[all-commits] [llvm/llvm-project] 2bdc1a: [LV] Use frozen start value for FindLastIV if need...

[Florian Hahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2bdc1a1337692a5743658ba6b680e5d914e684a4
      https://github.com/llvm/llvm-project/commit/2bdc1a1337692a5743658ba6b680e5d914e684a4
  Author: Florian Hahn <flo at fhahn.com>
  Date:   2025-04-04 (Fri, 04 Apr 2025)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
    M llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
    M llvm/test/Transforms/LoopVectorize/AArch64/epilog-iv-select-cmp.ll
    M llvm/test/Transforms/LoopVectorize/epilog-iv-select-cmp.ll

  Log Message:
  -----------
  [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



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