Implement explicit back pointers in transition tree.

src/heap.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 2002 matching lines @@
   map->set_instance_type(instance_type);
   map->set_visitor_id(
       StaticVisitorBase::GetVisitorId(instance_type, instance_size));
   map->set_prototype(null_value(), SKIP_WRITE_BARRIER);
   map->set_constructor(null_value(), SKIP_WRITE_BARRIER);
   map->set_instance_size(instance_size);
   map->set_inobject_properties(0);
   map->set_pre_allocated_property_fields(0);
   map->init_instance_descriptors();
   map->set_code_cache(empty_fixed_array(), SKIP_WRITE_BARRIER);
-  map->set_prototype_transitions(empty_fixed_array(), SKIP_WRITE_BARRIER);
+  map->init_prototype_transitions(undefined_value());
   map->set_unused_property_fields(0);
   map->set_bit_field(0);
   map->set_bit_field2(1 << Map::kIsExtensible);
   map->set_elements_kind(elements_kind);
 
   // If the map object is aligned fill the padding area with Smi 0 objects.
   if (Map::kPadStart < Map::kSize) {
     memset(reinterpret_cast<byte*>(map) + Map::kPadStart - kHeapObjectTag,
            0,
            Map::kSize - Map::kPadStart);
@@ skipping 118 matching lines @@
 
   // Allocate the empty descriptor array.
   { MaybeObject* maybe_obj = AllocateEmptyFixedArray();
     if (!maybe_obj->ToObject(&obj)) return false;
   }
   set_empty_descriptor_array(DescriptorArray::cast(obj));
 
   // Fix the instance_descriptors for the existing maps.
   meta_map()->init_instance_descriptors();
   meta_map()->set_code_cache(empty_fixed_array());
-  meta_map()->set_prototype_transitions(empty_fixed_array());
+  meta_map()->init_prototype_transitions(undefined_value());
 
   fixed_array_map()->init_instance_descriptors();
   fixed_array_map()->set_code_cache(empty_fixed_array());
-  fixed_array_map()->set_prototype_transitions(empty_fixed_array());
+  fixed_array_map()->init_prototype_transitions(undefined_value());
 
   oddball_map()->init_instance_descriptors();
   oddball_map()->set_code_cache(empty_fixed_array());
-  oddball_map()->set_prototype_transitions(empty_fixed_array());
+  oddball_map()->init_prototype_transitions(undefined_value());
 
   // Fix prototype object for existing maps.
   meta_map()->set_prototype(null_value());
   meta_map()->set_constructor(null_value());
 
   fixed_array_map()->set_prototype(null_value());
   fixed_array_map()->set_constructor(null_value());
 
   oddball_map()->set_prototype(null_value());
   oddball_map()->set_constructor(null_value());
@@ skipping 4948 matching lines @@
   } else {
     p ^= 0x1d1ed & (Page::kPageSize - 1);  // I died.
   }
   remembered_unmapped_pages_[remembered_unmapped_pages_index_] =
       reinterpret_cast<Address>(p);
   remembered_unmapped_pages_index_++;
   remembered_unmapped_pages_index_ %= kRememberedUnmappedPages;
 }
 
 } }  // namespace v8::internal