isatty" (func $isatty))
+))
 
-   (export "caml_unix_inet_addr_of_string" (func $unix_inet_addr_of_string))
-   (func $unix_inet_addr_of_string (export "unix_inet_addr_of_string")
+   (func (export "unix_getuid") (export "caml_unix_getuid")
+      (export "unix_geteuid") (export "caml_unix_geteuid")
+      (export "unix_getgid") (export "caml_unix_getgid")
+      (param (ref eq)) (result (ref eq))
+      (ref.i31 (i32.const 1)))
+
+   (func (export "unix_getpwnam") (export "caml_unix_getpwnam")
+      (export "unix_getpwuid") (export "caml_unix_getpwuid")
+      (export "unix_getgrnam") (export "caml_unix_getgrnam")
+      (export "unix_getgruid") (export "caml_unix_getgruid")
+      (param (ref eq)) (result (ref eq))
+      (call $caml_raise_not_found)
+      (ref.i31 (i32.const 0)))
+
+   (func (export "unix_inet_addr_of_string")
+      (export "caml_unix_inet_addr_of_string")
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
-   (export "caml_unix_filedescr_of_fd" (func $unix_filedescr_of_fd))
-   (func $unix_filedescr_of_fd (export "unix_filedescr_of_fd")
+
+   (func (export "win_handle_fd") (export "caml_unix_filedescr_of_fd")
       (param (ref eq)) (result (ref eq))
       (local.get 0))
-   (func $unix_cleanup (export "caml_unix_cleanup")
+
+   (func (export "win_cleanup") (export "caml_unix_cleanup")
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 )
diff --git a/runtime/wasm/version-dependent/post-5.1.wat b/runtime/wasm/version-dependent/post-5.1.wat
deleted file mode 100644
index 258505a5e9..0000000000
--- a/runtime/wasm/version-dependent/post-5.1.wat
+++ /dev/null
@@ -1,46 +0,0 @@
-;; Wasm_of_ocaml runtime support
-;; http://www.ocsigen.org/js_of_ocaml/
-;;
-;; This program is free software; you can redistribute it and/or modify
-;; it under the terms of the GNU Lesser General Public License as published by
-;; the Free Software Foundation, with linking exception;
-;; either version 2.1 of the License, or (at your option) any later version.
-;;
-;; This program is distributed in the hope that it will be useful,
-;; but WITHOUT ANY WARRANTY; without even the implied warranty of
-;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-;; GNU Lesser General Public License for more details.
-;;
-;; You should have received a copy of the GNU Lesser General Public License
-;; along with this program; if not, write to the Free Software
-;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
-
-(module
-   (import "obj" "caml_callback_1"
-      (func $caml_callback_1
-         (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "sync" "caml_ml_mutex_unlock"
-      (func $caml_ml_mutex_unlock (param (ref eq)) (result (ref eq))))
-   (import "domain" "caml_domain_latest_id"
-     (global $caml_domain_latest_id (mut i32)))
-   (import "domain" "caml_domain_id"
-     (global $caml_domain_id (mut i32)))
-
-   (func (export "caml_runtime_events_user_write")
-      (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.const 0)))
-
-   (func (export "caml_domain_spawn")
-      (param $f (ref eq)) (param $mutex (ref eq)) (result (ref eq))
-      (local $id i32) (local $old i32)
-      (local.set $id (global.get $caml_domain_latest_id))
-      (global.set $caml_domain_latest_id
-         (i32.add (local.get $id) (i32.const 1)))
-      (local.set $old (global.get $caml_domain_id))
-      (drop (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
-      (global.set $caml_domain_id (local.get $old))
-      (drop (call $caml_ml_mutex_unlock (local.get $mutex)))
-      (ref.i31 (local.get $id)))
-
-   (global (export "caml_marshal_header_size") i32 (i32.const 16))
-)
diff --git a/runtime/wasm/version-dependent/post-5.2.wat b/runtime/wasm/version-dependent/post-5.2.wat
deleted file mode 100644
index b4183d2dcb..0000000000
--- a/runtime/wasm/version-dependent/post-5.2.wat
+++ /dev/null
@@ -1,59 +0,0 @@
-;; Wasm_of_ocaml runtime support
-;; http://www.ocsigen.org/js_of_ocaml/
-;;
-;; This program is free software; you can redistribute it and/or modify
-;; it under the terms of the GNU Lesser General Public License as published by
-;; the Free Software Foundation, with linking exception;
-;; either version 2.1 of the License, or (at your option) any later version.
