[llvm] [RISCV] Merge shuffle sources if lanes are disjoint (PR #119401)

[Luke Lau via llvm-commits]

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@@ -5540,6 +5601,16 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
     ShuffleMaskRHS.push_back(IsLHSOrUndefIndex ? -1 : (MaskIndex - NumElts));
   }
 
+  // If the mask indices are disjoint between the two sources, we can lower it
+  // as a vselect + a single source vrgather.vv. Don't do this if the operands
+  // will be splatted since they will be lowered to something cheaper like
+  // vrgather.vi anyway.
+  if (!DAG.isSplatValue(V2) && !DAG.isSplatValue(V1) &&
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lukel97 wrote:

Added the guard for the identity shuffle.

If isSplatMask is true for either source then the original lowering would have been a vrgather.vi + vrgather.vv at worse. So I don't think there would be much benefit to replacing that with vmerge.vvm + vrgather.vv, I attached an example diff in https://github.com/llvm/llvm-project/pull/119401#discussion_r1879288787

https://github.com/llvm/llvm-project/pull/119401