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llvm / llvm-project / 2bdc1a1337692a5743658ba6b680e5d914e684a4
commit2bdc1a1337692a5743658ba6b680e5d914e684a4[log] [tgz]
authorFlorian Hahn <flo@fhahn.com>Fri Apr 04 11:48:01 2025 +0100
committerGitHub <noreply@github.com>Fri Apr 04 11:48:01 2025 +0100
tree22fcd3c32c9fad9adaee70085c00a88469f68b41
parentd6c076eeaa9d57363c6b64989d17fd094363bf9e [diff]
[LV] Use frozen start value for FindLastIV if needed. (#132691)

FindLastIV introduces multiple uses of the start value, where in the
original source there was only a single use, when the epilogue is
vectorized.

Each use of undef may produce a different result, so introducing
multiple uses can produce incorrect results when the input is
undef/poison.

If the start value may be undef or poison, freeze it and use the frozen
value, which will be the same at all uses.

See the following scenarios in Alive2:
* Both main and epilogue vector loops execute, go to exit block: https://alive2.llvm.org/ce/z/_TSvRr
* Both main and epilogue vector loops execute, go to scalar loop: https://alive2.llvm.org/ce/z/CsPj5v
* Only epilogue vector loop executes, go to exit block: https://alive2.llvm.org/ce/z/5XqkNV
* Only epilogue vector loop executes, go to scalar loop: https://alive2.llvm.org/ce/z/JUpqRN

The latter 2 show requiring freezing the resume phi. That means we cannot freeze 
in the preheader. We could move the freeze to the main iteration count check, but
that would be a bit fragile to find and other transforms can sink the freeze if needed.


Depends on https://github.com/llvm/llvm-project/pull/132689
and https://github.com/llvm/llvm-project/pull/132690.

Fixes https://github.com/llvm/llvm-project/issues/126836

PR: https://github.com/llvm/llvm-project/pull/132691
4 files changed
tree: 22fcd3c32c9fad9adaee70085c00a88469f68b41
  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-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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