[all-commits] [llvm/llvm-project] 700feb: [InstCombine] Fix incorrect is_zero_poison when fo...

[Justin Lebar via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 700febc04caa41923eaf4b3911db26cb13482a83
      https://github.com/llvm/llvm-project/commit/700febc04caa41923eaf4b3911db26cb13482a83
  Author: Justin Lebar <justin.lebar at gmail.com>
  Date:   2026-06-08 (Mon, 08 Jun 2026)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
    M llvm/test/Transforms/InstCombine/select-ctlz-to-cttz.ll

  Log Message:
  -----------
  [InstCombine] Fix incorrect is_zero_poison when folding select+ctlz to cttz (#202388)

foldSelectCtlzToCttz folds

    %lz = call i32 @llvm.ctlz.i32(i32 (x & -x), i1 is_zero_poison)
    %r  = select (icmp eq x, 0), i32 32, i32 (xor %lz, 31)

into

    %r = call i32 @llvm.cttz.i32(i32 x, i1 is_zero_poison)

The original select's result is defined when x is zero, even if
is_zero_poison is true.  Therefore in the new cttz call, we need to pass
false for the second param, we can't reuse is_zero_poison.



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