[RISCV] Use X0_Pair for storing 0 using Zilsd. (#141847)

When we're creating a Zilsd store from a split i64 value, check if both
inputs are 0 and use X0_Pair instead of a REG_SEQUENCE.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -1659,18 +1659,26 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
     SDValue Addr = Node->getOperand(3);
     SelectAddrRegImm(Addr, Base, Offset);
 
-    SDValue Ops[] = {
-        CurDAG->getTargetConstant(RISCV::GPRPairRegClassID, DL, MVT::i32),
-        Node->getOperand(1),
-        CurDAG->getTargetConstant(RISCV::sub_gpr_even, DL, MVT::i32),
-        Node->getOperand(2),
-        CurDAG->getTargetConstant(RISCV::sub_gpr_odd, DL, MVT::i32)};
+    SDValue Lo = Node->getOperand(1);
+    SDValue Hi = Node->getOperand(2);
+
+    SDValue RegPair;
+    // Peephole to use X0_Pair for storing zero.
+    if (isNullConstant(Lo) && isNullConstant(Hi)) {
+      RegPair = CurDAG->getRegister(RISCV::X0_Pair, MVT::Untyped);
+    } else {
+      SDValue Ops[] = {
+          CurDAG->getTargetConstant(RISCV::GPRPairRegClassID, DL, MVT::i32), Lo,
+          CurDAG->getTargetConstant(RISCV::sub_gpr_even, DL, MVT::i32), Hi,
+          CurDAG->getTargetConstant(RISCV::sub_gpr_odd, DL, MVT::i32)};
+
+      RegPair = SDValue(CurDAG->getMachineNode(TargetOpcode::REG_SEQUENCE, DL,
+                                               MVT::Untyped, Ops),
+                        0);
+    }
 
-    SDNode *RegPair = CurDAG->getMachineNode(TargetOpcode::REG_SEQUENCE, DL,
-                                             MVT::Untyped, Ops);
-    MachineSDNode *New =
-        CurDAG->getMachineNode(RISCV::SD_RV32, DL, MVT::Other,
-                               {SDValue(RegPair, 0), Base, Offset, Chain});
+    MachineSDNode *New = CurDAG->getMachineNode(RISCV::SD_RV32, DL, MVT::Other,
+                                                {RegPair, Base, Offset, Chain});
     CurDAG->setNodeMemRefs(New, {cast<MemSDNode>(Node)->getMemOperand()});
     ReplaceUses(SDValue(Node, 0), SDValue(New, 0));
     CurDAG->RemoveDeadNode(Node);

llvm/test/CodeGen/RISCV/zilsd.ll

@@ -110,10 +110,8 @@ entry:
 define void @store_g() nounwind {
 ; CHECK-LABEL: store_g:
 ; CHECK:       # %bb.0: # %entyr
-; CHECK-NEXT:    li a0, 0
-; CHECK-NEXT:    lui a2, %hi(g)
-; CHECK-NEXT:    li a1, 0
-; CHECK-NEXT:    sd a0, %lo(g)(a2)
+; CHECK-NEXT:    lui a0, %hi(g)
+; CHECK-NEXT:    sd zero, %lo(g)(a0)
 ; CHECK-NEXT:    ret
 entyr:
   store i64 0, ptr @g