[PATCH] D88432: [InstCombine] Fix pr47668

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune created this revision.
aqjune added reviewers: lebedev.ri, spatel, craig.topper.
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This patch fixes pr47668 (https://llvm.org/pr47668) by checking whether the shift amount is smaller than the bitwidth of variables.

This is done by checking whether Z is a constant and it is smaller than the bitwidth.

When the shift amount is undef, it is unsafe in theory because 1 << undef is poison, causing the miscompilation happen again.
To prevent this, I made the undef simply fold into zero, since shl 0 can be optimized later.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D88432

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

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