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

[Philip Reames 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()) {
----------------
preames wrote:

Would you mind filing a bug for this?  I don't want to forget it.

For the don't care lanes, maybe we should be deliberately using "If an element index is out of range ( vs1[i] ≥ VLMAX ) then zero is returned for the element value."

Something like UINT_MAX is a reasonable guess at > VLMAX for most index types on real hardware.

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