Promoting elements transitions to their own field.

src/bootstrapper.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 1614 matching lines @@
     array_function->shared()->set_construct_stub(
         isolate()->builtins()->builtin(Builtins::kArrayConstructCode));
     array_function->shared()->DontAdaptArguments();
 
     // InternalArrays should not use Smi-Only array optimizations. There are too
     // many places in the C++ runtime code (e.g. RegEx) that assume that
     // elements in InternalArrays can be set to non-Smi values without going
     // through a common bottleneck that would make the SMI_ONLY -> FAST_ELEMENT
     // transition easy to trap. Moreover, they rarely are smi-only.
     MaybeObject* maybe_map =
-        array_function->initial_map()->CopyDropTransitions();
+        array_function->initial_map()->CopyDropTransitions(
+            DescriptorArray::MAY_BE_SHARED);
     Map* new_map;
-    if (!maybe_map->To<Map>(&new_map)) return false;
+    if (!maybe_map->To(&new_map)) return false;
     new_map->set_elements_kind(FAST_HOLEY_ELEMENTS);
     array_function->set_initial_map(new_map);
 
     // Make "length" magic on instances.
     Handle<DescriptorArray> array_descriptors =
         factory()->CopyAppendForeignDescriptor(
             factory()->empty_descriptor_array(),
             factory()->length_symbol(),
             factory()->NewForeign(&Accessors::ArrayLength),
             static_cast<PropertyAttributes>(DONT_ENUM | DONT_DELETE));
@@ skipping 540 matching lines @@
           ASSERT(!to->HasFastProperties());
           // Add to dictionary.
           Handle<String> key = Handle<String>(descs->GetKey(i));
           Handle<Object> callbacks(descs->GetCallbacksObject(i));
           PropertyDetails d =
               PropertyDetails(details.attributes(), CALLBACKS, details.index());
           JSObject::SetNormalizedProperty(to, key, callbacks, d);
           break;
         }
         case MAP_TRANSITION:
-        case ELEMENTS_TRANSITION:
         case CONSTANT_TRANSITION:
         case NULL_DESCRIPTOR:
           // Ignore non-properties.
           break;
         case NORMAL:
           // Do not occur since the from object has fast properties.
         case HANDLER:
         case INTERCEPTOR:
           // No element in instance descriptors have proxy or interceptor type.
           UNREACHABLE();
@@ skipping 157 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   ASSERT(!IsActive());
 }
 
 } }  // namespace v8::internal