Moving prototype transitions into the header of the transition array.

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 1805 matching lines @@
 }
 
 
 // Force instantiation of template instances.
 template void Marker<IncrementalMarking>::MarkMapContents(Map* map);
 template void Marker<MarkCompactCollector>::MarkMapContents(Map* map);
 
 
 template <class T>
 void Marker<T>::MarkMapContents(Map* map) {
-  // Mark prototype transitions array but don't push it into marking stack.
-  // This will make references from it weak. We will clean dead prototype
-  // transitions in ClearNonLiveTransitions.
-  Object** proto_trans_slot =
-      HeapObject::RawField(map, Map::kPrototypeTransitionsOrBackPointerOffset);
-  HeapObject* prototype_transitions = HeapObject::cast(*proto_trans_slot);
-  if (prototype_transitions->IsFixedArray()) {
-    mark_compact_collector()->RecordSlot(proto_trans_slot,
-                                         proto_trans_slot,
-                                         prototype_transitions);
-    MarkBit mark = Marking::MarkBitFrom(prototype_transitions);
-    if (!mark.Get()) {
-      mark.Set();
-      MemoryChunk::IncrementLiveBytesFromGC(prototype_transitions->address(),
-                                            prototype_transitions->Size());
-    }
-  }
-
   // Make sure that the back pointer stored either in the map itself or inside
   // its prototype transitions array is marked. Treat pointers in the descriptor
   // array as weak and also mark that array to prevent visiting it later.
   base_marker()->MarkObjectAndPush(HeapObject::cast(map->GetBackPointer()));
 
   Object** descriptor_array_slot =
       HeapObject::RawField(map, Map::kInstanceDescriptorsOrBitField3Offset);
   Object* descriptor_array = *descriptor_array_slot;
   if (!descriptor_array->IsSmi()) {
     MarkDescriptorArray(reinterpret_cast<DescriptorArray*>(descriptor_array));
   }
 
   // Mark the Object* fields of the Map. Since the descriptor array has been
   // marked already, it is fine that one of these fields contains a pointer
   // to it. But make sure to skip back pointer and prototype transitions.

[Michael Starzinger, 2012/07/06 13:53:18]

Comment no longer applies, drop the "and prototype transitions".

[Toon Verwaest, 2012/07/09 12:45:27]

Done.

   STATIC_ASSERT(Map::kPointerFieldsEndOffset ==
-      Map::kPrototypeTransitionsOrBackPointerOffset + kPointerSize);
-  Object** start_slot = HeapObject::RawField(
-      map, Map::kPointerFieldsBeginOffset);
-  Object** end_slot = HeapObject::RawField(
-      map, Map::kPrototypeTransitionsOrBackPointerOffset);
+                Map::kBackPointerOffset + kPointerSize);
+  Object** start_slot =
+      HeapObject::RawField(map, Map::kPointerFieldsBeginOffset);
+  Object** end_slot = HeapObject::RawField(map, Map::kBackPointerOffset);
   for (Object** slot = start_slot; slot < end_slot; slot++) {
     Object* obj = *slot;
     if (!obj->NonFailureIsHeapObject()) continue;
     mark_compact_collector()->RecordSlot(start_slot, slot, obj);
     base_marker()->MarkObjectAndPush(reinterpret_cast<HeapObject*>(obj));
   }
 }
 
 
 template <class T>
@@ skipping 75 matching lines @@
 void Marker<T>::MarkTransitionArray(TransitionArray* transitions) {
   if (!base_marker()->MarkObjectWithoutPush(transitions)) return;
   Object** transitions_start = transitions->data_start();
 
   if (transitions->HasElementsTransition()) {
     mark_compact_collector()->RecordSlot(transitions_start,
                                          transitions->GetElementsSlot(),
                                          transitions->elements_transition());
   }
 
+  if (transitions->HasPrototypeTransitions()) {
+    // Mark prototype transitions array but don't push it into marking stack.
+    // This will make references from it weak. We will clean dead prototype
+    // transitions in ClearNonLiveTransitions.
+    Object** proto_trans_slot = transitions->GetPrototypeTransitionsSlot();
+    HeapObject* prototype_transitions = HeapObject::cast(*proto_trans_slot);
+    base_marker()->MarkObjectWithoutPush(prototype_transitions);
+    mark_compact_collector()->RecordSlot(
+        transitions_start, proto_trans_slot, prototype_transitions);
+  }
+
   for (int i = 0; i < transitions->number_of_transitions(); ++i) {
     Object** key_slot = transitions->GetKeySlot(i);
     Object* key = *key_slot;
     if (key->IsHeapObject()) {
       base_marker()->MarkObjectAndPush(HeapObject::cast(key));
       mark_compact_collector()->RecordSlot(transitions_start, key_slot, key);
     }
 
     Object** value_slot = transitions->GetValueSlot(i);
     if (!(*value_slot)->IsHeapObject()) continue;
@@ skipping 2190 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal