Add a comment about map collection into MarkCompactCollector::MarkUnmarkedObject.

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 1042 matching lines @@
 
 void MarkCompactCollector::MarkUnmarkedObject(HeapObject* object) {
   ASSERT(!object->IsMarked());
   ASSERT(HEAP->Contains(object));
   if (object->IsMap()) {
     Map* map = Map::cast(object);
     if (FLAG_cleanup_code_caches_at_gc) {
       map->ClearCodeCache(heap());
     }
     SetMark(map);
+
+    // When map collection is enabled we have to mark through map's transitions
+    // in a special way to make transition links weak.
+    // Only maps with instance types between FIRST_JS_OBJECT_TYPE and
+    // JS_FUNCTION_TYPE can have transitions.
     if (FLAG_collect_maps &&
         map->instance_type() >= FIRST_JS_OBJECT_TYPE &&
         map->instance_type() <= JS_FUNCTION_TYPE) {
       MarkMapContents(map);
     } else {
       marking_stack_.Push(map);
     }
   } else {
     SetMark(object);
     marking_stack_.Push(object);
@@ skipping 2061 matching lines @@
 }
 
 
 void MarkCompactCollector::Initialize() {
   StaticPointersToNewGenUpdatingVisitor::Initialize();
   StaticMarkingVisitor::Initialize();
 }
 
 
 } }  // namespace v8::internal