Reland "In-object double fields unboxing (for 64-bit only)."

src/json-stringifier.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_JSON_STRINGIFIER_H_
 #define V8_JSON_STRINGIFIER_H_
 
 #include "src/v8.h"
 
 #include "src/conversions.h"
@@ skipping 634 matching lines @@
     Handle<Map> map(object->map());
     for (int i = 0; i < map->NumberOfOwnDescriptors(); i++) {
       Handle<Name> name(map->instance_descriptors()->GetKey(i), isolate_);
       // TODO(rossberg): Should this throw?
       if (!name->IsString()) continue;
       Handle<String> key = Handle<String>::cast(name);
       PropertyDetails details = map->instance_descriptors()->GetDetails(i);
       if (details.IsDontEnum()) continue;
       Handle<Object> property;
       if (details.type() == FIELD && *map == object->map()) {
-        property = Handle<Object>(object->RawFastPropertyAt(
-            FieldIndex::ForDescriptor(*map, i)), isolate_);
+        FieldIndex field_index = FieldIndex::ForDescriptor(*map, i);
+        Isolate* isolate = object->GetIsolate();
+        if (object->IsUnboxedDoubleField(field_index)) {
+          double value = object->RawFastDoublePropertyAt(field_index);
+          property = isolate->factory()->NewHeapNumber(value);
+
+        } else {
+          property = handle(object->RawFastPropertyAt(field_index), isolate);
+        }
       } else {
         ASSIGN_RETURN_ON_EXCEPTION_VALUE(
             isolate_, property,
             Object::GetPropertyOrElement(object, key),
             EXCEPTION);
       }
       Result result = SerializeProperty(property, comma, key);
       if (!comma && result == SUCCESS) comma = true;
       if (result == EXCEPTION) return result;
     }
@@ skipping 203 matching lines @@
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_