[all-commits] [llvm/llvm-project] 8b22bb: [DivRemPairs] Do not freeze poisons that can't be ...

[Krzysztof Drewniak via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8b22bb8a62a259e35ccc49fb2f50077a2772cf2f
      https://github.com/llvm/llvm-project/commit/8b22bb8a62a259e35ccc49fb2f50077a2772cf2f
  Author: Krzysztof Drewniak <Krzysztof.Drewniak at amd.com>
  Date:   2024-05-20 (Mon, 20 May 2024)

  Changed paths:
    M llvm/lib/Transforms/Scalar/DivRemPairs.cpp
    M llvm/test/Transforms/DivRemPairs/AMDGPU/div-rem-pairs.ll

  Log Message:
  -----------
  [DivRemPairs] Do not freeze poisons that can't be undef (#92627)

Per comments in DivRemPairs, the rewrite from
```llvm
    %div = div %X, %Y
    %rem = rem %X, %Y
```
to
```llvm
    %div = div %X, %Y
    %.mul = mul %div, %Y
    %rem = sub %X, %mul
```
is unsound when %X or %Y are undef.

However, it is known to be sound if %X or %Y are poison but can't be
undef, since both the pre- and post-rewrite %rem are `poison`.

Additionally, proofs: https://alive2.llvm.org/ce/z/xtNQ8j

A comment in the pass listed a TODO for changing a usage of
isGuaranteedNotToBeUndefOrPoison() in the pass to something that only
detects undef. Such a function has been implemented since the time that
TODO was written, but has not been used.

Therefore, this commit updates DivRemPairs to use
isGuaranteedNotToBeUndef() instead.



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