First part of async support in native AOT

[MichalStrehovsky]

Following works with runtime async enabled in Roslyn on native AOT:

public class Async2Void
{
    public static void Main()
    {
        var t = AsyncTestEntryPoint(123, 456);
        t.Wait();
        Console.WriteLine(t.Result);
    }

    private static async Task<int> AsyncTestEntryPoint(int x, int y)
    {
        int result = await OtherAsync(x, y);
        return result;
    }

    private static async Task<int> OtherAsync(int x, int y)
    {
        Console.WriteLine(x);
        Console.WriteLine(y);
        return x + y;
    }
}

The implementation strategy is as follows:

• In the compiler, whenever someone holds an EcmaMethod of an asyncv2 method, it means they're holding the RuntimeAsync flavor of the method in CoreCLR VM parlance. The return value is a Task, MethodDesc.IsAsync is true, MethodSignature.IsAsyncCallConv is false. The IL is a thunk to the AsyncVariantImplMethod flavor.
• In the compiler, we have a new MethodDesc descendant: AsyncVariantImplMethod. This is the actual method with async calling convention that corresponds to some EcmaMethod with IsAsync==true. For this one: the return value is unwrapped from Task, MethodDesc.IsAsync is true, MethodSignature.IsAsyncCallConv is true. The IL is the actual IL that corresponds to the wrapped EcmaMethod on disk.

Cc @dotnet/ilc-contrib

[Copilot]

Pull Request Overview

This PR implements support for NativeAOT async method compilation by introducing an async calling convention and variant method infrastructure. The key changes enable async methods to be compiled with special handling in NativeAOT, mirroring the CoreCLR VM's AsyncVariantImpl concept.

Key Changes:

• Added NativeAOT-specific implementations for Task and Task<T> await methods in AsyncHelpers
• Introduced AsyncVariantImplMethod infrastructure to represent async methods callable with async calling convention
• Added support for unwrapping Task return types and marking methods with AsyncCallConv flag
• Extended JIT interface to handle async token resolution and calling convention flags

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
AsyncHelpers.cs	Added NativeAOT-specific await implementations for Task/Task that handle completed tasks synchronously
ILCompiler.Compiler.csproj	Registered new async variant method source files in the compiler project
AsyncThunks.cs	Added IL emitter for generating Task-returning thunks that wrap async methods
NativeAotILProvider.cs	Integrated async method detection and thunk generation into IL provider
CorInfoTypes.cs	Added CORINFO_CALLCONV_ASYNCCALL and CORINFO_TOKENKIND_Await enum values
CorInfoImpl.cs	Implemented async token resolution and propagated AsyncCallConv flag to JIT
AsyncMethodDesc.cs	Removed IsTaskReturning extension method (moved to MethodExtensions)
MethodExtensions.cs	Added IsTaskReturning helper method for identifying Task-returning methods
CompilerTypeSystemContext.Async.cs	Added async variant method hashtable and instantiation support
AsyncVariantImplMethod.cs	Core implementation of async variant method with unwrapped signature
AsyncVariantImplMethod.Sorting.cs	Added sorting/comparison support for async variant methods
AsyncVariantImplMethod.Mangling.cs	Added name mangling with "AsyncCallable" prefix

[MichalStrehovsky]

There are several shovel-ready workitems that can be started in parallel on the native AOT side after this merges (we can discuss on Teams).

I did a couple hacks on the crossgen side that makes things mostly compile in MichalStrehovsky@d395c1a (not part of this PR; was just looking around). I think the crossgen side will depend on what we want to compile. If we're fine with only precompiling the AsyncVariantImplMethod flavor and none of the thunks, the actual implementation could be different so as not to perturb the system too much. If we want to precompile the thunks as well, we'll have to take similar approach to my hack. The hacky part is that most of the crossgen2 codebase assumes that what we're working with are (potentially instantiated) EcmaMethods. It does not like when a method is not EcmaMethod (see the places I had to update, likely didn't hit them all yet, not sure if the fixes are correct). We cannot have EcmaMethod of the thunks.

Cc @dotnet/ilc-contrib

[jkotas]

There are several shovel-ready workitems that can be started in parallel on the native AOT side after this merges (we can discuss on Teams).

If you got a Teams channel going for this, would you mind adding me to it? I suspect that the questions that I have just asked might have been discussed there.

[MichalStrehovsky]

There are several shovel-ready workitems that can be started in parallel on the native AOT side after this merges (we can discuss on Teams).

If you got a Teams channel going for this, would you mind adding me to it? I suspect that the questions that I have just asked might have been discussed there.

It was just an ad hoc video meeting between Jackson, Eduardo, David and me. I'll add you if we have something written.

[MichalStrehovsky]

Rebased this on top of Jackson's PR that added the MethodDesc, so this is now only 130 lines. I validated the repro in top-post still works.

This is ready for review now.

[MichalStrehovsky]

/ba-g widespread Apple and Android infra issues