[llvm] [RISCV][ISel] Remove redundant vmerge for the vwadd. (PR #78403)

[Craig Topper via llvm-commits]

================
@@ -13457,6 +13457,56 @@ combineBinOp_VLToVWBinOp_VL(SDNode *N, TargetLowering::DAGCombinerInfo &DCI) {
   return InputRootReplacement;
 }
 
+// (vwadd y, (select cond, x, 0)) -> select cond (vwadd y, x), y
+static SDValue combineVWADDSelect(SDNode *N, SelectionDAG &DAG) {
+  unsigned Opc = N->getOpcode();
+  assert(Opc == RISCVISD::VWADD_VL || Opc == RISCVISD::VWADD_W_VL ||
+         Opc == RISCVISD::VWADDU_W_VL);
+
+  SDValue VL = N->getOperand(4);
+  SDValue Y = N->getOperand(0);
+  SDValue Merge = N->getOperand(1);
+
+  if (Merge.getOpcode() != RISCVISD::VMERGE_VL)
+    return SDValue();
+
+  SDValue Cond = Merge->getOperand(0);
+  SDValue X = Merge->getOperand(1);
+  SDValue Z = Merge->getOperand(2);
+
+  if (Z.getOpcode() != ISD::INSERT_SUBVECTOR ||
+      !isNullConstant(Z.getOperand(2)))
+    return SDValue();
+
+  if (!Merge.hasOneUse())
+    return SDValue();
+
+  SmallVector<SDValue, 6> Ops(N->op_values());
+  Ops[0] = Y;
+  Ops[1] = X;
+
+  SDLoc DL(N);
+  EVT VT = N->getValueType(0);
+
+  SDValue WX = DAG.getNode(Opc, DL, VT, Ops, N->getFlags());
+  return DAG.getNode(RISCVISD::VMERGE_VL, DL, VT, Cond, WX, Y, DAG.getUNDEF(VT),
----------------
topperc wrote:

Operand 2 of the original vmerge is the passthru operand for elements past VL if the VWADD is tail undisturbed. This VMERGE_VL has undef for its passthru. That corrupts the elements past VL.

https://github.com/llvm/llvm-project/pull/78403