-;;
-;; This program is distributed in the hope that it will be useful,
-;; but WITHOUT ANY WARRANTY; without even the implied warranty of
-;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-;; GNU Lesser General Public License for more details.
-;;
-;; You should have received a copy of the GNU Lesser General Public License
-;; along with this program; if not, write to the Free Software
-;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
-
-(module
-   (type $block (array (mut (ref eq))))
-   (import "obj" "caml_callback_1"
-      (func $caml_callback_1
-         (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "sync" "caml_ml_mutex_unlock"
-      (func $caml_ml_mutex_unlock (param (ref eq)) (result (ref eq))))
-   (import "domain" "caml_domain_latest_id"
-     (global $caml_domain_latest_id (mut i32)))
-   (import "domain" "caml_domain_id"
-     (global $caml_domain_id (mut i32)))
-
-   (func (export "caml_runtime_events_user_write")
-      (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.const 0)))
-
-   (func (export "caml_domain_spawn")
-      (param $f (ref eq)) (param $term_sync_v (ref eq)) (result (ref eq))
-      (local $id i32) (local $old i32) (local $ts (ref $block)) (local $res (ref eq))
-      (local.set $id (global.get $caml_domain_latest_id))
-      (global.set $caml_domain_latest_id
-         (i32.add (local.get $id) (i32.const 1)))
-      (local.set $old (global.get $caml_domain_id))
-      (local.set $res
-         (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
-      (global.set $caml_domain_id (local.get $old))
-      (local.set $ts (ref.cast (ref $block) (local.get $term_sync_v)))
-      (drop (call $caml_ml_mutex_unlock (array.get $block (local.get $ts) (i32.const 2))))
-      ;; TODO: fix exn case
-      (array.set
-         $block
-         (local.get $ts)
-         (i32.const 1)
-         (array.new_fixed
-            $block
-            2
-            (ref.i31 (i32.const 0))
-            (array.new_fixed $block 2 (ref.i31 (i32.const 0)) (local.get $res))))
-      (ref.i31 (local.get $id)))
-
-   (global (export "caml_marshal_header_size") i32 (i32.const 16))
-)
diff --git a/runtime/wasm/version-dependent/pre-5.1.wat b/runtime/wasm/version-dependent/pre-5.1.wat
deleted file mode 100644
index cc23b90ad7..0000000000
--- a/runtime/wasm/version-dependent/pre-5.1.wat
+++ /dev/null
@@ -1,46 +0,0 @@
-;; Wasm_of_ocaml runtime support
-;; http://www.ocsigen.org/js_of_ocaml/
-;;
-;; This program is free software; you can redistribute it and/or modify
-;; it under the terms of the GNU Lesser General Public License as published by
-;; the Free Software Foundation, with linking exception;
-;; either version 2.1 of the License, or (at your option) any later version.
-;;
-;; This program is distributed in the hope that it will be useful,
-;; but WITHOUT ANY WARRANTY; without even the implied warranty of
-;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-;; GNU Lesser General Public License for more details.
-;;
-;; You should have received a copy of the GNU Lesser General Public License
-;; along with this program; if not, write to the Free Software
-;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
-
-(module
-   (import "obj" "caml_callback_1"
-      (func $caml_callback_1
-         (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "sync" "caml_ml_mutex_unlock"
-      (func $caml_ml_mutex_unlock (param (ref eq)) (result (ref eq))))
-   (import "domain" "caml_domain_latest_id"
-     (global $caml_domain_latest_id (mut i32)))
-   (import "domain" "caml_domain_id"
-     (global $caml_domain_id (mut i32)))
-
-   (func (export "caml_runtime_events_user_write")
-      (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.const 0)))
-
-   (func (export "caml_domain_spawn")
-      (param $f (ref eq)) (param $mutex (ref eq)) (result (ref eq))
-      (local $id i32) (local $old i32)
-      (local.set $id (global.get $caml_domain_latest_id))
-      (global.set $caml_domain_latest_id
-         (i32.add (local.get $id) (i32.const 1)))
-      (local.set $old (global.get $caml_domain_id))
-      (drop (call $caml_callback_1 (local.get $f) (ref.i31 (i32.const 0))))
-      (global.set $caml_domain_id (local.get $old))
-      (drop (call $caml_ml_mutex_unlock (local.get $mutex)))
-      (ref.i31 (local.get $id)))
-
-   (global (export "caml_marshal_header_size") i32 (i32.const 20))
-)
