JIT: Validate jump threading phi inputs - #134205
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Jump threading could remove a phi without accounting for all remaining predecessors, producing an invalid SSA rewrite. Require complete predecessor coverage before replacing phi uses. Fixes dotnet#133981 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 19e3976f-8f3e-4ab8-973d-d568620770c1
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch |
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@EgorBo PTAL A couple of SPMI diffs. |
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🟡 Changes recommended
A critical finding remains: count-only phi coverage can accept a missing current predecessor when stale phi arguments remain.
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Pull request overview
This PR hardens JIT jump-threading SSA replacement against incomplete phi inputs and adds regression coverage.
Changes:
- Adds predecessor-coverage checks for block and successor rewrites.
- Adds the
Phi_01regression test.
File summaries
| File | Summary |
|---|---|
src/tests/JIT/opt/RedundantBranch/JumpThreadPhi.cs |
Adds regression coverage for incorrect output. |
src/coreclr/jit/redundantbranchopts.cpp |
Adds phi predecessor coverage checks. |
Review details
- Files reviewed: 2/2 changed files
- Comments generated: 1
- Review effort level: Lite
EgorBo
approved these changes
Sep 18, 2026
Compare predecessor identities when validating threaded PHI replacements. This prevents stale PHI arguments from masking missing current predecessors. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 19e3976f-8f3e-4ab8-973d-d568620770c1
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@EgorBo revised to a more exact check, ptal |
EgorBo
approved these changes
Sep 19, 2026
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Jump threading can see an incomplete phi after an earlier flow edit. The SSA
replacement logic only checked the phi arguments that remained, so it could
remove the phi and rewrite uses without accounting for every predecessor.
Require the block and successor replacement paths to cover every expected
predecessor before accepting a common SSA definition. Otherwise, conservatively
skip the rewrite.
Add a regression case to
JumpThreadPhithat returned 101 instead of 200 beforethe fix.
Validation:
JumpThreadPhi: 2 passed; new test fails before the fix and passes after itfour affected collections, +82 bytes over about 1.86 million sequentially
replayed contexts
Resolves #133981
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This pull request description was generated with GitHub Copilot.