[PATCH] D116191: [X86] isVectorShiftByScalarCheap - vXi8 select(shift(x,splat0),shift(x,splat1)) is better than shift(x,select(splat0,splat1))

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


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Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:32997-33001
   // XOP has v16i8/v8i16/v4i32/v2i64 variable vector shifts.
   // Splitting for v32i8/v16i16 on XOP+AVX2 targets is still preferred.
   if (Subtarget.hasXOP() &&
       (Bits == 8 || Bits == 16 || Bits == 32 || Bits == 64))
     return false;
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pengfei wrote:
> I'm confused by the summary and the code here. Do we prefer to shifts on both has XOP and not? Why do we always return false for XOP?
XOP has awesome per-element shift ops for all 128-bit vector types - so there's no need to prefer vector shifts by scalar amounts over per-element amounts.


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  https://reviews.llvm.org/D116191/new/

https://reviews.llvm.org/D116191