[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.

> No as we don't want to go though replacing the instruction by itself and we especially don't want to replace false by true in the program.


I don't understand the 2nd comment. For the first, I'm not suggesting that you replace the instruction by itself.
Looking around InstCombine, the common pattern I see is:

  I.setOperand(1, X);
  return &I;

This adheres to the 'visit' function comments:

  // Visitation implementation - Implement instruction combining for different
  // instruction types.  The semantics are as follows:
  // Return Value:
  //    null        - No change was made
  //     I          - Change was made, I is still valid, I may be dead though
  //   otherwise    - Change was made, replace I with returned instruction

I think we'd have to change the helper function signature to return an Instruction* for this to work, so I'll defer to @majnemer on whether the code here is an acceptable substitute. I have no further comments for this patch.


https://reviews.llvm.org/D23134