[PATCH] D29687: [TargetLowering] check for sign-bit comparisons in SimplifyDemandedBits

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
Herald added a subscriber: mcrosier.

I don't know if anything other than x86 vectors is affected by this change, but this may allow us to remove target-specific intrinsics for blendv* (vector selects). The simplification arises from the fact that blendv* instructions only use the sign-bit when deciding which vector element to choose for the destination vector. The mechanism to fold VSELECT into SHRUNKBLEND nodes already exists in x86 lowering; this demanded bits change just enables the transform to fire more often.

The original motivation starts with a bug for DSE of masked stores that seems completely unrelated, but I've explained the likely steps in this series here:
https://llvm.org/bugs/show_bug.cgi?id=11210


https://reviews.llvm.org/D29687

Files:
  lib/CodeGen/SelectionDAG/TargetLowering.cpp
  test/CodeGen/X86/vselect-pcmp.ll

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