[PATCH] D129650: [InstCombine] change conditions for transform of sub to xor

[Sanjay Patel via Phabricator via llvm-commits]

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As discussed in in D128123 <https://reviews.llvm.org/D128123>, converting sub to xor can cause codegen regressions, and it may not be reversible with respect to no-wrap as shown here:
https://alive2.llvm.org/ce/z/CgOlZU

The PhaseOrdering test is based on the example provided in the other review.

Generally, we prefer bitwise-logic to math ops in IR even if that means dropping no-wrap flags -- because that allows better bit-tracking. But as noted, we need to improve the backend before easing the conditions here to avoid sub-optimal codegen.

This makes the IR fold the same as SDAG other than avoiding the transform in the presence of nsw/nuw. We probably want to update SDAG too -- either with a similar check as here or with a TLI hook to restrict the fold for targets that prefer sub-from-constant form.


https://reviews.llvm.org/D129650

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/sub-xor.ll
  llvm/test/Transforms/PhaseOrdering/SystemZ/sub-xor.ll

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