Refactored iterative map traversal.

The main goal is to cleanly separate between the several parts involved in the traversal:

 * iterating over all transitions in a descriptor array

 * iterating over all prototype transitions

 * storing the parent and the current local traversal position in a map

 * the iterative traversal algorithm itself

The previous algorithm for iterating over prototype transitions did a little bit too much here, iterating over the whole array instead only the filled part. This has been fixed on the way, too.

With this CL, it will be much easier to make the necessary changes to the descriptor array iterator to correctly handle map transitions for accessor properties. Furthermore, perhaps we represent transitions a bit different in the future, making finding them a bit easier. This would make some code in this CL (and elsewhere) quite a bit shorter and more efficient.

Review URL: https://chromiumcodereview.appspot.com/9252007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10473 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -4935,75 +4935,191 @@ void Map::RemoveFromCodeCache(String* name, Code* code, int index) {
 }
 
 
-void Map::TraverseTransitionTree(TraverseCallback callback, void* data) {
-  // Traverse the transition tree without using a stack.  We do this by
-  // reversing the pointers in the maps and descriptor arrays.
-  Map* current = this;
-  Map* meta_map = GetHeap()->meta_map();
-  Object** map_or_index_field = NULL;
-  while (current != meta_map) {
-    DescriptorArray* d = reinterpret_cast<DescriptorArray*>(
-        *RawField(current, Map::kInstanceDescriptorsOrBitField3Offset));
-    if (!d->IsEmpty()) {
-      FixedArray* contents = reinterpret_cast<FixedArray*>(
-          d->get(DescriptorArray::kContentArrayIndex));
-      map_or_index_field = RawField(contents, HeapObject::kMapOffset);
-      Object* map_or_index = *map_or_index_field;
-      bool map_done = true;  // Controls a nested continue statement.
-      for (int i = map_or_index->IsSmi() ? Smi::cast(map_or_index)->value() : 0;
-           i < contents->length();
-           i += 2) {
-        PropertyDetails details(Smi::cast(contents->get(i + 1)));
-        if (details.IsTransition()) {
-          // Found a map in the transition array.  We record our progress in
-          // the transition array by recording the current map in the map field
-          // of the next map and recording the index in the transition array in
-          // the map field of the array.
-          Map* next = Map::cast(contents->get(i));
-          next->set_map_no_write_barrier(current);
-          *map_or_index_field = Smi::FromInt(i + 2);
-          current = next;
-          map_done = false;
-          break;
-        }
-      }
-      if (!map_done) continue;
-    } else {
-      map_or_index_field = NULL;
-    }
-    // That was the regular transitions, now for the prototype transitions.
-    FixedArray* prototype_transitions =
-        current->unchecked_prototype_transitions();
-    Object** proto_map_or_index_field =
-        RawField(prototype_transitions, HeapObject::kMapOffset);
-    Object* map_or_index = *proto_map_or_index_field;
-    const int start = kProtoTransitionHeaderSize + kProtoTransitionMapOffset;
-    int i = map_or_index->IsSmi() ? Smi::cast(map_or_index)->value() : start;
-    if (i < prototype_transitions->length()) {
-      // Found a map in the prototype transition array.  Record progress in
-      // an analogous way to the regular transitions array above.
-      Object* perhaps_map = prototype_transitions->get(i);
-      if (perhaps_map->IsMap()) {
-        Map* next = Map::cast(perhaps_map);
-        next->set_map_no_write_barrier(current);
-        *proto_map_or_index_field =
-            Smi::FromInt(i + kProtoTransitionElementsPerEntry);
-        current = next;
-        continue;
+// An iterator over all map transitions in an descriptor array, reusing the map
+// field of the contens array while it is running.
+class IntrusiveMapTransitionIterator {
+ public:
+  explicit IntrusiveMapTransitionIterator(DescriptorArray* descriptor_array)
+      : descriptor_array_(descriptor_array) { }
+
+  void Start() {
+    ASSERT(!IsIterating());
+    if (HasContentArray()) *ContentHeader() = Smi::FromInt(0);
+  }
+
+  bool IsIterating() {
+    return HasContentArray() && (*ContentHeader())->IsSmi();
+  }
+
+  Map* Next() {
+    ASSERT(IsIterating());
+    FixedArray* contents = ContentArray();
+    int index = Smi::cast(*ContentHeader())->value();
+    while (index < contents->length()) {
+      int next_index = index + 2;
+      PropertyDetails details(Smi::cast(contents->get(index + 1)));
+      if (details.IsTransition()) {
+        *ContentHeader() = Smi::FromInt(next_index);
+        return static_cast<Map*>(contents->get(index));
       }
+      index = next_index;
     }
-    *proto_map_or_index_field = GetHeap()->fixed_array_map();
-    if (map_or_index_field != NULL) {
-      *map_or_index_field = GetHeap()->fixed_array_map();
+    *ContentHeader() = descriptor_array_->GetHeap()->fixed_array_map();
+    return NULL;
+  }
+
+ private:
+  bool HasContentArray() {
+    return descriptor_array_-> length() > DescriptorArray::kContentArrayIndex;
+  }
+
+  FixedArray* ContentArray() {
+    Object* array = descriptor_array_->get(DescriptorArray::kContentArrayIndex);
+    return static_cast<FixedArray*>(array);
+  }
+
+  Object** ContentHeader() {
+    return HeapObject::RawField(ContentArray(), DescriptorArray::kMapOffset);
+  }
+
+  DescriptorArray* descriptor_array_;
+};
+
+
+// An iterator over all prototype transitions, reusing the map field of the
+// underlying array while it is running.
+class IntrusivePrototypeTransitionIterator {
+ public:
+  explicit IntrusivePrototypeTransitionIterator(FixedArray* proto_trans)
+      : proto_trans_(proto_trans) { }
+
+  void Start() {
+    ASSERT(!IsIterating());
+    if (HasTransitions()) *Header() = Smi::FromInt(0);
+  }
+
+  bool IsIterating() {
+    return HasTransitions() && (*Header())->IsSmi();
+  }
+
+  Map* Next() {
+    ASSERT(IsIterating());
+    int transitionNumber = Smi::cast(*Header())->value();
+    if (transitionNumber < NumberOfTransitions()) {
+      *Header() = Smi::FromInt(transitionNumber + 1);
+      return GetTransition(transitionNumber);
     }
+    *Header() = proto_trans_->GetHeap()->fixed_array_map();
+    return NULL;
+  }
+
+ private:
+  bool HasTransitions() {
+    return proto_trans_->length() >= Map::kProtoTransitionHeaderSize;
+  }
+
+  Object** Header() {
+    return HeapObject::RawField(proto_trans_, FixedArray::kMapOffset);
+  }
+
+  int NumberOfTransitions() {
+    Object* num = proto_trans_->get(Map::kProtoTransitionNumberOfEntriesOffset);
+    return Smi::cast(num)->value();
+  }
 
