[PATCH] D85687: [Instcombine] Fix uses of undef (PR46940)

[Kazu Hirata via Phabricator via llvm-commits]

kazu created this revision.
kazu added reviewers: spatel, aqjune, nikic, lebedev.ri.
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Without this patch, we attempt to distribute And over Xor even in
unsafe circumstances like so:

  undef & (true ^ true)  ==>  (undef & true) ^ (undef & true)

and evaluate it to undef instead of false.  Note that "true ^ true"
may show up implicitly with one true being part of a PHI node.

This patch fixes the problem by teaching SimplifyAndInst and
SimplifyUsingDistributiveLaws to not use undef as part of
simplifications when they distribute And over Xor.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D85687

Files:
  llvm/lib/Analysis/InstructionSimplify.cpp
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/dont-distribute-phi.ll

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