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

[Luke Lau via llvm-commits]

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@@ -5197,6 +5197,67 @@ static bool isCompressMask(ArrayRef<int> Mask) {
   return true;
 }
 
+/// Given a shuffle where the indices are disjoint between the two sources,
+/// e.g.:
+///
+/// t2:v4i8 = vector_shuffle t0:v4i8, t1:v4i8, <2, 7, 1, 4>
+///
+/// Merge the two sources into one and do a single source shuffle:
+///
+/// t2:v4i8 = vselect t1:v4i8, t0:v4i8, <0, 1, 0, 1>
+/// t3:v4i8 = vector_shuffle t2:v4i8, undef, <2, 3, 1, 0>
+///
+/// A vselect will either be merged into a masked instruction or be lowered as a
+/// vmerge.vvm, which is cheaper than a vrgather.vv.
+static SDValue lowerDisjointIndicesShuffle(ShuffleVectorSDNode *SVN,
+                                           SelectionDAG &DAG,
+                                           const RISCVSubtarget &Subtarget) {
+  MVT VT = SVN->getSimpleValueType(0);
+  MVT XLenVT = Subtarget.getXLenVT();
+  SDLoc DL(SVN);
+
+  const ArrayRef<int> Mask = SVN->getMask();
+
+  // Work out which source each lane will come from.
+  SmallVector<int, 16> Srcs(Mask.size(), -1);
+
+  for (int Idx : Mask) {
+    if (Idx == -1)
+      continue;
+    unsigned SrcIdx = Idx % Mask.size();
+    int Src = (uint32_t)Idx < Mask.size() ? 0 : 1;
+    if (Srcs[SrcIdx] == -1)
+      // Mark this source as using this lane.
+      Srcs[SrcIdx] = Src;
+    else if (Srcs[SrcIdx] != Src)
+      // The other source is using this lane: not disjoint.
+      return SDValue();
+  }
+
+  SmallVector<SDValue> SelectMaskVals;
+  for (int Lane : Srcs) {
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lukel97 wrote:

Good point, I've reversed it to match the generic fallthrough when SwapOps is false. But it's worth pointing out that this simultaneously removes some mask diffs and introduces some others too

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