[PATCH] D99481: [InstCombine] Fix miscompile on GEP+load to icmp fold (PR45210)

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

I investigated a bit and the reason was as follows.

The reason was that `foldCmpLoadFromIndexedGlobal` can simply give up folding load+cmp, causing the new instructions created by these lines to be dummy ones:

  if (countTrailingZeros(ElementSize) != 0) {
    Value *Mask = ConstantInt::getSigned(Idx->getType(), -1);
    Mask = Builder.CreateLShr(Mask, countTrailingZeros(ElementSize));
    Idx = Builder.CreateAnd(Idx, Mask);
  }

In InstCombine's next iteration, they are found as a dead code. They are removed & InstCombine proceeds to the next iteration because there was something to remove.

Then, `foldCmpLoadFromIndexedGlobal` is called again, creating the dummy instructions, then dead code is removed, and so on.

To avoid this, the lshr+and should be created only when they are necessary.


Repository:
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https://reviews.llvm.org/D99481