[llvm] [RISCV] Lower disjoint shuffle of shuffles to MERGE-GATHER sequence (PR #178095)

[Ryan Buchner via llvm-commits]

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@@ -5439,6 +5439,107 @@ static SDValue lowerVECTOR_SHUFFLEAsVSlideup(const SDLoc &DL, MVT VT,
   return convertFromScalableVector(VT, Res, DAG, Subtarget);
 }
 
+// A shuffle of shuffles where:
+// - the first level of shuffles both only used data from a single input
+// - the final output doesn't use data from the same lane of both operands
+// This can be lowered to a MERGE followed by a GATHER
+static SDValue lowerVECTOR_SHUFFLEAsMergeGather(const SDLoc &DL, MVT VT,
+                                                SDValue V1, SDValue V2,
+                                                ArrayRef<int> Mask,
+                                                const RISCVSubtarget &Subtarget,
+                                                SelectionDAG &DAG) {
+  if (V1.getOpcode() != ISD::VECTOR_SHUFFLE ||
+      V2.getOpcode() != ISD::VECTOR_SHUFFLE)
+    return SDValue();
+
+  if (!V1.hasOneUse() || !V2.hasOneUse())
+    return SDValue();
+
+  // Can just be reduced into a single gather operation
+  if (V1.getOperand(0) == V2.getOperand(0))
+    return SDValue();
+
+  unsigned NumElts = VT.getVectorNumElements();
+  auto *SVN1 = cast<ShuffleVectorSDNode>(V1.getNode());
+  auto *SVN2 = cast<ShuffleVectorSDNode>(V2.getNode());
+  auto V1Mask = SVN1->getMask();
+  auto V2Mask = SVN2->getMask();
+  // 0: Not set, 1: Set by V1, 2: Set by V2
+  SmallVector<unsigned> ShuffleLaneUses(NumElts, 0);
+  for (unsigned Idx : seq<unsigned>(NumElts)) {
+    int Lane = Mask[Idx];
+    auto LanePoisonOrOOB = [](int Lane, unsigned NumElts) -> bool {
+      return Lane < 0 || Lane > (int)NumElts;
+    };
+    // Don't handle if the index is poison or out of bounds
+    if (LanePoisonOrOOB(Lane, 2 * NumElts))
+      return SDValue();
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bababuck wrote:

Thanks for the explanation!

https://github.com/llvm/llvm-project/pull/178095