Remember and reuse derived map for external arrays

src/api.cc

 // Copyright 2010 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 2757 matching lines @@
   // If the object already has external elements, create a new, unique
   // map if the element type is now changing, because assumptions about
   // generated code based on the receiver's map will be invalid.
   i::Handle<i::HeapObject> elements(object->elements());
   bool force_unique_map =
       elements->map()->IsUndefined() ||
       !elements->map()->has_external_array_elements() ||
       elements->map() != i::Heap::MapForExternalArrayType(array_type);
   if (force_unique_map) {
     i::Handle<i::Map> external_array_map =
-        i::Factory::NewExternalArrayElementsMap(
-            i::Handle<i::Map>(object->map()));
+        i::Factory::GetExternalArrayElementsMap(
+            i::Handle<i::Map>(object->map()), array_type);
     object->set_map(*external_array_map);
   }
   object->set_elements(*array);
 }
 
 }  // namespace
 
 
 void v8::Object::SetIndexedPropertiesToPixelData(uint8_t* data, int length) {
   ON_BAILOUT("v8::SetElementsToPixelData()", return);
@@ skipping 2434 matching lines @@
 
 
 char* HandleScopeImplementer::Iterate(ObjectVisitor* v, char* storage) {
   HandleScopeImplementer* thread_local =
       reinterpret_cast<HandleScopeImplementer*>(storage);
   thread_local->IterateThis(v);
   return storage + ArchiveSpacePerThread();
 }
 
 } }  // namespace v8::internal