[llvm] [RISCV] Add combines to form binop from tail insert idioms (PR #72675)

[Luke Lau via llvm-commits]

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@@ -14159,6 +14159,75 @@ static SDValue performBUILD_VECTORCombine(SDNode *N, SelectionDAG &DAG,
                      DAG.getBuildVector(VT, DL, RHSOps));
 }
 
+static SDValue performINSERT_VECTOR_ELTCombine(SDNode *N, SelectionDAG &DAG,
+                                               const RISCVSubtarget &Subtarget,
+                                               const RISCVTargetLowering &TLI) {
+  SDValue InVec = N->getOperand(0);
+  SDValue InVal = N->getOperand(1);
+  SDValue EltNo = N->getOperand(2);
+  SDLoc DL(N);
+
+  EVT VT = InVec.getValueType();
+  if (VT.isScalableVector())
+    return SDValue();
+
+  if (!InVec.hasOneUse())
+    return SDValue();
+
+  // Given insert_vector_elt (binop a, VecC), (same_binop b, C2), Elt
+  // move the insert_vector_elts into the arms of the binop.  Note that
+  // the new RHS must be a constant.
+  const unsigned InVecOpcode = InVec->getOpcode();
+  if (InVecOpcode == InVal->getOpcode() && TLI.isBinOp(InVecOpcode) &&
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lukel97 wrote:

I think this combine would also work on scalable vectors too, is it worthwhile moving it up before the scalable vector check?

https://github.com/llvm/llvm-project/pull/72675