Save async contexts for ValueTask methods#129890
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We were not properly saving/restoring async contexts for ValueTask methods because of an incorrect flag check.
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch |
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Pull request overview
This PR fixes Runtime Async’s ExecutionContext save/restore behavior for ValueTask-returning async variants by correcting an overly strict async-flag check in the VM. It also adds/updates tests to cover the regression and re-enables a previously skipped System.Net.Http diagnostics test.
Changes:
- Fix VM async-flag logic to treat ValueTask async variants as requiring async-context save/restore.
- Add an ExecutionContext regression test covering
async ValueTaskflows and re-enable the affected diagnostics functional test. - Add DEBUG-only JIT knobs to selectively enable/break on await-optimization by hash range.
Reviewed changes
Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.
Show a summary per file
| File | Description |
|---|---|
| src/tests/async/execution-context/execution-context.cs | Adds a new ValueTask-focused ExecutionContext regression test. |
| src/libraries/System.Net.Http/tests/FunctionalTests/DiagnosticsTests.cs | Removes ActiveIssue to re-enable the diagnostics test previously associated with the bug. |
| src/coreclr/vm/method.hpp | Fixes async-flag check for ValueTask variants; adds operator~ helper for flag masking. |
| src/coreclr/jit/jitconfigvalues.h | Introduces DEBUG-only config entries for await optimization range/break. |
| src/coreclr/jit/importer.cpp | Adds an await-level “optimize or not” check (DEBUG-configurable) when selecting async variants. |
| src/coreclr/jit/compiler.h | Declares the new importer helper (impCheckOptimizeAwait). |
jakobbotsch
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Jun 26, 2026
| AsyncMethodFlags asyncFlags = GetAddrOfAsyncMethodData()->flags; | ||
| // asynccall that is also async variant, but not a thunk | ||
| return asyncFlags == (AsyncMethodFlags::AsyncCall | AsyncMethodFlags::IsAsyncVariant); | ||
| return (asyncFlags & ~AsyncMethodFlags::IsAsyncVariantForValueTask) == (AsyncMethodFlags::AsyncCall | AsyncMethodFlags::IsAsyncVariant); |
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We were not properly saving/restoring async contexts for ValueTask methods because of an incorrect flag check.
Fix #129771
Also add a mechanism to allow a bisection-like search of optimized awaits in the JIT.
This is a surprising bug/test hole. It is also surprising that it was exposed by #128384, but the test seems to be timing sensitive -- it also stops reproducing if I introduce an
await Task.Yield()inWaitForConnectionWithTelemetryAsync.