[PATCH] D47348: [LoopIdiomRecognize] Only convert loops to ctlz if we can prove that the input is non-negative.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: spatel, hfinkel, efriedma.
Herald added a reviewer: javed.absar.

Loop idiom recognize tries to convert loops like

  int foo(int x) {
    int cnt = 0;
    while (x) {
      x >>= 1;
      ++cnt;
    }
    return cnt;
  }

into calls to ctlz, but if x is initially negative this loop should be infinite.

It happens that the cases that motivated this change have an absolute value of x before the loop. So this patch restricts the transform to cases where we know x is positive. Note: We are relying on the absolute value of INT_MIN to be undefined so we can assume that the result is always positive.

Fixes PR37479


https://reviews.llvm.org/D47348

Files:
  test/Transforms/LoopIdiom/ARM/ctlz.ll
  test/Transforms/LoopIdiom/X86/ctlz.ll

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