The x86 path for UnmanagedCallersOnlyAttribute should be improved.

[AaronRobinsonMSFT]

The making public of NativeCallableAttribute UnmanagedCallersOnlyAttribute done in #33005, provided some additional optimizations. However the Windows-x86 path was deemed lower priority so wasn't optimized. There are various locations where the Windows-x86 platform opted out of the optimization.

Examples:

runtime/src/coreclr/src/vm/jitinterface.cpp

Lines 9212 to 9224 in a1af0f2

	#if defined(TARGET_X86) && defined(TARGET_WINDOWS) && !defined(CROSSGEN_COMPILE)
	// Deferring X86 support until a need is observed or
	// time permits investigation into all the potential issues.
	if (pMD->HasNativeCallableAttribute())
	{
	pResult->addr = (void*)COMDelegate::ConvertToCallback(pMD);
	}
	else
	{
	pResult->addr = (void*)pMD->GetMultiCallableAddrOfCode();
	}
	#else

runtime/src/coreclr/src/vm/jitinterface.cpp

Lines 12389 to 12392 in a1af0f2

	#if !defined(TARGET_X86) || !defined(TARGET_WINDOWS)
	if (ftn->HasNativeCallableAttribute())
	flags.Set(CORJIT_FLAGS::CORJIT_FLAG_REVERSE_PINVOKE);
	#endif // !TARGET_X86 || !TARGET_WINDOWS

runtime/src/coreclr/src/tools/Common/JitInterface/CorInfoImpl.cs

Lines 2887 to 2891 in a1af0f2

	if (targetArchitecture == TargetArchitecture.X86
	&& _compilation.TypeSystemContext.Target.OperatingSystem == TargetOS.Windows)
	{
	throw new RequiresRuntimeJitException("ReadyToRun: Methods with NativeCallableAttribute not implemented");
	}

runtime/src/coreclr/src/tools/crossgen2/ILCompiler.ReadyToRun/JitInterface/CorInfoImpl.ReadyToRun.cs

Lines 1663 to 1669 in a1af0f2

	var targetDetails = _compilation.TypeSystemContext.Target;
	if (targetDetails.Architecture == TargetArchitecture.X86
	&& targetDetails.OperatingSystem == TargetOS.Windows
	&& targetMethod.IsNativeCallable)
	{
	throw new RequiresRuntimeJitException("ReadyToRun: References to methods with NativeCallableAttribute not implemented");
	}

[jkotas]

Generalizing the issue to both Windows and Linux. It turns our both Windows x86 and Linux x86 need work.

[AaronRobinsonMSFT]

See #34251 (comment) for Linux x86 issues.

[jkotas]

See #34548 for (subset of) places that are affected.

[AaronRobinsonMSFT]

@jkoritzinsky I assume some of the work you are doing is addressing this.