Perform string add in generated code on X64 platform...

src/x64/macro-assembler-x64.h

 // Copyright 2009 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 500 matching lines @@
   // un-done.
   void UndoAllocationInNewSpace(Register object);
 
   // Allocate a heap number in new space with undefined value. Returns
   // tagged pointer in result register, or jumps to gc_required if new
   // space is full.
   void AllocateHeapNumber(Register result,
                           Register scratch,
                           Label* gc_required);
 
+  // Allocate a sequential string. All the header fields of the string object
+  // are initialized.
+  void AllocateTwoByteString(Register result,
+                             Register length,
+                             Register scratch1,
+                             Register scratch2,
+                             Register scratch3,
+                             Label* gc_required);
+  void AllocateAsciiString(Register result,
+                           Register length,
+                           Register scratch1,
+                           Register scratch2,
+                           Register scratch3,
+                           Label* gc_required);
+
+  // Allocate a raw cons string object. Only the map field of the result is
+  // initialized.
+  void AllocateConsString(Register result,
+                          Register scratch1,
+                          Register scratch2,
+                          Label* gc_required);
+  void AllocateAsciiConsString(Register result,
+                               Register scratch1,
+                               Register scratch2,
+                               Label* gc_required);
+
   // ---------------------------------------------------------------------------
   // Support functions.
 
   // Check if result is zero and op is negative.
   void NegativeZeroTest(Register result, Register op, Label* then_label);
 
   // Check if result is zero and op is negative in code using jump targets.
   void NegativeZeroTest(CodeGenerator* cgen,
                         Register result,
                         Register op,
@@ skipping 19 matching lines @@
 
   // Find the function context up the context chain.
   void LoadContext(Register dst, int context_chain_length);
 
   // ---------------------------------------------------------------------------
   // Runtime calls
 
   // Call a code stub.
   void CallStub(CodeStub* stub);
 
+  // Tail call a code stub (jump).
+  void TailCallStub(CodeStub* stub);
+
   // Return from a code stub after popping its arguments.
   void StubReturn(int argc);
 
   // Call a runtime routine.
   // Eventually this should be used for all C calls.
   void CallRuntime(Runtime::Function* f, int num_arguments);
 
   // Convenience function: Same as above, but takes the fid instead.
   void CallRuntime(Runtime::FunctionId id, int num_arguments);
 
@@ skipping 146 matching lines @@
   }                                                                       \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_