diff --git a/runtime/wasm/wasi_errors.wat b/runtime/wasm/wasi_errors.wat
new file mode 100644
index 0000000000..577fb410fa
--- /dev/null
+++ b/runtime/wasm/wasi_errors.wat
@@ -0,0 +1,86 @@
+(module
+(@if wasi
+(@then
+   (type $block (array (mut (ref eq))))
+   (type $bytes (array (mut i8)))
+   (global (export "error_messages") (ref $block)
+      (array.new_fixed $block 77
+         (@string "Success")
+         (@string "Argument list too long")
+         (@string "Permission denied")
+         (@string "Address in use")
+         (@string "Address not available")
+         (@string "Address family not supported")
+         (@string "Resource unavailable, or operation would block")
+         (@string "Connection already in progress")
+         (@string "Bad file descriptor")
+         (@string "Bad message")
+         (@string "Device or resource busy")
+         (@string "Operation canceled")
+         (@string "No child processes")
+         (@string "Connection aborted")
+         (@string "Connection refused")
+         (@string "Connection reset")
+         (@string "Resource deadlock would occur")
+         (@string "Destination address required")
+         (@string "Mathematics argument out of domain of function")
+         (@string "Reserved")
+         (@string "File exists")
+         (@string "Bad address")
+         (@string "File too large")
+         (@string "Host is unreachable")
+         (@string "Identifier removed")
+         (@string "Illegal byte sequence")
+         (@string "Operation in progress")
+         (@string "Interrupted function")
+         (@string "Invalid argument")
+         (@string "I/O error")
+         (@string "Socket is connected")
+         (@string "Is a directory")
+         (@string "Too many levels of symbolic links")
+         (@string "File descriptor value too large")
+         (@string "Too many links")
+         (@string "Message too large")
+         (@string "Reserved")
+         (@string "Filename too long")
+         (@string "Network is down")
+         (@string "Connection aborted by network")
+         (@string "Network unreachable")
+         (@string "Too many files open in system")
+         (@string "No buffer space available")
+         (@string "No such device")
+         (@string "No such file or directory")
+         (@string "Executable file format error")
+         (@string "No locks available")
+         (@string "Reserved")
+         (@string "Not enough space")
+         (@string "No message of the desired type")
+         (@string "Protocol not available")
+         (@string "No space left on device")
+         (@string "Function not supported")
+         (@string "The socket is not connected")
+         (@string "Not a directory or a symbolic link to a directory")
+         (@string "Directory not empty")
+         (@string "State not recoverable")
+         (@string "Not a socket")
+         (@string "Not supported, or operation not supported on socket")
+         (@string "Inappropriate I/O control operation")
+         (@string "No such device or address")
+         (@string "Value too large to be stored in data type")
+         (@string "Previous owner died")
+         (@string "Operation not permitted")
+         (@string "Broken pipe")
+         (@string "Protocol error")
+         (@string "Protocol not supported")
+         (@string "Protocol wrong type for socket")
+         (@string "Result too large")
+         (@string "Read-only file system")
+         (@string "Invalid seek")
+         (@string "No such process")
+         (@string "Reserved")
+         (@string "Connection timed out")
+         (@string "Text file busy")
+         (@string "Cross-device link")
+         (@string "Capabilities insufficient")))
+))
+)
diff --git a/runtime/wasm/wasi_memory.wat b/runtime/wasm/wasi_memory.wat
new file mode 100644
index 0000000000..0e737a46db
--- /dev/null
+++ b/runtime/wasm/wasi_memory.wat
@@ -0,0 +1,98 @@
+(module
+(@if wasi
+(@then
+   (import "libc" "memory" (memory 2))
+   (import "libc" "malloc" (func $malloc (param i32) (result i32)))
+   (import "libc" "free" (func $free (param i32)))
+   (import "io" "IO_BUFFER_SIZE" (global $IO_BUFFER_SIZE i32))
+   (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
+
+   (type $bytes (array (mut i8)))
+
+   (func (export "checked_malloc") (param $size i32) (result i32)
+      (local $p i32)
