Remember and reuse derived map for external arrays

src/mark-compact.cc

 // Copyright 2006-2008 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 1082 matching lines @@
   ASSERT(descriptors != HEAP->raw_unchecked_empty_descriptor_array());
   SetMark(descriptors);
 
   FixedArray* contents = reinterpret_cast<FixedArray*>(
       descriptors->get(DescriptorArray::kContentArrayIndex));
   ASSERT(contents->IsHeapObject());
   ASSERT(!contents->IsMarked());
   ASSERT(contents->IsFixedArray());
   ASSERT(contents->length() >= 2);
   SetMark(contents);
-  // Contents contains (value, details) pairs.  If the details say that
-  // the type of descriptor is MAP_TRANSITION, CONSTANT_TRANSITION, or
-  // NULL_DESCRIPTOR, we don't mark the value as live.  Only for
-  // MAP_TRANSITION and CONSTANT_TRANSITION is the value an Object* (a
-  // Map*).
+  // Contents contains (value, details) pairs.  If the details say that the type
+  // of descriptor is MAP_TRANSITION, CONSTANT_TRANSITION,
+  // EXTERNAL_ARRAY_TRANSITION or NULL_DESCRIPTOR, we don't mark the value as
+  // live.  Only for MAP_TRANSITION, EXTERNAL_ARRAY_TRANSITION and
+  // CONSTANT_TRANSITION is the value an Object* (a Map*).
   for (int i = 0; i < contents->length(); i += 2) {
     // If the pair (value, details) at index i, i+1 is not
     // a transition or null descriptor, mark the value.
     PropertyDetails details(Smi::cast(contents->get(i + 1)));
     if (details.type() < FIRST_PHANTOM_PROPERTY_TYPE) {
       HeapObject* object = reinterpret_cast<HeapObject*>(contents->get(i));
       if (object->IsHeapObject() && !object->IsMarked()) {
         SetMark(object);
         marking_stack_.Push(object);
       }
@@ skipping 1948 matching lines @@
 }
 
 
 void MarkCompactCollector::Initialize() {
   StaticPointersToNewGenUpdatingVisitor::Initialize();
   StaticMarkingVisitor::Initialize();
 }
 
 
 } }  // namespace v8::internal