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

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 12f6c171d1a1588b1b5baffb2cf1ed78933fe914..f6cfb4fbc7b3630cbd9484272fbc5d812593d0f4 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -4644,6 +4644,7 @@ void StringAddStub::GenerateConvertArgument(MacroAssembler* masm,
   __ j(below, &done);
 
   // Check the number to string cache.
+  Label not_cached;
   __ bind(&not_string);
   // Puts the cached result into scratch1.
   NumberToStringStub::GenerateLookupNumberStringCache(masm,
@@ -4651,9 +4652,22 @@ void StringAddStub::GenerateConvertArgument(MacroAssembler* masm,
                                                       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);
 }