[llvm] r295782 - DAG: Check if extract_vector_elt is legal or custom

[Friedman, Eli via llvm-commits]

On 2/21/2017 2:47 PM, Matt Arsenault via llvm-commits wrote:
> Author: arsenm
> Date: Tue Feb 21 16:47:27 2017
> New Revision: 295782
>
> URL: http://llvm.org/viewvc/llvm-project?rev=295782&view=rev
> Log:
> DAG: Check if extract_vector_elt is legal or custom
>
> Avoids test regressions in future AMDGPU commits when
> more vector types are custom lowered.
>
> Modified:
>      llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=295782&r1=295781&r2=295782&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Tue Feb 21 16:47:27 2017
> @@ -7882,7 +7882,7 @@ SDValue DAGCombiner::visitTRUNCATE(SDNod
>       EVT SrcVT = VecSrc.getValueType();
>       if (SrcVT.isVector() && SrcVT.getScalarType() == VT &&
>           (!LegalOperations ||
> -         TLI.isOperationLegal(ISD::EXTRACT_VECTOR_ELT, SrcVT))) {
> +         TLI.isOperationLegalOrCustom(ISD::EXTRACT_VECTOR_ELT, SrcVT))) {
>         SDLoc SL(N);
>   
>         EVT IdxVT = TLI.getVectorIdxTy(DAG.getDataLayout());
>

This doesn't work.  If LegalOperations is true, we're creating an 
operation which isn't legal after the legalizer has run, so a operation 
marked CUSTOM could fail to match.  (For example, on AArch64, we 
custom-lower EXTRACT_VECTOR_ELT on 64-bit vectors by widening them to 
128-bit vectors.)

If you're running into trouble with this, maybe it makes sense to add a 
specific helper here, along the lines of isShuffleMaskLegal?

-Eli

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