[PATCH] D118843: [x86] avoid false dependency stall on 'sbb' with same source reg

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
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This is effectively inverting the transform added with D116804 <https://reviews.llvm.org/D116804> because the downside of the false dependency of something like "sbb %eax, %eax" is much greater than the upside of eliminating a zeroing instruction on (all?) Intel CPUs.

I modeled the tuning flag on similar existing false dependency flags, but we could invert the logic and make this a positive flag for AMD instead. That would require less changes and not affect the default CPU models.


https://reviews.llvm.org/D118843

Files:
  llvm/lib/Target/X86/X86.td
  llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
  llvm/lib/Target/X86/X86Subtarget.h
  llvm/test/CodeGen/X86/pr32588.ll
  llvm/test/CodeGen/X86/sbb-false-dep.ll
  llvm/test/CodeGen/X86/select.ll

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