Cache multiple ElementsKind map transition per map.

src/mark-compact.cc

 // Copyright 2011 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 1608 matching lines @@
     // 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)));
 
     Object** slot = contents->data_start() + i;
     Object* value = *slot;
     if (!value->IsHeapObject()) continue;
 
     RecordSlot(slot, slot, *slot);
 
-    if (details.type() < FIRST_PHANTOM_PROPERTY_TYPE) {
+    PropertyType type = details.type();
+    if (type < FIRST_PHANTOM_PROPERTY_TYPE) {
       HeapObject* object = HeapObject::cast(value);
       MarkBit mark = Marking::MarkBitFrom(HeapObject::cast(object));
       if (!mark.Get()) {
         SetMark(HeapObject::cast(object), mark);
         marking_deque_.PushBlack(object);
       }
+    } else if (type == ELEMENTS_TRANSITION && value->IsFixedArray()) {
+      // For maps with multiple elements transitions, the transition maps are
+      // stored in a FixedArray. Keep the fixed array alive but not the maps
+      // that it refers to.
+      HeapObject* object = HeapObject::cast(value);
+      MarkBit mark = Marking::MarkBitFrom(HeapObject::cast(object));
+      if (!mark.Get()) {
+        SetMark(HeapObject::cast(object), mark);
+      }
     }
   }
   // The DescriptorArray descriptors contains a pointer to its contents array,
   // but the contents array is already marked.
   marking_deque_.PushBlack(descriptors);
 }
 
 
 void MarkCompactCollector::CreateBackPointers() {
   HeapObjectIterator iterator(heap()->map_space());
@@ skipping 2172 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal