JIT: Skip bitcast transformation for promoted locals #120435
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The bitcast transformation produces `BITCAST(LCL_VAR)`, but for SIMDs the local can be promoted and this ends up resulting in invalid IR. We would normally go through the `LCL_FLD` path that DNERs the local, so make sure we still do that.
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Pull Request Overview
This PR fixes a JIT bug where bitcast transformations on promoted SIMD locals resulted in invalid IR. The fix prevents bitcast transformation for promoted locals, ensuring they go through the proper LCL_FLD path that handles promoted locals correctly.
Key changes:
- Added a condition to skip bitcast transformation when the local variable is promoted
- Added a regression test to verify the fix works correctly
Reviewed Changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| src/coreclr/jit/lclmorph.cpp | Added check to prevent bitcast transformation on promoted locals |
| src/tests/JIT/Regression/JitBlue/Runtime_120414/Runtime_120414.cs | Regression test using Vector2/Vector128 operations to reproduce the bug |
| src/tests/JIT/Regression/JitBlue/Runtime_120414/Runtime_120414.csproj | Project file for the regression test |
EgorBo
approved these changes
Oct 6, 2025
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/ba-g Deadletter that succeeded on retry but is not being picked up by build analysis |
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/backport to release/10.0 |
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Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/18313339759 |
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The bitcast transformation produces
BITCAST(LCL_VAR), but for SIMDs the local can be promoted and this ends up resulting in invalid IR. We would normally go through theLCL_FLDpath that DNERs the local, so make sure we still do that.Fix #120414