[all-commits] [llvm/llvm-project] ca7eaa: [InstSimplify] allow undef element match in vector...

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  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ca7eaa0a54938bfe9aeed7ebc64202f0b6f2ce0d
      https://github.com/llvm/llvm-project/commit/ca7eaa0a54938bfe9aeed7ebc64202f0b6f2ce0d
  Author: Sanjay Patel <spatel at rotateright.com>
  Date:   2021-05-25 (Tue, 25 May 2021)

  Changed paths:
    M llvm/lib/Analysis/InstructionSimplify.cpp
    M llvm/test/Transforms/InstCombine/select.ll
    M llvm/test/Transforms/InstSimplify/select-inseltpoison.ll
    M llvm/test/Transforms/InstSimplify/select.ll

  Log Message:
  -----------
  [InstSimplify] allow undef element match in vector select condition value

The semantics of select with undefined/poison condition
are not explicitly stated in the LangRef, but this matches
comments in the code and Alive2 appears to concur:
https://alive2.llvm.org/ce/z/KXytmd

We can find this pattern after demanded elements transforms.

As noted in D101191, fuzzers are finding infinite loops because
we may not account for this pattern in other passes.