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JIT: check LIR unused values in the post-phase checks - #132612
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jakobbotsch merged 2 commits intoAug 24, 2026
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CheckLIR's checkUnusedValues checking was only done by direct calls from Rationalizer, DecomposeLongs and Lowering, while the post-phase checks ran it without. Meanwhile codegen, fgUpdateFlowGraph and fgDebugCheckNodeLinks called CheckLIR again with checking that the post-phase checks already do. Add CHECK_LIR_UNUSED_VALUES, enabled from rationalization until register allocation, and have fgDebugCheckLinks pass it on to CheckLIR. All six direct CheckLIR calls can then go away. The check cannot stay on for LSRA since it inserts nodes without users to model register saves/restores. Throughput on benchmarks.run for the checked JIT: -6.03% instructions retired, -5.59% wall clock. Co-authored-by: Copilot <[email protected]> Copilot-Session: 064035bf-fb1f-450e-afd2-08a7700aa736
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Pull request overview
This PR refactors how LIR unused-value validation is enabled and executed in the JIT’s debug/post-phase checking pipeline, so that the “unused value” invariant is checked consistently after phases (where appropriate) rather than via scattered direct CheckLIR(..., true) call sites.
Changes:
- Introduces
PhaseChecks::CHECK_LIR_UNUSED_VALUESand plumbs it intofgDebugCheckLinksso post-phaseCheckLIRcan optionally validate unused-value marking. - Enables the unused-value check from just before rationalization through lowering, and disables it prior to LSRA (since LSRA may intentionally insert nodes without users).
- Removes redundant direct
CheckLIRcalls from Rationalizer/Lowering/DecomposeLongs/CodeGen and some debug helpers, relying on post-phase checks instead.
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| src/coreclr/jit/compiler.h | Adds a new PhaseChecks bit to represent enabling the LIR unused-value validation. |
| src/coreclr/jit/compiler.cpp | Turns the new flag on before rationalization and off before LSRA. |
| src/coreclr/jit/fgdiagnostic.cpp | Passes the new flag through fgDebugCheckLinks to CheckLIR. |
| src/coreclr/jit/rationalize.cpp | Removes a direct CheckLIR(..., true) assert now covered by post-phase checks. |
| src/coreclr/jit/lower.cpp | Removes a direct CheckLIR(..., true) assert now covered by post-phase checks. |
| src/coreclr/jit/decomposelongs.cpp | Removes a direct CheckLIR(..., true) assert now covered by post-phase checks. |
| src/coreclr/jit/fgopt.cpp | Removes a redundant debug-only CheckLIR sweep from fgUpdateFlowGraph. |
| src/coreclr/jit/codegenlinear.cpp | Removes a redundant per-block CheckLIR assert in codegen. |
Member
Author
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PTAL @dotnet/jit-contrib |
jakobbotsch
commented
Aug 21, 2026
jakobbotsch
commented
Aug 21, 2026
Co-authored-by: Jakob Botsch Nielsen <[email protected]>
AndyAyersMS
approved these changes
Aug 21, 2026
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CheckLIR's checkUnusedValues checking was only done by direct calls from Rationalizer, DecomposeLongs and Lowering, while the post-phase checks ran it without. Meanwhile codegen, fgUpdateFlowGraph and fgDebugCheckNodeLinks called CheckLIR again with checking that the post-phase checks already do.
Add CHECK_LIR_UNUSED_VALUES, enabled from rationalization until register allocation, and have fgDebugCheckLinks pass it on to CheckLIR. All six direct CheckLIR calls can then go away.
The check cannot stay on for LSRA since it inserts nodes without users to model register saves/restores.
Throughput on benchmarks.run for the checked JIT: -6.03% instructions retired, -5.59% wall clock.