Revert "Clean up after r16292 (disable optimization for StringWrappers)."

src/x64/code-stubs-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 4626 matching lines @@
                                             Register scratch2,
                                             Register scratch3,
                                             Label* slow) {
   // First check if the argument is already a string.
   Label not_string, done;
   __ JumpIfSmi(arg, &not_string);
   __ CmpObjectType(arg, FIRST_NONSTRING_TYPE, scratch1);
   __ j(below, &done);
 
   // Check the number to string cache.
+  Label not_cached;
   __ bind(&not_string);
   // Puts the cached result into scratch1.
   NumberToStringStub::GenerateLookupNumberStringCache(masm,
                                                       arg,
                                                       scratch1,
                                                       scratch2,
                                                       scratch3,
-                                                      slow);
+                                                      &not_cached);
   __ movq(arg, scratch1);
   __ movq(Operand(rsp, stack_offset), arg);
+  __ jmp(&done);
+
+  // Check if the argument is a safe string wrapper.
+  __ bind(&not_cached);
+  __ JumpIfSmi(arg, slow);
+  __ CmpObjectType(arg, JS_VALUE_TYPE, scratch1);  // map -> scratch1.
+  __ j(not_equal, slow);
+  __ testb(FieldOperand(scratch1, Map::kBitField2Offset),
+           Immediate(1 << Map::kStringWrapperSafeForDefaultValueOf));
+  __ j(zero, slow);
+  __ movq(arg, FieldOperand(arg, JSValue::kValueOffset));
+  __ movq(Operand(rsp, stack_offset), arg);
+
   __ bind(&done);
 }
 
 
 void StringHelper::GenerateCopyCharacters(MacroAssembler* masm,
                                           Register dest,
                                           Register src,
                                           Register count,
                                           bool ascii) {
   Label loop;
@@ skipping 1938 matching lines @@
   __ bind(&fast_elements_case);
   GenerateCase(masm, FAST_ELEMENTS);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64