+      (local.set $p (call $malloc (local.get $size)))
+      (if (i32.eqz (local.get $p))
+         (then (call $caml_raise_out_of_memory)))
+      (local.get $p))
+
+   (func (export "blit_substring_to_memory")
+      (param $buf i32) (param $s (ref $bytes)) (param $ofs i32) (param $len i32)
+      (local $i i32)
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (i32.store8 (i32.add (local.get $buf) (local.get $i))
+                  (array.get $bytes (local.get $s)
+                     (i32.add (local.get $ofs) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func $blit_string_to_memory (export "blit_string_to_memory")
+      (param $buf i32) (param $s (ref $bytes))
+      (local $i i32) (local $len i32)
+      (local.set $len (array.len (local.get $s)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (i32.store8 (i32.add (local.get $buf) (local.get $i))
+                  (array.get $bytes (local.get $s) (local.get $i)))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func (export "blit_memory_to_substring")
+      (param $buf i32) (param $s (ref $bytes)) (param $ofs i32) (param $len i32)
+      (local $i i32)
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $s)
+                  (i32.add (local.get $ofs) (local.get $i))
+                  (i32.load8_u (i32.add (local.get $buf) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop)))))
+
+   (func $blit_memory_to_string (export "blit_memory_to_string")
+      (param $buf i32) (param $len i32) (result (ref $bytes))
+      (local $s (ref $bytes))
+      (local $i i32)
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (array.set $bytes (local.get $s) (local.get $i)
+                  (i32.load8_u (i32.add (local.get $buf) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (local.get $s))
+
+   (func (export "write_string_to_memory")
+      (param $buf i32) (param $avail i32) (param $v (ref eq))
+      (result i32)
+      (local $s (ref $bytes)) (local $i i32) (local $len i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get $v)))
+      (local.set $len (array.len (local.get $s)))
+      (if (i32.lt_u (local.get $avail) (i32.add (local.get $len) (i32.const 1)))
+         (then
+            (local.set $buf
+               (call $checked_malloc (i32.add (local.get $len) (i32.const 1))))))
+      (call $blit_string_to_memory (local.get $buf) (local.get $s))
+      (i32.store8 (i32.add (local.get $buf) (local.get $len)) (i32.const 0))
+      (local.get $buf))
+
+   (func (export "release_memory") (param $initial_buffer i32) (param $buf i32)
+      (if (i32.ne (local.get $initial_buffer) (local.get $buf))
+         (then
+            (call $free (local.get $buf)))))
+
+   (global $buffer (mut i32) (i32.const 0))
+
+   (func $get_buffer (export "get_buffer") (result i32)
+      (if (i32.eqz (global.get $buffer))
+         (then
+            (global.set $buffer
+               (call $checked_malloc
+                  (i32.add (global.get $IO_BUFFER_SIZE) (i32.const 12))))))
+      (global.get $buffer))
+))
+)
diff --git a/runtime/wasm/weak.wat b/runtime/wasm/weak.wat
index 8f1403606d..68d5328c04 100644
--- a/runtime/wasm/weak.wat
+++ b/runtime/wasm/weak.wat
@@ -21,6 +21,19 @@
       (func $caml_obj_dup (param (ref eq)) (result (ref eq))))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param $arg (ref eq))))
+
+(@if wasi
+(@then
+   (func $wrap (param (ref eq)) (result (ref eq))
+      (local.get 0))
+   (func $unwrap (param (ref eq)) (result (ref eq))
+      (local.get 0))
+   (func $weak_new (param $v (ref eq)) (result (ref eq))
+      (local.get $v))
+   (func $weak_deref (param $r (ref eq)) (result (ref eq))
+      (local.get $r))
+)
+(@else
    (import "bindings" "weak_new"
       (func $weak_new (param (ref eq)) (result anyref)))
    (import "bindings" "weak_deref"
@@ -32,6 +45,8 @@
       (func $map_set (param (ref any)) (param (ref eq)) (param (ref any))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
+))
+
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
    (type $js (struct (field anyref)))
@@ -62,6 +77,8 @@
          (block $released
             (br_if $no_data
                (ref.eq (local.get $d) (global.get $caml_ephe_none)))
+(@if (not wasi)
+(@then
             (local.set $i (global.get $caml_ephe_key_offset))
             (local.set $len (array.len (local.get $x)))
             (local.set $m (ref.as_non_null (call $unwrap (local.get $d))))
