[all-commits] [llvm/llvm-project] e196a7: [X86] Fix CCMP miscompile for flag-source-folded S...

[Evgenii Kudriashov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e196a7e873c01c7176345541167ecd3fcb60f2e4
      https://github.com/llvm/llvm-project/commit/e196a7e873c01c7176345541167ecd3fcb60f2e4
  Author: Evgenii Kudriashov <evgenii.kudriashov at intel.com>
  Date:   2026-09-29 (Tue, 29 Sep 2026)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/X86/apx/ccmp-flag-source.ll

  Log Message:
  -----------
  [X86] Fix CCMP miscompile for flag-source-folded SETCC leaves (#217682)

EmitCmp does not always return a comparison: where
it can it reuses the EFLAGS of an instruction that is computed anyway,
so (X | Y) == 0 comes back as the flags of an X86ISD::OR, (X & Y) == 0
as those of an X86ISD::AND, and (0 - X) == Y is rewritten into
(X + Y) == 0 and comes back as the flags of an X86ISD::ADD.

Only a subtract (CCMP) and an AND (CTEST) have conditional forms, so
build the link from the comparison's own operands rather than from a
flag source:

A comparison with zero can set neither OF nor CF, and TEST clears both,
so the last two are flag-equivalent for every condition code. An equality
comparison is a test of the operands' XOR, so an already live XOR is still
reused instead of keeping both operands live.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply at anthropic.com>



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