[PATCH] D36498: [InstCombine] Teach foldSelectICmpAnd to recognize a (icmp slt trunc X, 0) and (icmp sgt trunc X, -1) as equivalent to an and with the sign bit of the truncated type

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

I believe the issue is that this needs to handle And slightly differently. I think if MaskedValueIsZero returns true for the And, we should reset the BitsToClear to 0 before returning true.

  // If the operation is an AND/OR/XOR and the bits to clear are zero in the
  // other side, BitsToClear is ok.
  if (Tmp == 0 && I->isBitwiseLogicOp()) {
    // We use MaskedValueIsZero here for generality, but the case we care
    // about the most is constant RHS.
    unsigned VSize = V->getType()->getScalarSizeInBits();
    if (IC.MaskedValueIsZero(I->getOperand(1),
                             APInt::getHighBitsSet(VSize, BitsToClear),
                             0, CxtI))
      return true;
  }


https://reviews.llvm.org/D36498