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[ValueTracking] Check round-trip cast in lookThroughCast - #9118
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Steven Perron (s-perron) wants to merge 1 commit into
Open
Steven Perron (s-perron) wants to merge 1 commit into
Steven Perron (s-perron) wants to merge 1 commit into
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lookThroughCast truncated the constant arm of a select without verifying that casting back reproduces the original constant, causing InstCombine to narrow selects whose constants do not fit in the smaller integer type. Route ZExtInst, SExtInst, and TruncInst through a round-trip cast check to ensure no bits are lost. Backports llvm/llvm-project@d2a074b1f463 and llvm/llvm-project@826e9831a7f6.
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🟡 Changes recommended
The user-visible optimizer miscompile fix is missing the required Bug Fixes entry in docs/ReleaseNotes.md.
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What changed in this PR
Backports LLVM fixes to prevent InstCombine from rewriting cast-based selects when the constant cannot be reconstructed exactly.
Changes:
- Adds round-trip validation for zero-extension, sign-extension, and truncation in
lookThroughCast. - Adds regression tests for an out-of-range constant and a vector containing an undefined lane.
| File | Description |
|---|---|
tools/clang/test/DXC/Passes/InstructionCombining/select-zext-minmax.ll |
Tests rejection of select rewrites that fail round-trip validation. |
lib/Analysis/ValueTracking.cpp |
Requires exact constant reconstruction before recognizing cast-based select patterns. |
Review effort: Balanced
Comment on lines
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| // Make sure the cast doesn't lose any information. | ||
| if (CastedBack != C) | ||
| return nullptr; |
Steven Perron (s-perron)
requested review from
Chris B (llvm-beanz) and
Natalie Chouinard (sudonatalie)
November 13, 2026 18:36
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lookThroughCast truncated the constant arm of a select without
verifying that casting back reproduces the original constant, causing
InstCombine to narrow selects whose constants do not fit in the smaller
integer type. Route ZExtInst, SExtInst, and TruncInst through a round-trip
cast check to ensure no bits are lost.
Backports llvm/llvm-project@d2a074b1f463 and
llvm/llvm-project@826e9831a7f6.