Lower simplified StringAdd to stub call.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 9ccd4ed5ef7d303c156898c6aa138f952e3719ff..0d2be7eff85103731e3beabbccea190a6b7b756b 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -507,7 +507,7 @@ class RepresentationSelector {
       }
       case IrOpcode::kStringAdd: {
         VisitBinop(node, kMachAnyTagged, kMachAnyTagged);
-        // TODO(titzer): lower StringAdd to stub/runtime call.
+        if (lower()) lowering->DoStringAdd(node);
         break;
       }
       case IrOpcode::kLoadField: {
@@ -807,6 +807,19 @@ void SimplifiedLowering::DoStoreElement(Node* node) {
 }
 
 
+void SimplifiedLowering::DoStringAdd(Node* node) {
+  StringAddStub stub(zone()->isolate(), STRING_ADD_CHECK_NONE, NOT_TENURED);
+  CodeStubInterfaceDescriptor* d = stub.GetInterfaceDescriptor();
+  CallDescriptor::Flags flags = CallDescriptor::kNoFlags;
+  CallDescriptor* desc = Linkage::GetStubCallDescriptor(d, 0, flags, zone());
+  node->set_op(jsgraph()->common()->Call(desc));
+  node->InsertInput(zone(), 0, jsgraph()->HeapConstant(stub.GetCode()));
+  node->AppendInput(zone(), jsgraph()->UndefinedConstant());
+  node->AppendInput(zone(), graph()->start());
+  node->AppendInput(zone(), graph()->start());
+}
+
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8