[RISCV] Form vredsum from explode_vector + scalar (left) reduce

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -11122,6 +11122,85 @@ void RISCVTargetLowering::ReplaceNodeResults(SDNode *N,
   }
 }
 
+/// Perform two related transforms whose purpose is to incrementally recognize
+/// an explode_vector followed by scalar reduction as a vector reduction node.
+/// This exists to recover from a deficiency in SLP which can't handle
+/// forests with multiple roots sharing common nodes.  In some cases, one
+/// of the trees will be vectorized, and the other will remain (unprofitably)
+/// scalarized.
+static SDValue
+combineBinOpOfExtractToReduceTree(SDNode *N, SelectionDAG &DAG,
+                                  const RISCVSubtarget &Subtarget) {
+
+  // This transforms need to run before all integer types have been legalized
+  // to i64 (so that the vector element type matches the add type), and while
+  // it's safe to introduce odd sized vector types.
+  if (DAG.NewNodesMustHaveLegalTypes)
+    return SDValue();
+
+  const SDLoc DL(N);
+  const EVT VT = N->getValueType(0);
+  [[maybe_unused]] const unsigned Opc = N->getOpcode();
+  assert(Opc == ISD::ADD && "extend this to other reduction types");
+  const SDValue LHS = N->getOperand(0);
+  const SDValue RHS = N->getOperand(1);
+
+  if (!LHS.hasOneUse() || !RHS.hasOneUse())
+    return SDValue();
+
+  if (RHS.getOpcode() != ISD::EXTRACT_VECTOR_ELT ||
+      !isa<ConstantSDNode>(RHS.getOperand(1)))
+    return SDValue();
+
+  SDValue SrcVec = RHS.getOperand(0);
+  EVT SrcVecVT = SrcVec.getValueType();
+  assert(SrcVecVT.getVectorElementType() == VT);
+  if (SrcVecVT.isScalableVector())
+    return SDValue();
+
+  if (SrcVecVT.getScalarSizeInBits() > Subtarget.getELen())
+    return SDValue();
+
+  // match binop (extract_vector_elt V, 0), (extract_vector_elt V, 1) to
+  // reduce_op (extract_subvector [2 x VT] from V).  This will form the
+  // root of our reduction tree. TODO: We could extend this to any two
+  // adjacent constant indices if desired.
+  if (LHS.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
+      LHS.getOperand(0) == SrcVec && isNullConstant(LHS.getOperand(1)) &&
+      isOneConstant(RHS.getOperand(1))) {
+    EVT ReduceVT = EVT::getVectorVT(*DAG.getContext(), VT, 2);
+    SDValue Vec = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, ReduceVT, SrcVec,
+                              DAG.getVectorIdxConstant(0, DL));
+    return DAG.getNode(ISD::VECREDUCE_ADD, DL, VT, Vec);
+  }
+
+  // Match (binop (reduce (extract_subvector V, 0),
+  //                      (extract_vector_elt V, sizeof(SubVec))))
+  // into a reduction of one more element from the original vector V.
+  if (LHS.getOpcode() != ISD::VECREDUCE_ADD)
+    return SDValue();
+
+  SDValue ReduceVec = LHS.getOperand(0);
+  if (ReduceVec.getOpcode() == ISD::EXTRACT_SUBVECTOR &&
+      ReduceVec.hasOneUse() && ReduceVec.getOperand(0) == RHS.getOperand(0) &&
+      isNullConstant(ReduceVec.getOperand(1))) {
+    uint64_t Idx = cast<ConstantSDNode>(RHS.getOperand(1))->getLimitedValue();
+    if (ReduceVec.getValueType().getVectorNumElements() == Idx) {
+      // For illegal types (e.g. 3xi32), most will be combined again into a
+      // wider (hopefully legal) type.  If this is a terminal state, we are
+      // relying on type legalization here to produce something reasonable
+      // and this lowering quality could probably be improved. (TODO)
+      EVT ReduceVT = EVT::getVectorVT(*DAG.getContext(), VT, Idx + 1);
+      SDValue Vec = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, ReduceVT, SrcVec,
+                                DAG.getVectorIdxConstant(0, DL));
+      return DAG.getNode(ISD::VECREDUCE_ADD, DL, VT, Vec);
+    }
+  }
+
+  return SDValue();
+}
+
+
 // Try to fold (<bop> x, (reduction.<bop> vec, start))
 static SDValue combineBinOpToReduce(SDNode *N, SelectionDAG &DAG,
                                     const RISCVSubtarget &Subtarget) {
@@ -11449,6 +11528,9 @@ static SDValue performADDCombine(SDNode *N, SelectionDAG &DAG,
     return V;
   if (SDValue V = combineBinOpToReduce(N, DAG, Subtarget))
     return V;
+  if (SDValue V = combineBinOpOfExtractToReduceTree(N, DAG, Subtarget))
+    return V;
+
   // fold (add (select lhs, rhs, cc, 0, y), x) ->
   //      (select lhs, rhs, cc, x, (add x, y))
   return combineSelectAndUseCommutative(N, DAG, /*AllOnes*/ false, Subtarget);