[llvm] [RISCV] Extend zvqdot matching to handle reduction trees (PR #138965)

[Luke Lau via llvm-commits]

================
@@ -18131,6 +18131,29 @@ static MVT getQDOTXResultType(MVT OpVT) {
   return MVT::getVectorVT(MVT::i32, OpEC.divideCoefficientBy(4));
 }
 
+/// Given fixed length vectors A and B with equal element types, but possibly
+/// different number of elements, return A + B where either A or B is zero
+/// padded to the larger number of elements.
+static SDValue getZeroPaddedAdd(const SDLoc &DL, SDValue A, SDValue B,
+                                SelectionDAG &DAG) {
+  // NOTE: Manually doing the extract/add/insert scheme produces
+  // significantly better coegen than the naive pad with zeros
+  // and add scheme.
+  EVT AVT = A.getValueType();
+  EVT BVT = B.getValueType();
+  assert(AVT.getVectorElementType() == BVT.getVectorElementType());
+  if (AVT.getVectorNumElements() > BVT.getVectorNumElements()) {
+    std::swap(A, B);
+    std::swap(AVT, BVT);
+  }
+
+  SDValue BPart = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, AVT, B,
+                              DAG.getVectorIdxConstant(0, DL));
+  SDValue Res = DAG.getNode(ISD::ADD, DL, AVT, A, BPart);
+  return DAG.getNode(ISD::INSERT_SUBVECTOR, DL, BVT, B, Res,
+                     DAG.getVectorIdxConstant(0, DL));
----------------
lukel97 wrote:

Can you use the new DAG.get{Insert,Extract}Subvector utils you added recently here?

https://github.com/llvm/llvm-project/pull/138965