[PATCH] D144671: [InstCombine] prevent miscompiles from select-of-div/rem transform

[Sanjay Patel via Phabricator via llvm-commits]

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This avoids the danger shown in issue #60906 <https://github.com/llvm/llvm-project/issues/60906>. There were no regression tests for these patterns, so these potential failures have been around for a long time.

We could carve out some exceptions to this bailout. For example, the transform is actually ok with udiv/urem and a common divisor because poison in the dividend would not cause immediate UB - the divisor has to be zero, and that would be UB in the original code too. That doesn't work with **signed** div/rem because we can choose the poison dividend to be MinSignedValue, and that overflows when divided by -1 causing UB that doesn't exist in the original code.

Here are a couple of examples that can be corrected with 'noundef':
https://alive2.llvm.org/ce/z/sXHTTC


https://reviews.llvm.org/D144671

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/select-divrem.ll

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