Copy-on-write arrays.

src/heap.cc

 // Copyright 2009 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 1439 matching lines @@
   // 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());
 
+  obj = AllocateMap(FIXED_ARRAY_TYPE, FixedArray::kHeaderSize);
+  if (obj->IsFailure()) return false;
+  set_fixed_cow_array_map(Map::cast(obj));
+  ASSERT(fixed_array_map() != fixed_cow_array_map());
+
   obj = AllocateMap(HEAP_NUMBER_TYPE, HeapNumber::kSize);
   if (obj->IsFailure()) return false;
   set_heap_number_map(Map::cast(obj));
 
   obj = AllocateMap(PROXY_TYPE, Proxy::kSize);
   if (obj->IsFailure()) return false;
   set_proxy_map(Map::cast(obj));
 
   for (unsigned i = 0; i < ARRAY_SIZE(string_type_table); i++) {
     const StringTypeTable& entry = string_type_table[i];
@@ skipping 1433 matching lines @@
     // the contents without worrying about updating the write barrier.
     CopyBlock(HeapObject::cast(clone)->address(),
               source->address(),
               object_size);
   }
 
   FixedArray* elements = FixedArray::cast(source->elements());
   FixedArray* properties = FixedArray::cast(source->properties());
   // Update elements if necessary.
   if (elements->length() > 0) {
-    Object* elem = CopyFixedArray(elements);
+    Object* elem =
+        (elements->map() == fixed_cow_array_map()) ?
+        elements : CopyFixedArray(elements);
     if (elem->IsFailure()) return elem;
     JSObject::cast(clone)->set_elements(FixedArray::cast(elem));
   }
   // Update properties if necessary.
   if (properties->length() > 0) {
     Object* prop = CopyFixedArray(properties);
     if (prop->IsFailure()) return prop;
     JSObject::cast(clone)->set_properties(FixedArray::cast(prop));
   }
   // Return the new clone.
@@ skipping 2034 matching lines @@
 void ExternalStringTable::TearDown() {
   new_space_strings_.Free();
   old_space_strings_.Free();
 }
 
 
 List<Object*> ExternalStringTable::new_space_strings_;
 List<Object*> ExternalStringTable::old_space_strings_;
 
 } }  // namespace v8::internal