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14 changes: 14 additions & 0 deletions crates/wasm-encoder/src/component/builder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -206,6 +206,20 @@ impl ComponentBuilder {
})
}

/// Creates an alias to a previous core instance's exported item.
///
/// The `instance` provided is the instance to access and the `name` is the
/// item to access.
///
/// Returns the index of the new item defined.
pub fn alias_core_export(&mut self, instance: u32, name: &str, kind: ExportKind) -> u32 {
self.alias(Alias::CoreInstanceExport {
instance,
kind,
name,
})
}

fn inc_kind(&mut self, kind: ComponentExportKind) -> u32 {
match kind {
ComponentExportKind::Func => inc(&mut self.funcs),
Expand Down
56 changes: 56 additions & 0 deletions crates/wit-component/src/encoding.rs
Original file line number Diff line number Diff line change
Expand Up @@ -631,6 +631,8 @@ impl<'a> EncodingState<'a> {
self.instantiate_adapter_module(&shims, name, adapter);
}

self.encode_initialize_with_start()?;

Ok(())
}

Expand Down Expand Up @@ -1689,6 +1691,60 @@ impl<'a> EncodingState<'a> {
});
self.adapter_import_reallocs.insert(name, realloc);
}

/// Generates component bits that are responsible for executing
/// `_initialize`, if found, in the original component.
///
/// The `_initialize` function was a part of WASIp1 where it generally is
/// intended to run after imports and memory and such are all "hooked up"
/// and performs other various initialization tasks. This is additionally
/// specified in https://github.com/WebAssembly/component-model/pull/378
/// to be part of the component model lowerings as well.
///
/// This implements this functionality by encoding a core module that
/// imports a function and then registers a `start` section with that
/// imported function. This is all encoded after the
/// imports/lowerings/tables/etc are all filled in above meaning that this
/// is the last piece to run. That means that when this is running
/// everything should be hooked up for all imported functions to work.
///
/// Note that at this time `_initialize` is only detected in the "main
/// module", not adapters/libraries.
fn encode_initialize_with_start(&mut self) -> Result<()> {
let initialize = match self.info.info.initialize {
Some(name) => name,
// If this core module didn't have `_initialize` or similar, then
// there's nothing to do here.
None => return Ok(()),
};
let initialize_index = self.component.alias_core_export(
self.instance_index.unwrap(),
initialize,
ExportKind::Func,
);
let mut shim = Module::default();
let mut section = TypeSection::new();
section.function([], []);
shim.section(&section);
let mut section = ImportSection::new();
section.import("", "", EntityType::Function(0));
shim.section(&section);
shim.section(&StartSection { function_index: 0 });

// Declare the core module within the component, create a dummy core
// instance with one export of our `_initialize` function, and then use
// that to instantiate the module we emit to run the `start` function in
// core wasm to run `_initialize`.
let shim_module_index = self.component.core_module(&shim);
let shim_args_instance_index =
self.component
.core_instantiate_exports([("", ExportKind::Func, initialize_index)]);
self.component.core_instantiate(
shim_module_index,
[("", ModuleArg::Instance(shim_args_instance_index))],
);
Ok(())
}
}

/// A list of "shims" which start out during the component instantiation process
Expand Down
11 changes: 10 additions & 1 deletion crates/wit-component/src/validation.rs
Original file line number Diff line number Diff line change
Expand Up @@ -100,6 +100,10 @@ pub struct ValidatedModule<'a> {
/// Post-return functions annotated with `cabi_post_*` in their function
/// name.
pub post_returns: IndexSet<String>,

/// Exported function like `_initialize` which needs to be run after
/// everything else has been instantiated.
pub initialize: Option<&'a str>,
}

#[derive(Default)]
Expand Down Expand Up @@ -146,6 +150,7 @@ pub fn validate_module<'a>(
metadata: &metadata.metadata,
required_resource_funcs: Default::default(),
post_returns: Default::default(),
initialize: None,
};

for payload in Parser::new(0).parse_all(bytes) {
Expand Down Expand Up @@ -194,7 +199,11 @@ pub fn validate_module<'a>(
}
}

assert!(export_funcs.insert(export.name, export.index).is_none())
if export.name == "_initialize" {
ret.initialize = Some(export.name);
} else {
assert!(export_funcs.insert(export.name, export.index).is_none())
}
}
ExternalKind::Memory => {
if export.name == "memory" {
Expand Down
38 changes: 38 additions & 0 deletions crates/wit-component/tests/components/initialize/component.wat
Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
(component
(core module (;0;)
(type (;0;) (func))
(func (;0;) (type 0)
unreachable
)
(func (;1;) (type 0)
unreachable
)
(export "a" (func 0))
(export "_initialize" (func 1))
(@producers
(processed-by "wit-component" "$CARGO_PKG_VERSION")
(processed-by "my-fake-bindgen" "123.45")
)
)
(core instance (;0;) (instantiate 0))
(alias core export 0 "_initialize" (core func (;0;)))
(core module (;1;)
(type (;0;) (func))
(import "" "" (func (;0;) (type 0)))
(start 0)
)
(core instance (;1;)
(export "" (func 0))
)
(core instance (;2;) (instantiate 1
(with "" (instance 1))
)
)
(type (;0;) (func))
(alias core export 0 "a" (core func (;1;)))
(func (;0;) (type 0) (canon lift (core func 1)))
(export (;1;) "a" (func 0))
(@producers
(processed-by "wit-component" "$CARGO_PKG_VERSION")
)
)
Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
package root:component;

world root {
export a: func();
}
6 changes: 6 additions & 0 deletions crates/wit-component/tests/components/initialize/module.wat
Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
;; This test ensures that the `_initialize` function is hooked up to execute
;; when the component is instantiated.
(module
(func (export "a") unreachable)
(func (export "_initialize") unreachable)
)
5 changes: 5 additions & 0 deletions crates/wit-component/tests/components/initialize/module.wit
Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
package foo:foo;

world module {
export a: func();
}