[PATCH] D93065: [InstCombine] Disable optimizations of select instructions that causes propagation of poison values

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

I wanted to make ScalarEvolution recognize select pattern, and became a bit uncertain about its validity.

Take this example: https://alive2.llvm.org/ce/z/NsP9ue
SCEV's computeExitLimit can return min(n, m) as ExactNotTaken value, so I put llvm.assume to show its validity.
But it fails because the exit limit becomes poison if n is zero and m is poison. This will make e.g. replacing the last value of i with min(n, m) invalid.
If `and` is used instead, this becomes okay: https://alive2.llvm.org/ce/z/K9rbJk

If there is a guard about n and m at the loop header it would be safe I think. Is this what SCEV assumes? Then, how should the input look like?


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