WebAssembly ABI mismatch between clang and rust

[tomaka]

When I compile this C code to wasm32-wasi:

typedef struct Vector {
    int a;
    int b;
} Vector;

extern int extract_a(Vector v) {
    return v.a;
}

And this Rust code to wasm32-wasi:

#[repr(C)]
struct Vector {
    a: i32,
    b: i32,
}

extern "C" {
    fn extract_a(v: Vector) -> i32;
}

fn main() {
    unsafe {
        extract_a(Vector { a: 5, b: 4 });
    }
}

And try link them together, I'm getting linking errors:

  = note: lld: error: function signature mismatch: extract_a
          >>> defined as (i32, i32) -> i32 in /home/pierre/Projets/wasm-abi-test/target/wasm32-wasi/debug/deps/wasm_abi_test.67xpw7l331r9o5x.rcgu.o
          >>> defined as (i32) -> i32 in /home/pierre/Projets/wasm-abi-test/target/wasm32-wasi/debug/build/wasm-abi-test-9c49ce7f6c5ca031/out/libfoo.a(test.o)

It seems that, according to clang, passing a struct by value should inline the fields, while for Rust it should be passed by pointer.

I uploaded an example project here: https://github.com/tomaka/wasm-abi-test
It can be built with something like:

AR_wasm32_wasi=/path/to/wasi-sdk-10.0/bin/ar CC_wasm32_wasi=/path/to/wasi-sdk-10.0/bin/clang cargo run --target=wasm32-wasi

Rust version: rustc 1.44.0-nightly (38114ff 2020-03-21)

[tomaka]

Apparently this is a known issue:

• Pass small structs in parameters instead of memory WebAssembly/tool-conventions#88 (comment)
• Clang and Rustc use incompatible Wasm signatures for struct arguments passed by value WebAssembly/tool-conventions#130

[TheBlueMatt]

This is mostly tracked at rustwasm/team#291 and would be fixed by #79998.

[TheBlueMatt]

This should now work for wasm32-wasi (on current git head).

[programmerjake]

Note that #83763 changed the default, so only wasm32-unknown-unknown uses the mismatched ABI, all other wasm targets (including custom targets) now default to using the ABI that matches clang:

rust/compiler/rustc_target/src/spec/wasm32_unknown_unknown.rs

Lines 22 to 30 in 482a3d0

	// This is a default for backwards-compatibility with the original
	// definition of this target oh-so-long-ago. Once the "wasm" ABI is
	// stable and the wasm-bindgen project has switched to using it then there's
	// no need for this and it can be removed.
	//
	// Currently this is the reason that this target's ABI is mismatched with
	// clang's ABI. This means that, in the limit, you can't merge C and Rust
	// code on this target due to this ABI mismatch.
	options.default_adjusted_cabi = Some(Abi::Wasm);

[temeddix]

There are discussions going on at wasm-bindgen to extend the support to wasm32-wasi in addition to the current wasm32-unknown-unknown.

• WASI support rustwasm/wasm-bindgen#3421
• Align with the "C" ABI rustwasm/wasm-bindgen#3454

In short, wasm-bindgen needs a consistent C ABI between wasm32-unknown-unknown and wasm32-wasi to support both of them.

Note that #83763 changed the default, so only wasm32-unknown-unknown uses the mismatched ABI, all other wasm targets (including custom targets) now default to using the ABI that matches clang:

rust/compiler/rustc_target/src/spec/wasm32_unknown_unknown.rs

Lines 22 to 30 in 482a3d0

	// This is a default for backwards-compatibility with the original
	// definition of this target oh-so-long-ago. Once the "wasm" ABI is
	// stable and the wasm-bindgen project has switched to using it then there's
	// no need for this and it can be removed.
	//
	// Currently this is the reason that this target's ABI is mismatched with
	// clang's ABI. This means that, in the limit, you can't merge C and Rust
	// code on this target due to this ABI mismatch.
	options.default_adjusted_cabi = Some(Abi::Wasm);

Can we have ABI that matches clang on wasm32-unknown-unknown as well? I can help with this, but would it be a hard task?

[bjorn3]

The change itself would be removing options.default_adjusted_cabi = Some(Abi::Wasm) from the target spec and that's it. Because it breaks older wasm-bindgen versions it will probably require waiting months to years before we can actually make this change. And preferably we would introduce a future compat warning for this change. Either against older wasm-bindgen versions or against any extern "C" function for which changing the abi actually has any effect.