Directly set the value in the descriptors pointer to avoid checks.

src/objects.cc

 // Copyright 2012 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 1792 matching lines @@
 
   // TODO(verwaest): From here on we lose existing map transitions, causing
   // invalid back pointers. This will change once we can store multiple
   // transitions with the same key.
 
   if (old_map->owns_descriptors()) {
     // If the old map owns its own descriptors, transfer ownership to the
     // new_map and install its descriptors in the old_map. Since the old_map
     // stores the descriptors for the new_map, remove the transition array of
     // the new_map that is only in place to store the descriptors.
-    old_map->transitions()->set_descriptors(new_map->instance_descriptors());
+    old_map->transitions()->descriptors_pointer()->set_value(
+        new_map->instance_descriptors());
     new_map->ClearTransitions(GetHeap());
     old_map->set_owns_descriptors(false);
     Map* map;
     JSGlobalPropertyCell* pointer =
         old_map->transitions()->descriptors_pointer();
     for (Object* current = old_map;
          !current->IsUndefined();
          current = map->GetBackPointer()) {
       map = Map::cast(current);
       if (!map->HasTransitionArray()) break;
@@ skipping 11667 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal