Clang vs wasm32-{emscripten,wasi} rustc C ABI mismatch w.r.t. "singleton" unions

[hanna-kruppe]

On wasm32-unknown-emscripten and wasm32-wasi, rustc implements the C ABI for some unions incorrectly, i.e., different from Clang. Minimized example:

#[repr(C)]
pub union U {
    a: u32,
    b: u32,
}

#[no_mangle]
pub extern "C" fn unwrap_union(u: U) -> u32 {
    unsafe { u.a }
}

#[no_mangle]
pub extern "C" fn make_union() -> U {
    U { a: 0 }
}

I expected to see this happen: the resulting wasm code should pass and return the union indirectly, i.e. by pointers, as described in the C ABI document and implemented in Clang (compiler explorer).

Instead, this happened: the union is passed and returned as a single scalar (i32). See the previous compiler explorer link, and I also see it locally for wasm32-wasi (too lazy to install a whole emscripten toolchain):

Details

$ rustc +nightly -O cabi-union.rs --crate-type=cdylib --target wasm32-wasi
$ wasm-tools print cabi_union.wasm
(module $cabi_union.wasm
  (type (;0;) (func (param i32) (result i32)))
  (type (;1;) (func (result i32)))
  (type (;2;) (func))
  (func $unwrap_union (;0;) (type 0) (param i32) (result i32)
    local.get 0
  )
  (func $make_union (;1;) (type 1) (result i32)
    i32.const 0
  )
  (func $dummy (;2;) (type 2))
  (func $__wasm_call_dtors (;3;) (type 2)
    call $dummy
    call $dummy
  )
  (func $unwrap_union.command_export (;4;) (type 0) (param i32) (result i32)
    local.get 0
    call $unwrap_union
    call $__wasm_call_dtors
  )
  (func $make_union.command_export (;5;) (type 1) (result i32)
    call $make_union
    call $__wasm_call_dtors
  )
  (table (;0;) 1 1 funcref)
  (memory (;0;) 16)
  (global $__stack_pointer (;0;) (mut i32) i32.const 1048576)
  (export "memory" (memory 0))
  (export "unwrap_union" (func $unwrap_union.command_export))
  (export "make_union" (func $make_union.command_export))
  (@producers
    (language "Rust" "")
    (processed-by "rustc" "1.78.0-nightly (2bf78d12d 2024-02-18)")
    (processed-by "clang" "16.0.4 (https://github.com/llvm/llvm-project ae42196bc493ffe877a7e3dff8be32035dea4d07)")
  )
)

The definition of "singleton" union in the C ABI document ("recursively contains just a single scalar value") may be considered ambiguous, but clearly Clang interprets it differently from rustc, so something will have to give. I have not tried to exhaustively explore in which cases they differ, the above example may not be the only one.

Compare and contrast #71871 - as discussed there, the emscripten and wasi targets have long since been fixed to match Clang's ABI, with only wasm32-unknown-unknown lagging behind. However, it seems that the fixed C ABI on emscripten and wasi targets is still incorrect in some cases around unions.

cc @curiousdannii, who encountered this in a real project (rust-lang/cc-rs#954)

Meta

rustc +nightly --version --verbose:

rustc 1.78.0-nightly (2bf78d12d 2024-02-18)
binary: rustc
commit-hash: 2bf78d12d33ae02d10010309a0d85dd04e7cff72
commit-date: 2024-02-18
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

(Also happens on 1.76 stable.)

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[curiousdannii]

I've run into this problem again, but this time it's a C callback that's returning the union. As I don't control it I can't change its signature to manually match the ABI. I think I'll have to move more processing code into my C support library.

[hanna-kruppe]

A possible workaround might be to use a technically incorrect function signature on the Rust side that is lowered to the correct ABI on wasm targets by current (and future) rustc - pretending the callbacks get an out-pointer instead of returning by value / receive arguments by reference instead of by value. Pretty ugly and wasm-specific, but maybe less ugly than additional C code that is pointless on non-wasm platforms?

[curiousdannii]

@hanna-kruppe Hmm, that might work, but it would mean I'd need to add platform conditional code in a number of places, vs a small C shim which works everywhere.

Edit: I'm having trouble even getting a shim working. It works fine when compiled to x86_64, but I can't get it to work in wasm - it's like the DispatchRock is having its value modified.

@hanna-kruppe On the chance that you have any time to take a look, I pushed my broken code to https://github.com/curiousdannii/emglken/tree/remglk_rs_broken_unions

Details

This builds and runs glulxe ``` ./src/build.sh ./bin/emglken.js tests/glulxercise.ulx ``` Then enter anything. You should see something like this (60664 turns into 60684 and then into 54940)

gidispatch_call_array_register 0xef0e0 &+#!Cn 60664 0xed1c
retain_array 0xef0e0 0xed1c 977032
print_disprock 60684

unretain_array 0xef0e0 977032
gidispatch_call_array_unregister 0xef0e0 &+#!Cn 54940

Glulxe fatal error: Mismatched array reference in Glk call.

Where as in x86_64 it works (280594112 is retained through it all):

gidispatch_call_array_register 0x55f910b98d60 &+#!Cn 280594112 0x7ffeb52fd1d0
retain_array 0x55f910b98d60 0x7ffeb52fd1d0 94528215811776
print_disprock 280594112
unretain_array 0x55f910b98d60 94528215811776
gidispatch_call_array_unregister 0x55f910b98d60 &+#!Cn 280594112

[curiousdannii]

Hmm, just found a potential solution: add an dummy union variant on the rust side that is 64 bits. I think that means it won't be considered a singleton union, but C just ignores the extra word. I might be able to remove all my manual shimming this way?

Yep, that seems to work perfectly! And when this bug eventually gets fixed, all I'll need to do is remove the dummy variant. Much cleaner. :)