[PATCH] D111410: [InstCombine] generalize fold for mask-with-signbit-splat

[Sanjay Patel via Phabricator via llvm-commits]

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(iN X s>> (N-1)) & Y --> (X < 0) ? Y : 0

https://alive2.llvm.org/ce/z/qeYhdz

I was looking at a missing abs() transform and found my way to this generalization of an existing fold that was added with D67799 <https://reviews.llvm.org/D67799>. As discussed in that review, we want to make sure codegen handles this difference well, and for all of the targets/types that I spot-checked, it looks good.

There's a difference on an existing test that shows a potentially unnecessary use limit on an icmp fold.

That fold is in InstCombinerImpl::foldICmpSubConstant(), and IIRC there was some back-and-forth on it and similar folds because it could cause analysis/passes (SCEV, LSR?) to miss optimizations.

So I think this patch makes things more consistent. But if the more specific transform was here as a work-around, we could leave it in as a special-case before we do the more general fold (...but that would be pretty hacky).


https://reviews.llvm.org/D111410

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  llvm/test/Transforms/InstCombine/and.ll
  llvm/test/Transforms/InstCombine/icmp.ll
  llvm/test/Transforms/InstCombine/mul-inseltpoison.ll
  llvm/test/Transforms/InstCombine/mul.ll
  llvm/test/Transforms/InstCombine/sub-ashr-and-to-icmp-select.ll

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