[release/11.0] JIT: Fix promoted liveness in forward substitution - #134349
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Multi-use forward substitution could leave stale last-use flags on promoted struct parents, allowing a live struct copy to be removed. Make last-use invalidation promotion-aware and reuse the same logic for single- and multi-use substitution. Add a regression test. Fixes #133641 > [!NOTE] > This PR description was generated with GitHub Copilot. --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch |
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@dotnet/jit-contrib ptal |
JulieLeeMSFT
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Sep 21, 2026
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Backport of #133703 to release/11.0
/cc @AndyAyersMS
Customer Impact
Forward substitution can leave stale last-use flags on promoted struct parents and remove a live struct copy. The reported case returns
6instead of the correct value8. This reproduces on .NET 11 RC2; .NET 8, 9, and 10 are correct. See #133641.Regression
Introduced during the .NET 11 cycle by #129312.
Testing
Added a focused regression test that exercises multi-use forward substitution of a promoted struct and verifies that the defining copy remains live. The source PR was validated through JIT CI and SuperPMI analysis; only a small number of expected code-generation differences were observed.
Risk
Low. The fix makes last-use invalidation promotion-aware and reuses the same logic for single- and multi-use substitution. It preserves liveness conservatively rather than enabling a new optimization.