JIT: Add an AsyncHelpers.TailAwait intrinsic#124175
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jakobbotsch wants to merge 4 commits intodotnet:mainfrom
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JIT: Add an AsyncHelpers.TailAwait intrinsic#124175jakobbotsch wants to merge 4 commits intodotnet:mainfrom
jakobbotsch wants to merge 4 commits intodotnet:mainfrom
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Can be used to instruct the JIT that an upcoming await should not create an explicit suspension point. This can be used to optimize instantiating/unboxing stubs and will simplify the NAOT side of the implementation of those.
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Pull request overview
This PR introduces a new AsyncHelpers.TailAwait intrinsic to let the runtime/JIT treat certain awaits as “tail awaits”, avoiding creation of an additional suspension point/continuation in forwarding stubs (notably instantiating/unboxing stubs), and updates the runtime async codegen documentation accordingly.
Changes:
- Emit
AsyncHelpers.TailAwait()from instantiating/unboxing IL stubs when the target method is async. - Add JIT recognition of
AsyncHelpers.TailAwait(named intrinsic + importer handling) and plumb a “tail await” flag into async call metadata. - Extend async transformation to rewrite tail awaits to directly return the callee’s continuation (no new continuation allocation), plus small comment/doc updates.
Reviewed changes
Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| src/coreclr/vm/prestub.cpp | Emits TailAwait before async calli in generated instantiating/unboxing IL stubs. |
| src/coreclr/vm/corelib.h | Adds CoreLib binder entry for AsyncHelpers.TailAwait. |
| src/coreclr/vm/asyncthunks.cpp | Updates comment to refer to AsyncHelpers.AsyncCallContinuation. |
| src/coreclr/jit/namedintrinsiclist.h | Adds named intrinsic enum value for AsyncHelpers.TailAwait. |
| src/coreclr/jit/importercalls.cpp | Maps and imports TailAwait, setting a “next await is tail” flag that’s consumed by async-call setup. |
| src/coreclr/jit/gentree.h | Adds IsTailAwait to AsyncCallInfo. |
| src/coreclr/jit/compiler.h | Adds m_nextAwaitIsTail importer state. |
| src/coreclr/jit/async.h | Adds tail-await transformation helpers and adjusts liveness plumbing signatures. |
| src/coreclr/jit/async.cpp | Implements tail-await transformation and refactors continuation temp allocation to be lazy. |
| src/coreclr/System.Private.CoreLib/.../AsyncHelpers.CoreCLR.cs | Adds intrinsic stub for TailAwait. |
| docs/design/coreclr/botr/runtime-async-codegen.md | Updates documentation to reference AsyncHelpers.AsyncCallContinuation. |
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Folded into #122126 |
MichalStrehovsky
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Feb 12, 2026
We need to: * Compile the unboxing/instantiating thunk as `CORJIT_FLAG_ASYNC` when the target is AsyncCall * Use the TailAwait intrinsic to just return the suspended state from the called method instead of trying to suspend the instantiating thunk too. Also includes a JIT change from #124175. Fixes #121781.
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Can be used to instruct the JIT that an upcoming await should not create an explicit suspension point. This can be used to optimize instantiating/unboxing stubs and will simplify the NAOT side of the implementation of those (#122126).