[all-commits] [llvm/llvm-project] 2b7c38: [IR][InstSimplify] Clarify poison elements in cttz...

[Luke Lau via All-commits]

  Branch: refs/heads/users/lukel97/lang-ref-poison-elts-reductions-cttz-elts
  Home:   https://github.com/llvm/llvm-project
  Commit: 2b7c382b338bf1be2e5731d8aa14060d5803ca72
      https://github.com/llvm/llvm-project/commit/2b7c382b338bf1be2e5731d8aa14060d5803ca72
  Author: Luke Lau <luke at igalia.com>
  Date:   2026-09-16 (Wed, 16 Sep 2026)

  Changed paths:
    M llvm/docs/LangRef.md
    M llvm/lib/Analysis/ConstantFolding.cpp
    M llvm/test/Transforms/InstSimplify/ConstProp/cttz-elts.ll

  Log Message:
  -----------
  [IR][InstSimplify] Clarify poison elements in cttz.elts results in poison

The generic expansion for cttz.elts in LegalizeVectorOps is

    (reduce.umax (select input, step_vector, 0))

Because reductions produce poison if any element is poison, this will also produce poison.

ConstantFolding currently has short circuiting behaviour, e.g. `cttz.elts <0, 1, poison, poison> -> 1`, but we should probably make this consistent with reductions and just return poison. This updates LangRef and ConstantFolding to be more explicit about this.



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