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

[Philip Reames via llvm-commits]

================
@@ -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) &&
----------------
preames wrote:

I believe you're right.  I'd prefer to leave this as is for this change as the interaction of the combines is rather fiddly.  Happy to revisit in a follow up change though.  

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