[PATCH] D38023: [X86] Prefer MOVSS/SD over BLENDI during legalization. Remove BLENDI versions of scalar arithmetic patterns

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

We currently disable some converting of shuffles to MOVSS/MOVSD during legalization if SSE41 is enabled. But later during shuffle combining we go back to prefering MOVSS/MOVSD.

Additionally we have patterns that look for BLENDIs to detect scalar arithmetic operations. I believe due to the combining using MOVSS/MOVSD these are unnecessary.

Interestingly, we still codegen blend instructions even though lowering/isel emit movss/movsd instructions. Turns out machine CSE commutes them to blend, and then commuting those blends back into blends that are equivalent to the original movss/movsd.

This patch fixes the inconsistency in legalization to prefer MOVSS/MOVSD. The one test change was caused by this change. The problem is that we have integer types and are mostly selecting integer instructions except for the shufps. This shufps forced the execution domain, but the vpblendw couldn't have its domain changed with a naive instruction swap. We could fix this by special casing VPBLENDW based on the immediate to widen the element type.

The rest of the patch is removing all the excess scalar patterns.

Long term we should probably add isel patterns to make MOVSS/MOVSD emit blends directly instead of relying on the double commute. We may also want to consider emitting movss/movsd for optsize. I also wonder if we should still use the VEX encoded blendi instructions even with AVX512. Blends have better throughput, and that may outweigh the register constraint.


https://reviews.llvm.org/D38023

Files:
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86InstrAVX512.td
  lib/Target/X86/X86InstrSSE.td
  test/CodeGen/X86/vector-shuffle-256-v8.ll

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