Get rid of the unused native code StringAddStub.

src/x64/lithium-codegen-x64.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 4370 matching lines @@
   Register temp = ToRegister(instr->temp());
   Label no_memento_found;
   __ TestJSArrayForAllocationMemento(object, temp, &no_memento_found);
   DeoptimizeIf(equal, instr->environment());
   __ bind(&no_memento_found);
 }
 
 
 void LCodeGen::DoStringAdd(LStringAdd* instr) {
   ASSERT(ToRegister(instr->context()).is(rsi));
-  if (FLAG_new_string_add) {
-    ASSERT(ToRegister(instr->left()).is(rdx));
-    ASSERT(ToRegister(instr->right()).is(rax));
-    NewStringAddStub stub(instr->hydrogen()->flags(),
-                          isolate()->heap()->GetPretenureMode());
-    CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
-  } else {
-    EmitPushTaggedOperand(instr->left());
-    EmitPushTaggedOperand(instr->right());
-    StringAddStub stub(instr->hydrogen()->flags());
-    CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
-  }
+  ASSERT(ToRegister(instr->left()).is(rdx));
+  ASSERT(ToRegister(instr->right()).is(rax));
+  StringAddStub stub(instr->hydrogen()->flags(),
+                     isolate()->heap()->GetPretenureMode());
+  CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
 }
 
 
 void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
   class DeferredStringCharCodeAt V8_FINAL : public LDeferredCode {
    public:
     DeferredStringCharCodeAt(LCodeGen* codegen, LStringCharCodeAt* instr)
         : LDeferredCode(codegen), instr_(instr) { }
     virtual void Generate() V8_OVERRIDE {
       codegen()->DoDeferredStringCharCodeAt(instr_);
@@ skipping 1223 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64