[RuntimeAsync] Follow up for unboxing and instantiating stubs.

[VSadov]

Re: #117266

Adding support for async target methods in unboxing and instantiating stubs.

As far as stubs are concerned, async is a passive participant. The main reason for this change is to make sure that whenever arguments are propagated/shuffled by a stub and the target is using async calling convention, the implicit continuation arguments are not lost or corrupted.
Also need to ensure that returned continuation is handled correctly. Since it is a managed object, if stub is managed code, then it needs to be aware of continuation return, at very least for GC purposes.

• ASM stubs only need to be sure that continuation argument is not lost in the shuffle. Since all ASM stubs tailcall the target, they do not need to worry about continuation on the return side.
• on x86 the generic context is the last one, so it can be set/pushed without disturbing or even knowing about other args.
  That seems sufficient for the continuation to be handled correctly.
• Portable shuffler maps each arg move thus needs to know about continuation arg offset, so that continuation can be moved, if needed. This change adds such knowledge.
• IL stubs:
  This is a bit more complicated. The IL stubs typically come into play when we cannot tailcall. Even if we immediately return from the stub, a non-async stub method would have two problems: a) ensure that implicit continuation return is not trashed in the epilog, b) ensure that continuation return is GC protected.
  Thus we go with a different solution - we introduce async IL stubs. Since IL stubs are fully normal methods sent through the JIT, we can ask the JIT to treat them as AsyncMethodKind::AsyncExplicitImpl.

One subtle part is that in the async stubs' CALLI we pass the continuation argument explicitly, similar to what we do with generic contexts. The JIT expects this pattern when seeing async CALLI. Since only stubs use async CALLI, this convention is sufficient.

The reason we cannot rely on normal inserting of continuation arg by the JIT is that in stub calls the generic context is passed as an ordinary parameter and JIT would not know how to order continuation argument with respect to that.
(the knowledge that one of the parameter is a generic context stays on stub's side, means the continuation needs to be inserted on the stub's side as well).

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[VSadov]

@jakobbotsch Thanks!!

[VSadov]

I think this is ready for review.

[Copilot]

Pull Request Overview

This PR adds support for async target methods in unboxing and instantiating stubs, ensuring that implicit continuation arguments are properly handled during argument propagation and that returned continuations are managed correctly for GC purposes.

Key changes include:

• Introduction of async IL stubs that handle continuation arguments explicitly
• Updates to portable shuffler to handle async continuation argument mapping
• Configuration changes to enable runtime async support by default

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/tests/async/inst-unbox-thunks/inst-unbox-thunks.csproj	Test project configuration for async unboxing/instantiating stub tests
src/tests/async/inst-unbox-thunks/inst-unbox-thunks.cs	Comprehensive test cases covering async unboxing and instantiating stubs
src/tests/async/Directory.Build.targets	Enables async testing by uncommenting DisableProjectBuild
src/coreclr/vm/siginfo.cpp	Adds PermitUninstDefOrRef flag to type loading
src/coreclr/vm/prestub.cpp	Major updates to support async in unboxing and instantiating stubs
src/coreclr/vm/jitinterface.cpp	Adds async calling convention translation and removes unnecessary token reset
src/coreclr/vm/ilstubcache.h	Extends IL stub cache to support async stubs
src/coreclr/vm/ilstubcache.cpp	Implements async stub creation with proper async method data
src/coreclr/vm/comdelegate.cpp	Adds async continuation argument handling to portable shuffle array
src/coreclr/vm/callingconvention.h	Adds GetAsyncContinuatioLoc method for argument location
src/coreclr/jit/importercalls.cpp	Updates async call handling for CALLI instructions
src/coreclr/inc/corhdr.h	Defines new IMAGE_CEE_CS_CALLCONV_ASYNC calling convention
src/coreclr/inc/clrconfigvalues.h	Enables RuntimeAsync support by default

[VSadov]

The network timeouts on Android are unlikely related to the changes

[VSadov]

/ba-g the failures are known - #87783 and #118350