JIT: Optimize always suspending helpers

[jakobbotsch]

Several async helpers are known to always suspend. We can skip the "returned a continuation?" check for those.

Fix #128324

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[Copilot]

Pull request overview

This PR teaches the CoreCLR JIT’s runtime-async transformation to treat certain async helper calls as always suspending, allowing it to skip generating the “did the call return a continuation?” null check and instead unconditionally branch to the suspension path. To enable this, the relevant helpers are marked [Intrinsic] and recognized by lookupNamedIntrinsic, and a new AsyncCallInfo::AlwaysSuspends bit flows from import to async transformation.

Changes:

• Mark AsyncHelpers.AwaitAwaiter / UnsafeAwaitAwaiter and CoreCLR’s Suspend* / TransparentSuspend* helpers as [Intrinsic] so the JIT can identify them reliably.
• Add new named intrinsics for these helpers and plumb NamedIntrinsic into impSetupAsyncCall to set AsyncCallInfo.AlwaysSuspends.
• Update AsyncTransformation::CreateCheckAndSuspendAfterCall to omit the null-continuation check when AlwaysSuspends is set.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.cs	Marks AwaitAwaiter / UnsafeAwaitAwaiter as [Intrinsic] to enable JIT recognition.
src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.CoreCLR.cs	Marks Suspend* / TransparentSuspend* helpers as [Intrinsic] for JIT recognition.
src/coreclr/jit/namedintrinsiclist.h	Adds new NamedIntrinsic IDs for always-suspending async helpers.
src/coreclr/jit/importercalls.cpp	Recognizes new helper intrinsics and plumbs NamedIntrinsic into async-call setup to set AlwaysSuspends.
src/coreclr/jit/gentree.h	Extends AsyncCallInfo with AlwaysSuspends.
src/coreclr/jit/compiler.h	Updates impSetupAsyncCall signature to accept the callee NamedIntrinsic.
src/coreclr/jit/async.cpp	Skips null-continuation check and changes CFG wiring for AlwaysSuspends calls.

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[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Sadly cannot see diffs here because it requires the APIs to be marked Intrinsic.

[jakobbotsch]

/ba-g Failure is #128907