Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 9080 matching lines @@
   HValue* context = environment()->LookupContext();
   HGlobalObject* global_object = Add<HGlobalObject>(context);
   HRandom* result = new(zone()) HRandom(global_object);
   return ast_context()->ReturnInstruction(result, call->id());
 }
 
 
 // Fast support for StringAdd.
 void HOptimizedGraphBuilder::GenerateStringAdd(CallRuntime* call) {
   ASSERT_EQ(2, call->arguments()->length());
-  CHECK_ALIVE(VisitArgumentList(call->arguments()));
+  CHECK_ALIVE(VisitForValue(call->arguments()->at(0)));
+  CHECK_ALIVE(VisitForValue(call->arguments()->at(1)));
+  HValue* right = Pop();
+  HValue* left = Pop();
   HValue* context = environment()->LookupContext();
-  HCallStub* result = new(zone()) HCallStub(context, CodeStub::StringAdd, 2);
-  Drop(2);
+  HInstruction* result = HStringAdd::New(
+      zone(), context, left, right, NO_STRING_ADD_FLAGS);
   return ast_context()->ReturnInstruction(result, call->id());
 }
 
 
 // Fast support for SubString.
 void HOptimizedGraphBuilder::GenerateSubString(CallRuntime* call) {
   ASSERT_EQ(3, call->arguments()->length());
   CHECK_ALIVE(VisitArgumentList(call->arguments()));
   HValue* context = environment()->LookupContext();
   HCallStub* result = new(zone()) HCallStub(context, CodeStub::SubString, 3);
@@ skipping 815 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal