In-place shrinking of descriptor arrays with non-live transitions.

src/mark-compact.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 1872 matching lines @@
 
   if (descriptors->elements_transition_map() != NULL) {
     Object** transitions_slot = descriptors->GetTransitionsSlot();
     Object* transitions = *transitions_slot;
     mark_compact_collector()->RecordSlot(descriptor_start,
                                          transitions_slot,
                                          transitions);
   }
 
   // If the descriptor contains a transition (value is a Map), we don't mark the
-  // value as live. It might be set to the NULL_DESCRIPTOR in
-  // ClearNonLiveTransitions later.
+  // value as live. It might be removed by ClearNonLiveTransitions later.
   for (int i = 0; i < descriptors->number_of_descriptors(); ++i) {
     Object** key_slot = descriptors->GetKeySlot(i);
     Object* key = *key_slot;
     if (key->IsHeapObject()) {
       base_marker()->MarkObjectAndPush(reinterpret_cast<HeapObject*>(key));
       mark_compact_collector()->RecordSlot(descriptor_start, key_slot, key);
     }
 
     Object** value_slot = descriptors->GetValueSlot(i);
     if (!(*value_slot)->IsHeapObject()) continue;
@@ skipping 17 matching lines @@
         if (!value->IsAccessorPair()) {
           base_marker()->MarkObjectAndPush(value);
         } else if (base_marker()->MarkObjectWithoutPush(value)) {
           AccessorPair* accessors = AccessorPair::cast(value);
           MarkAccessorPairSlot(accessors, AccessorPair::kGetterOffset);
           MarkAccessorPairSlot(accessors, AccessorPair::kSetterOffset);
         }
         break;
       case MAP_TRANSITION:
       case CONSTANT_TRANSITION:
-      case NULL_DESCRIPTOR:
+        break;
+      case NONEXISTENT:
+        UNREACHABLE();
         break;
     }
   }
 }
 
 
 template <class T>
 void Marker<T>::MarkAccessorPairSlot(AccessorPair* accessors, int offset) {
   Object** slot = HeapObject::RawField(accessors, offset);
   HeapObject* accessor = HeapObject::cast(*slot);
@@ skipping 2176 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal