[NativeAOT] Fix Activator.CreateInstance for shared generic structs

[jkotas]

The default constructor has to be invoked using fat pointer in shared generic structs.

[jkotas]

/azp run runtime-nativeaot-outerloop

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[jkotas]

cc @AaronRobinsonMSFT Bug that I have noticed as I was replying to your email

[MichalStrehovsky]

Thanks! I wanted to ask by how much this makes things slower, but optimizing the "no default ctor" case can be a separate thing. I think I can add a RuntimeHelpers.MightHaveDefaultConstructor<T> that can be used to avoid this extra work (and more) in the 99% common case.

[jkotas]

optimizing the "no default ctor"

This should be taken care of by

runtime/src/coreclr/tools/Common/TypeSystem/IL/NativeAotILProvider.cs

Lines 114 to 130 in fa1164c

	if (methodName == "CreateInstance" && method.Signature.Length == 0 && method.HasInstantiation
	&& method.Instantiation[0] is TypeDesc activatedType
	&& activatedType != context.UniversalCanonType
	&& activatedType.IsValueType
	&& activatedType.GetParameterlessConstructor() == null)
	{
	ILEmitter emit = new ILEmitter();
	ILCodeStream codeStream = emit.NewCodeStream();
	var t = emit.NewLocal(context.GetSignatureVariable(0, method: true));
	codeStream.EmitLdLoca(t);
	codeStream.Emit(ILOpcode.initobj, emit.NewToken(context.GetSignatureVariable(0, method: true)));
	codeStream.EmitLdLoc(t);
	codeStream.Emit(ILOpcode.ret);
	return new InstantiatedMethodIL(method, emit.Link(method.GetMethodDefinition()));
	}

. I am not sure what else is there to optimize.

[MichalStrehovsky]

This should be taken care of by

Keep forgetting how this works. I guess the throw on line 46 might be unreachable for value types then?

[jkotas]

I guess the throw on line 46 might be unreachable for value types then?

It may be reachable for value types with private default constructor.