JIT: Renable SSA-aware PHI jump threading#127103
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AndyAyersMS merged 1 commit intodotnet:mainfrom Apr 20, 2026
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Teach redundant branch elimination to keep jump threading through PHI-based blocks when the PHI uses can be fully accounted for in the block and its immediate successors. Rewrite the affected successor SSA/VN uses, keep dominating-block threading conservative, and add focused regression coverage for the new PHI-based cases. Fix included here that was not in dotnet#126812: ensure that field uses of locals get the proper VN updates. Fixes dotnet#126976.
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Pull request overview
This PR updates CoreCLR JIT redundant branch elimination (jump threading) to better handle SSA-aware PHI cases by allowing threading when PHI uses can be safely resolved and by keeping SSA/VN state consistent when rewriting those uses.
Changes:
- Adjust jump-thread viability checks to treat “global PHI uses” as potentially resolvable (rather than an immediate bail-out), enabling PHI-based threading in more cases.
- Update successor SSA uses during threading and fix VN rewriting for
GT_LCL_FLDso field loads get the correct derived field VN from the replacement SSA def.
EgorBo
approved these changes
Apr 20, 2026
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Teach redundant branch elimination to keep jump threading through PHI-based blocks when the PHI uses can be fully accounted for in the block and its immediate successors. Rewrite the affected successor SSA/VN uses, keep dominating-block threading conservative, and add focused regression coverage for the new PHI-based cases.
Fix included here that was not in #126812: ensure that field uses of locals get the proper VN updates.
Fixes #126976.