[RISCV] Remove custom legalization of v2i16/v4i8 loads for P extension. (#167651)

We can use the default legalization which will create an i32 load
followed by a v2i32 scalar_to_vector followed by a bitcast. We can isel
the scalar_to_vector like a bitcast and not generate any instructions
for it.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -2691,6 +2691,16 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
     }
     break;
   }
+  case ISD::SCALAR_TO_VECTOR:
+    if (Subtarget->enablePExtCodeGen()) {
+      MVT SrcVT = Node->getOperand(0).getSimpleValueType();
+      if (VT == MVT::v2i32 && SrcVT == MVT::i64) {
+        ReplaceUses(SDValue(Node, 0), Node->getOperand(0));
+        CurDAG->RemoveDeadNode(Node);
+        return;
+      }
+    }
+    break;
   case ISD::INSERT_SUBVECTOR:
   case RISCVISD::TUPLE_INSERT: {
     SDValue V = Node->getOperand(0);

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -516,8 +516,6 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
       setTruncStoreAction(MVT::v8i16, MVT::v8i8, Expand);
       setTruncStoreAction(MVT::v2i32, MVT::v2i16, Expand);
       setTruncStoreAction(MVT::v4i16, MVT::v4i8, Expand);
-      setOperationAction(ISD::LOAD, MVT::v2i16, Custom);
-      setOperationAction(ISD::LOAD, MVT::v4i8, Custom);
     } else {
       VTs.append({MVT::v2i16, MVT::v4i8});
     }
@@ -14757,21 +14755,6 @@ void RISCVTargetLowering::ReplaceNodeResults(SDNode *N,
       return;
     }
 
-    if (Subtarget.is64Bit() && Subtarget.enablePExtCodeGen()) {
-      SDLoc DL(N);
-      SDValue ExtLoad =
-          DAG.getExtLoad(ISD::SEXTLOAD, DL, MVT::i64, Ld->getChain(),
-                         Ld->getBasePtr(), MVT::i32, Ld->getMemOperand());
-      if (N->getValueType(0) == MVT::v2i16) {
-        Results.push_back(DAG.getBitcast(MVT::v4i16, ExtLoad));
-        Results.push_back(ExtLoad.getValue(1));
-      } else if (N->getValueType(0) == MVT::v4i8) {
-        Results.push_back(DAG.getBitcast(MVT::v8i8, ExtLoad));
-        Results.push_back(ExtLoad.getValue(1));
-      }
-      return;
-    }
-
     assert(N->getValueType(0) == MVT::i32 && Subtarget.is64Bit() &&
            "Unexpected custom legalisation");