Preserve scalar negation types when folding FMA operands - #134522
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tannergooding merged 1 commit intoSep 24, 2026
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Copilot review overview
🔵 Needs a closer look
Cross-architecture JIT lowering changes warrant final human review.
Review effort: Lite
Findings: None
What changed in this PR
Fixes incorrect FMA negation folding across scalar type reinterprets on ARM64 and xarch.
Changes:
- Require matching scalar and FMA element types before negation folding.
- Add regressions for mismatched, same-type, and signed-zero cases.
| File | Description |
|---|---|
src/tests/JIT/Regression_ro_2/Runtime_134053.cs |
Adds focused regression tests and controls. |
src/coreclr/jit/lowerxarch.cpp |
Restricts scalar negation folding by type. |
src/coreclr/jit/lowerarmarch.cpp |
Applies the matching-type safeguard on ARM64. |
EgorBo
approved these changes
Sep 24, 2026
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Require the scalar
GT_NEGtype to match the FMA element type before folding it into a negated FMA variant on ARM64 and xarch.Vector bit reinterprets can expose integer negation or a different floating-point width to these folds. Integer negation is not a floating-point sign change, and double negation does not change the sign of a float in the low 32 bits. Preserve those operations while retaining same-type fusion, without changing operand evaluation order.
Add focused regressions for both mismatch classes and the analogous xarch path, plus same-type finite and signed-zero controls.
Validation:
JitStress=2.negor doublefnegbeforefmaddfor the repros. Same-type control disassembly is unchanged. Native ARM64 execution was unavailable.Resolves #134053
Note
This PR description was drafted with GitHub Copilot.