[Wasm] Write barriers

[kg]

Sufficient for the R2R'd version of this to run without crashing:

    static string? TestField;

    [MethodImpl(MethodImplOptions.NoOptimization | MethodImplOptions.NoInlining)]
    public static void ConcatTwoStrings (string a, string b) {
        TestField = a + b;
    }

    [MethodImpl(MethodImplOptions.NoOptimization | MethodImplOptions.NoInlining)]
    public static int Main () {
        Console.WriteLine("Hello world!");
        ConcatTwoStrings("A", "B");
        Console.WriteLine(TestField);
        return 42;
    }

Currently the write barrier in ConcatTwoStrings fails due to the calling convention being wrong and then once the calling convention is fixed, it fails because we didn't implement the write barriers yet.

This PR:

• Updates the calling convention of the write barriers to match that expected by the JIT (bare calling convention without sp/pep args) by adding new FCDECL2_RAW / FCIMPL2_RAW macros
• Fixes RhpByRefAssignRef's signature being incorrect (missing *)
• Implements the checked and unchecked write barriers in inline wasm assembly in the new wasm/writebarriers.cpp file

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[Copilot]

Pull request overview

Initial scaffolding to make the WASM portable write barriers (RhpAssignRef, RhpCheckedAssignRef, RhpByRefAssignRef) use a "raw" calling convention that matches what RyuJIT expects for write barriers, rather than the trampolined WASM FCALL convention that threads callersStackPointer / portableEntryPointContext through. The actual barrier implementations still assert and will be filled in later.

Changes:

• Add new FCDECL2_RAW / FCIMPL2_RAW macros that bypass the WASM stack-pointer/entrypoint-context plumbing and define the function with the plain (a1, a2) signature; provide non-WASM fallbacks that alias to FCDECL2 / FCIMPL2.
• Switch the three Rhp*AssignRef declarations in jitinterface.h and definitions in WriteBarriers.cpp to the new _RAW variants.
• Trivial whitespace cleanup in jitinterface.h.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/vm/fcall.h	Adds WASM-specific FCDECL2_RAW / FCIMPL2_RAW macros and non-WASM fallbacks aliasing to the standard FCDECL2 / FCIMPL2.
src/coreclr/vm/jitinterface.h	Switches Rhp{Checked,ByRef,}AssignRef declarations to FCDECL2_RAW; trims trailing whitespace.
src/coreclr/runtime/portable/WriteBarriers.cpp	Updates the three portable write-barrier stubs to use FCDECL2_RAW / FCIMPL2_RAW.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[jkotas]

This can use comment about why we have inline wasm implementation and not just regular C/C++ implementation.

[jkotas]

Can we move this file to src/coreclr/runtime/wasm ? We will want to use this implementation for naot. src/coreclr/runtime/wasm is the directory where files shared between regular coreclr and naot live.

[jkotas]

Suggested change

	EXTERN_C FCDECL2_RAW(VOID, RhpAssignRef, Object **dst, Object *ref);
	ASM_HELPER_2(VOID, RhpAssignRef, Object **dst, Object *ref)
	__attribute__((alias("JIT_WriteBarrier")));
	EXTERN_C FCDECL2_RAW(VOID, RhpCheckedAssignRef, Object **dst, Object *ref);
	ASM_HELPER_2(VOID, RhpCheckedAssignRef, Object **dst, Object *ref)
	__attribute__((alias("JIT_CheckedWriteBarrier")));

Do we need these aliases?

It seems that things should work fine if the functions implementing this are called RhpAssignRef and RhpCheckedAssignRef, and JIT_WriteBarrier does not need to exist. Or is JIT_WriteBarrier referenced from somewhere in wasm builds?

[jkotas]

There are comments that refer to JIT_WriteBarrier but the thing that the comment is attached to is actually RhpAssignRef, e.g.

runtime/src/coreclr/runtime/arm64/WriteBarriers.asm

Lines 218 to 231 in ed8987f

	;; JIT_CheckedWriteBarrier(Object** dst, Object* src)
	;;
	;; Write barrier for writes to objects that may reside
	;; on the managed heap.
	;;
	;; On entry:
	;; x14 : the destination address (LHS of the assignment).
	;; May not be a heap location (hence the checked).
	;; x15 : the object reference (RHS of the assignment)
	;;
	;; On exit:
	;; x12, x17 : trashed
	;; x14 : incremented by 8
	LEAF_ENTRY RhpCheckedAssignRefArm64

. It would be nice to clean those us. JIT_WriteBarrier should only be used with the dynamically runtime generated write barriers.

[jkotas]

Suggested change

	EXTERN_C FCDECL2_RAW(VOID, RhpByRefAssignRef, Object **dst, Object **ref);
	ASM_HELPER_2(VOID, RhpByRefAssignRef, Object **dst, Object **ref)
	{
	GC_ASM("unreachable"
	);
	}

We can define this helper as not implemented for wasm in src\coreclr\inc\jithelpers.h (notice that this file has similar arch-specific ifdefs for number of other helpers):

#ifdef TARGET_WASM
    JITHELPER(CORINFO_HELP_ASSIGN_BYREF, NULL,METHOD__NIL)
#else
    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_BYREF, RhpByRefAssignRef, METHOD__NIL)
#endif

[jkotas]

Suggested change

	#define ASM_HELPER_2(rettype, funcname, a1, a2) \
	#ifdef FEATURE_MULTITHREADING
	#error The assembly implemementaiton of write barriers assumes single threaded wasm
	#endif
	#define ASM_HELPER_2(rettype, funcname, a1, a2) \