[PATCH] D92745: [LoopIdiomRecognize] Teach detectShiftUntilZeroIdiom to recognize loops where the counter is decrementing.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: rengolin, joerg, lebedev.ri.
Herald added a subscriber: hiraditya.
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Herald added a project: LLVM.

This adds support for loops like

unsigned clz(unsigned x) {

  unsigned w = sizeof (x) * 8;
  while (x) {
      w--;
      x >>= 1;
  }
  
  return w;

}

and

unsigned clz(unsigned x) {

  unsigned w = sizeof (x) * 8 - 1;
  while (x >>= 1) {
      w--;
  }
  
  return w;

}

To support these we look for add x, -1 as well as add x, 1 that
we already matched.  If the value was -1 we need to subtract from
the initial counter value instead of adding to it.

Fixes PR48404.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D92745

Files:
  llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
  llvm/test/Transforms/LoopIdiom/X86/ctlz.ll
  llvm/test/Transforms/LoopIdiom/X86/cttz.ll

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