[dart2wasm] Implement structs backed by TypedData

[dcharkes]

Currently, dart2wasm assumes that all structs are backed by a Pointer.

sdk/pkg/dart2wasm/lib/intrinsics.dart

Lines 186 to 193 in 42ccd79

	// _Compound._typedDataBase
	if (cls == translator.ffiCompoundClass && name == '_typedDataBase') {
	// A compound (subclass of Struct or Union) is represented by its i32
	// address. The _typedDataBase field contains a Pointer pointing to the
	// compound, whose representation is the same.
	codeGen.wrap(receiver, w.NumType.i32);
	return w.NumType.i32;
	}

dart2wasm can currently assume this, because it does not implement support for:

• Returning struct by value from FFI calls.
• Struct.create.

If either of these features is implemented in dart2wasm, compounds cannot be represented as the int address of the pointer.

Also, it would break nested compounds if the suggestion in #54892 is implemented, as the _offsetInBytes would not be saved anywhere.

[dcharkes]

Also, it would break nested compounds if the suggestion in #54892 is implemented, as the _offsetInBytes would not be saved anywhere.

@mkustermann @osa1 @eyebrowsoffire Are you aware of any uses of nested structs or arrays in current dart:ffi usage (only skwasm right?) in dart2wasm?

CL breaking nested compounds in dart2wasm: https://dart-review.googlesource.com/c/sdk/+/354226 (We have no dart2wasm tests exercising nested compounds. So nothing goes red.)

[mkustermann]

We currently don't consider dart:ffi as a supported thing that end users will use. There's a limited use in flutter web engine, which is supported. I believe that limited use doesn't use structs by value and is also restricted to static @Native interop usages. None of the ffi tests in the tests/ffi test suite run on wasm - we only have tests/web/wasm/ffi_native_test.dart.

Longer term we'll require multiple memories for wasm. We'll have to see what functionality of dart:ffi we want to support and how to expose it to users - maybe through dart:ffi library - maybe not.

So for now we should ensure that the ffi features which the flutter web engine requires are tested in tests/web/wasm/ffi_native_test.dart and we keep that supported.

[mkustermann]

I'm going to close this in favor of the more general Implement dart:ffi for the web.