[PATCH] D104574: AMDGPU: Fix high 16-bit optimization on gfx9

[Matt Arsenault via Phabricator via llvm-commits]

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We can do this optimization in the majority of cases, but we currently
don't have a way to do it. We do not track/model which instructions
have which behavior, the control bit to change the high bit behavior,
or making use of preserved bits at all. This is a bit fuzzy since we
don't know precisely how the source instruction will be lowered, but
that only really matters in one case (for fma_mixlo).

      

We do need to fixup some of these cases after selection, but the
pattern helps eliminate many of these zexts.


https://reviews.llvm.org/D104574

Files:
  llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
  llvm/lib/Target/AMDGPU/SIInstructions.td
  llvm/test/CodeGen/AMDGPU/fptrunc.f16.ll
  llvm/test/CodeGen/AMDGPU/preserve-hi16.ll

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