@@ -81,9 +98,11 @@
                         (br_on_null $released
                            (call $map_get (local.get $m) (local.get $v))))
                      (br $loop))))
+            (local.set $d (ref.cast (ref eq) (local.get $m)))
+))
             (return
               (array.new_fixed $block 2 (ref.i31 (i32.const 0))
-                 (ref.cast (ref eq) (local.get $m)))))
+                 (local.get $d))))
          (array.set $block (local.get $x) (global.get $caml_ephe_data_offset)
             (global.get $caml_ephe_none)))
       (ref.i31 (i32.const 0)))
@@ -110,6 +129,8 @@
       (local $m (ref any)) (local $m' (ref any))
       (local $i i32)
       (local.set $x (ref.cast (ref $block) (local.get $vx)))
+(@if (not wasi)
+(@then
       (local.set $i (array.len (local.get $x)))
       (local.set $m (local.get $data))
       (loop $loop
@@ -133,8 +154,10 @@
                (array.set $block (local.get $x) (local.get $i)
                   (global.get $caml_ephe_none))
                (br $loop))))
+      (local.set $data (call $wrap (local.get $m)))
+))
       (array.set $block (local.get $x) (global.get $caml_ephe_data_offset)
-         (call $wrap (local.get $m)))
+         (local.get $data))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ephe_unset_data")
@@ -275,7 +298,7 @@
       (call $caml_ephe_set_data_opt (local.get $vx) (local.get $d))
       (ref.i31 (i32.const 0)))
 
-   (data $Weak_create "Weak.create")
+   (@string $Weak_create "Weak.create")
 
    (export "caml_weak_create" (func $caml_ephe_create))
    (func $caml_ephe_create (export "caml_ephe_create")
@@ -284,10 +307,7 @@
       (local $res (ref $block))
       (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
       (if (i32.lt_s (local.get $len) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $bytes $Weak_create
-                  (i32.const 0) (i32.const 11)))))
+         (then (call $caml_invalid_argument (global.get $Weak_create))))
       (local.set $res
          (array.new $block (global.get $caml_ephe_none)
             (i32.add (local.get $len) (global.get $caml_ephe_key_offset))))
diff --git a/tools/node_wrapper.ml b/tools/node_wrapper.ml
index 15979975a8..dfa7a8bf47 100644
--- a/tools/node_wrapper.ml
+++ b/tools/node_wrapper.ml
@@ -1,6 +1,20 @@
+let wizard_args = [ "-ext:stack-switching"; "-stack-size=2M"; "--dir=."; "--dir=/tmp" ]
+
+let wasmtime_args =
+  [ (* "-C"; "collector=null"; *) "-W=all-proposals=y"; "--dir=."; "--dir=/tmp" ]
+
+let wasmedge_args =
+  [ "--enable-gc"
+  ; "--enable-exception-handling"
+  ; "--enable-tail-call"
+  ; "--dir=."
+  ; "--dir=/tmp"
+  ]
+
 let extra_args_for_wasoo =
   [ "--experimental-wasm-imported-strings"
   ; "--experimental-wasm-stack-switching"
+  ; "--experimental-wasm-exnref"
   ; "--stack-size=10000"
   ]
 
@@ -23,16 +37,32 @@ let env =
       else e)
     env
 
-let args =
+let environment_args () =
+  List.filter
+    (fun e -> not (String.contains e ','))
+    (Array.to_list (Array.map (fun e -> "--env=" ^ e) env))
+
+let wasm_file file =
+  Filename.concat (Filename.chop_extension file ^ ".assets") "code.wasm"
+
+let common_args file argv = environment_args () @ (wasm_file file :: List.tl argv)
+
+let exe, args =
   match Array.to_list Sys.argv with
   | exe :: argv ->
-      let argv =
+      let exe', argv =
         match argv with
-        | file :: _ when Filename.check_suffix file ".wasm.js" ->
-            extra_args_for_wasoo @ argv
-        | _ -> extra_args_for_jsoo @ argv
+        | file :: _ when Filename.check_suffix file ".wasm.js" -> (
+            match Sys.getenv_opt "WASM_ENGINE" with
+            | Some "wizard" -> "wizeng.x86-linux", wizard_args @ common_args file argv
+            | Some "wizard-fast" ->
+                "wizeng.x86-64-linux", wizard_args @ common_args file argv
+            | Some "wasmtime" -> "wasmtime", wasmtime_args @ common_args file argv
+            | Some "wasmedge" -> "wasmedge", wasmedge_args @ common_args file argv
+            | _ -> "node", extra_args_for_wasoo @ argv)
+        | _ -> "node", extra_args_for_jsoo @ argv
       in
-      Array.of_list (exe :: argv)
+      exe', Array.of_list (exe :: argv)
   | [] -> assert false
 
 let () =
@@ -45,4 +75,4 @@ let () =
     | _, WEXITED n -> exit n
     | _, WSIGNALED _ -> exit 9
     | _, WSTOPPED _ -> exit 9
-  else Unix.execvpe "node" args env
+  else Unix.execvpe exe args env