[PATCH] D60333: [X86][SSE] SimplifyDemandedBitsForTargetNode - Add PACKSS support

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

This looks right to me, but I think we should be able to do better. The sign bit of the result is determined only by the sign bit of the input. Any other bit is determined by the same bit in the input, as well as the high half of the input. I'm thinking something along these lines:

  APInt DemandedBits;
  if (OriginalDemandedBits.isSignMask()) {
      DemandedBits = APInt::getSignMask(BitWidth * 2);
  } else {
      DemandedBits = OriginalDemandedBits.zext(BitWidth * 2);
      DemandedBits.setHighBits(BitWidth);
  }


Repository:
  rL LLVM

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  https://reviews.llvm.org/D60333/new/

https://reviews.llvm.org/D60333