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llvm / llvm-project / 30cabdda45a0f2e48aaa19bded9de9bb50bb83cb
commit30cabdda45a0f2e48aaa19bded9de9bb50bb83cb[log] [tgz]
authorMarius Kamp <msk@posteo.org>Tue Jun 10 18:12:10 2025 +0200
committerGitHub <noreply@github.com>Tue Jun 10 17:12:10 2025 +0100
treee00b941963fffea62e21a606bc746737b4e2b805
parent94c241614f491c0cd832614bde6130e315e53467 [diff]
[X86] Distribute Certain Bitwise Operations over SELECT (#136555)

InstCombine canonicalizes `(select P (and X (- X)) X)` to
`(and (select P (- X) umax) X)`. This is counterproductive for the X86
backend when BMI is available because we can encode `(and X (- X))`
using the `BLSI` instruction. A similar situation arises if we have
`(select P (and X (sub X 1)) X)` (prevents use of `BLSR` instruction) or
`(select P (xor X (sub X 1)) X)` (prevents use of `BLSMSK` instruction).
    
Trigger the inverse transformation in the X86 backend if BMI is
available and we can use the mentioned BMI instructions. This is done by
overriding the appropriate `shouldFoldSelectWithIdentityConstant()`
overload. In this way, we get `(select P (and X (- X)) X)` again, which
enables the use of `BLSI` (similar for the other cases described above).
    
Alive proofs: https://alive2.llvm.org/ce/z/MT_pKi
    
Fixes #131587, fixes #133848.

---------

Co-authored-by: Simon Pilgrim <llvm-dev@redking.me.uk>
2 files changed
tree: e00b941963fffea62e21a606bc746737b4e2b805
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
README.md

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