[llvm] r277959 - [X86] lowerVectorShuffle - ensure that undefined mask elements only use SM_SentinelUndef

[Simon Pilgrim via llvm-commits]

No, but we don’t seem to have a hard rule that VECTOR_SHUFFLE can only use (-1) to define undefined mask elements - AFAICT it can be any negative value. This is preventing us from sharing a number of chunks of x86 shuffle matching code that current use Mask[i] < 0 to match all undefineds. 

Simon.

> On 7 Aug 2016, at 16:56, Craig Topper <craig.topper at gmail.com> wrote:
> 
> I don't quite understand this. Does combining come through lowerVectorShuffle? Or when would a VECTOR_SHUFFLE come through where this wasn't true?
> 
> On Sunday, August 7, 2016, Simon Pilgrim via llvm-commits <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
> Author: rksimon
> Date: Sun Aug  7 10:29:12 2016
> New Revision: 277959
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=277959&view=rev <http://llvm.org/viewvc/llvm-project?rev=277959&view=rev>
> Log:
> [X86] lowerVectorShuffle - ensure that undefined mask elements only use SM_SentinelUndef
> 
> Help lowering and combining (which can specify SM_SentinelZero mask elements) share more shuffle matching code.
> 
> Modified:
>     llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
> 
> Modified: llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelLowering.cpp?rev=277959&r1=277958&r2=277959&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelLowering.cpp?rev=277959&r1=277958&r2=277959&view=diff>
> ==============================================================================
> --- llvm/trunk/lib/Target/X86/X86ISelLowering.cpp (original)
> +++ llvm/trunk/lib/Target/X86/X86ISelLowering.cpp Sun Aug  7 10:29:12 2016
> @@ -12281,6 +12281,15 @@ static SDValue lowerVectorShuffle(SDValu
>          return DAG.getVectorShuffle(VT, DL, V1, V2, NewMask);
>        }
> 
> +  // Ensure that undefined mask elements only use SM_SentinelUndef.
> +  if (llvm::any_of(Mask, [](int M) { return M < SM_SentinelUndef; })) {
> +    SmallVector<int, 8> NewMask(Mask.begin(), Mask.end());
> +    for (int &M : NewMask)
> +      if (M < SM_SentinelUndef)
> +        M = SM_SentinelUndef;
> +    return DAG.getVectorShuffle(VT, DL, V1, V2, NewMask);
> +  }
> +
>    // We actually see shuffles that are entirely re-arrangements of a set of
>    // zero inputs. This mostly happens while decomposing complex shuffles into
>    // simple ones. Directly lower these as a buildvector of zeros.
> 
> 
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> 
> 
> -- 
> ~Craig

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