[llvm] [RISCV] Recurse on first operand of two operand shuffles (PR #79180)

[Craig Topper via llvm-commits]

================
@@ -4949,56 +4949,60 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
   MVT IndexContainerVT =
       ContainerVT.changeVectorElementType(IndexVT.getScalarType());
 
-  SDValue Gather;
-  // TODO: This doesn't trigger for i64 vectors on RV32, since there we
-  // encounter a bitcasted BUILD_VECTOR with low/high i32 values.
-  if (SDValue SplatValue = DAG.getSplatValue(V1, /*LegalTypes*/ true)) {
-    Gather = lowerScalarSplat(SDValue(), SplatValue, VL, ContainerVT, DL, DAG,
-                              Subtarget);
-  } else {
+  // Base case for the recursion just below - handle the worst case
+  // single source permutation.  Note that all the splat variants
+  // are handled above.
+  if (V2.isUndef()) {
     V1 = convertToScalableVector(ContainerVT, V1, DAG, Subtarget);
-    // If only one index is used, we can use a "splat" vrgather.
-    // TODO: We can splat the most-common index and fix-up any stragglers, if
-    // that's beneficial.
-    if (LHSIndexCounts.size() == 1) {
-      int SplatIndex = LHSIndexCounts.begin()->getFirst();
-      Gather = DAG.getNode(GatherVXOpc, DL, ContainerVT, V1,
-                           DAG.getConstant(SplatIndex, DL, XLenVT),
-                           DAG.getUNDEF(ContainerVT), TrueMask, VL);
-    } else {
-      SDValue LHSIndices = DAG.getBuildVector(IndexVT, DL, GatherIndicesLHS);
-      LHSIndices =
-          convertToScalableVector(IndexContainerVT, LHSIndices, DAG, Subtarget);
-
-      Gather = DAG.getNode(GatherVVOpc, DL, ContainerVT, V1, LHSIndices,
-                           DAG.getUNDEF(ContainerVT), TrueMask, VL);
+    SDValue LHSIndices = DAG.getBuildVector(IndexVT, DL, GatherIndicesLHS);
+    LHSIndices = convertToScalableVector(IndexContainerVT, LHSIndices, DAG,
+                                         Subtarget);
+    SDValue Gather = DAG.getNode(GatherVVOpc, DL, ContainerVT, V1, LHSIndices,
+                                 DAG.getUNDEF(ContainerVT), TrueMask, VL);
+    return convertFromScalableVector(VT, Gather, DAG, Subtarget);
+  }
+
+  // Translate the gather index we computed above (and possibly swapped)
+  // back to a shuffle mask.  This step should disappear once we complete
+  // the migration to recursive design.
+  SmallVector<int> ShuffleMaskLHS;
+  ShuffleMaskLHS.reserve(GatherIndicesLHS.size());
+  for (SDValue GatherIndex : GatherIndicesLHS) {
+    if (GatherIndex.isUndef()) {
----------------
topperc wrote:

Not related to this patch, but passing undef indices to gather might not be a good idea. The undef will probably become 0 during build_vector lowering. If the type is LMUL > 1, and the undef index occurs in the second or higher VLEN piece, this may make that VLEN piece of the destination falsely depend on the first VLEN of the source.

https://github.com/llvm/llvm-project/pull/79180