[PATCH] D23134: Make cltz and cttz zero undef when the operand cannot be zero in InstCombine

[Sanjay Patel via llvm-commits]

spatel added a comment.

> I has no idea there was an assertion to make sure of this. I'd still go for the logic "if it is anything but true, make it true" rather than it "if it false, make it true".


I think that's fair - how about this:

  if (KnownOne != 0 || isKnownNonZero(Op0, IC.getDataLayout()))
    if (!match(II.getArgOperand(1), m_One()))
      II.setArgOperand(1, ConstantInt::getTrue(II.getContext()));

But I think we need a comment to explain the transform and motivation. Something like

  // If the input to cttz/ctlz is known to be non-zero, 
  // then change the 'ZeroIsUndef' parameter to 'true'
  // because we know the zero behavior can't affect the result.


https://reviews.llvm.org/D23134