[PATCH] D104156: [DAGCombine][X86][ARM] EXTRACT_SUBVECTOR(VECTOR_SHUFFLE(?,?,Mask)) -> VECTOR_SHUFFLE(EXTRACT_SUBVECTOR(?, ?), EXTRACT_SUBVECTOR(?, ?), Mask')

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:20684
+      !TLI.isOperationLegalOrCustom(ISD::VECTOR_SHUFFLE, NarrowVT))
+    return SDValue();
+
----------------
lebedev.ri wrote:
> RKSimon wrote:
> > Shouldn't this be done earlier as soon as we know NarrowVT to early-out ?
> Well, yes and no. As it can be seen in the comment for the previous code block,
> we then would loose `return DAG.getUNDEF(NarrowVT);` constant-fold case.
> Should we not have it?
I suppose it depends if these UNDEF paths are actually active?


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  https://reviews.llvm.org/D104156/new/

https://reviews.llvm.org/D104156