[all-commits] [llvm/llvm-project] 1e5f81: [InstCombine] Fix infinite recursion in ashr/xor v...

[David Green via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1e5f814302f8c24bdcc6f79623b86bd32231278f
      https://github.com/llvm/llvm-project/commit/1e5f814302f8c24bdcc6f79623b86bd32231278f
  Author: David Green <david.green at arm.com>
  Date:   2021-11-04 (Thu, 04 Nov 2021)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
    M llvm/test/Transforms/InstCombine/xor-ashr.ll

  Log Message:
  -----------
  [InstCombine] Fix infinite recursion in ashr/xor vector fold.

The added test has poison lanes due to the vector shuffle. This can
cause an infinite loop of combines in instcombine where it folds
xor(ashr, -1) -> select (icmp slt 0), -1, 0 -> sext (icmp slt 0) -> xor(ashr, -1).
We usually prevent this by checking that the xor constant is not -1,
but with vectors some of the lanes may be -1, some may be poison. So
this changes the way we detect that from "!C1->isAllOnesValue()" to
"!match(C1, m_AllOnes())", which is more able to detect that some of the
lanes are poison.

Fixes PR52397