Implement map collection for incremental marking.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index 5b58c9d7877bb8d211da7a4375fd0d1569b55c79..651b66cecee0745a6276f881860224f3a8ecb5ea 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -807,12 +807,6 @@ void IncrementalMarking::Step(intptr_t allocated_bytes,
       Map* map = obj->map();
       if (map == filler_map) continue;
 
-      if (obj->IsMap()) {
-        Map* map = Map::cast(obj);
-        heap_->ClearCacheOnMap(map);
-      }
-
-
       int size = obj->SizeFromMap(map);
       bytes_to_process -= size;
       MarkBit map_mark_bit = Marking::MarkBitFrom(map);
@@ -830,6 +824,32 @@ void IncrementalMarking::Step(intptr_t allocated_bytes,
         MarkObjectGreyDoNotEnqueue(ctx->normalized_map_cache());
 
         VisitGlobalContext(ctx, &marking_visitor);
+      } else if (map->instance_type() == JS_MAP_TYPE) {

[Vyacheslav Egorov (Chromium), 2012/05/11 12:58:35]

s/JS_MAP_TYPE/MAP_TYPE/

[Michael Starzinger, 2012/05/11 14:51:53]

Already fixed in second patch set.

+        Map* map = Map::cast(obj);
+        heap_->ClearCacheOnMap(map);
+
+        // When map collection is enabled we have to mark through map's
+        // transitions and back pointers in a special way to make these links
+        // weak.  Only maps for subclasses of JSReceiver can have transitions.
+        STATIC_ASSERT(LAST_TYPE == LAST_JS_RECEIVER_TYPE);
+        if (FLAG_collect_maps && map->instance_type() >= FIRST_JS_RECEIVER_TYPE) {
+          heap_->mark_compact_collector()->MarkWeakMapContents(map);
+
+          // 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.
+          STATIC_ASSERT(Map::kPointerFieldsEndOffset ==
+              Map::kPrototypeTransitionsOrBackPointerOffset + kPointerSize);
+          marking_visitor.VisitPointers(
+              HeapObject::RawField(map, Map::kPointerFieldsBeginOffset),
+              HeapObject::RawField(
+                  map, Map::kPrototypeTransitionsOrBackPointerOffset));
+        } else {
+          marking_visitor.VisitPointers(
+              HeapObject::RawField(map, Map::kPointerFieldsBeginOffset),
+              HeapObject::RawField(map, Map::kPointerFieldsEndOffset));
+        }
       } else if (map->instance_type() == JS_FUNCTION_TYPE) {
         marking_visitor.VisitPointers(
             HeapObject::RawField(obj, JSFunction::kPropertiesOffset),