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[Reassociate] Bail out when 16-bit rank fields overflow - #9113
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Skip reassociation on functions whose argument count, basic-block count, or per-block unmovable instruction count exceeds the 16-bit fields packed into 32-bit ranks by BuildRankMap. In rank-overflow.ll, without bailing out, rank wraparound gives %l1 and %l2 the same rank across an intervening store, causing Reassociate to cancel %l1 - %l2 and fold (%base + %l1) - %l2 to %base.
Steven Perron (s-perron)
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Natalie Chouinard (sudonatalie)
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a balanced review from Copilot
November 10, 2026 15:25
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🟡 Changes recommended
The argument-count bailout lacks regression coverage for overflow and partial-map cleanup, and the user-visible wrong-code fix is missing a required release note.
2 open issues
What changed in this PR
This PR prevents incorrect compiler results from Reassociate rank overflow by skipping functions whose ranks exceed the packed 16-bit fields.
Changes:
- Add overflow checks for argument, basic-block, and per-block unmovable-instruction ranks.
- Clear partially built rank maps when skipping reassociation.
- Add instruction- and block-overflow regressions, checking that subsequent small functions still optimize.
| File | Description |
|---|---|
tools/clang/test/DXC/Passes/reassociate/rank-overflow.ll |
Tests overflow bailouts and continued optimization of a small function. |
lib/Transforms/Scalar/Reassociate.cpp |
Detects rank overflow and skips reassociation with map cleanup. |
Review effort: Balanced
Natalie Chouinard (sudonatalie)
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Nov 11, 2026
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Skip reassociation on functions whose argument count, basic-block count,
or per-block unmovable instruction count exceeds the 16-bit fields packed
into 32-bit ranks by BuildRankMap.
In rank-overflow.ll, without bailing out, rank wraparound gives %l1 and
%l2 the same rank across an intervening store, causing Reassociate to
cancel %l1 - %l2 and fold (%base + %l1) - %l2 to %base.