-    // The callback expects a map to have a real map as its map, so we save
-    // the map field, which is being used to track the traversal and put the
-    // correct map (the meta_map) in place while we do the callback.
-    Map* prev = current->map();
-    current->set_map_no_write_barrier(meta_map);
-    callback(current, data);
-    current = prev;
+  Map* GetTransition(int transitionNumber) {
+    return Map::cast(proto_trans_->get(IndexFor(transitionNumber)));
+  }
+
+  int IndexFor(int transitionNumber) {
+    return Map::kProtoTransitionHeaderSize +
+        Map::kProtoTransitionMapOffset +
+        transitionNumber * Map::kProtoTransitionElementsPerEntry;
+  }
+
+  FixedArray* proto_trans_;
+};
+
+
+// To traverse the transition tree iteratively, we have to store two kinds of
+// information in a map: The parent map in the traversal and which children of a
+// node have already been visited. To do this without additional memory, we
+// temporarily reuse two maps with known values:
+//
+//  (1) The map of the map temporarily holds the parent, and is restored to the
+//      meta map afterwards.
+//
+//  (2) The info which children have already been visited depends on which part
+//      of the map we currently iterate:
+//
+//    (a) If we currently follow normal map transitions, we temporarily store
+//        the current index in the map of the FixedArray of the desciptor
+//        array's contents, and restore it to the fixed array map afterwards.
+//        Note that a single descriptor can have 0, 1, or 2 transitions.
+//
+//    (b) If we currently follow prototype transitions, we temporarily store
+//        the current index in the map of the FixedArray holding the prototype
+//        transitions, and restore it to the fixed array map afterwards.
+//
+// Note that the child iterator is just a concatenation of two iterators: One
+// iterating over map transitions and one iterating over prototype transisitons.
+class TraversableMap : public Map {
+ public:
+  // Record the parent in the traversal within this map. Note that this destroys
+  // this map's map!
+  void SetParent(TraversableMap* parent) { set_map_no_write_barrier(parent); }
+
+  // Reset the current map's map, returning the parent previously stored in it.
+  TraversableMap* GetAndResetParent() {
+    TraversableMap* old_parent = static_cast<TraversableMap*>(map());
+    set_map_no_write_barrier(GetHeap()->meta_map());
+    return old_parent;
+  }
+
+  // Start iterating over this map's children, possibly destroying a FixedArray
+  // map (see explanation above).
+  void ChildIteratorStart() {
+    IntrusiveMapTransitionIterator(instance_descriptors()).Start();
+    IntrusivePrototypeTransitionIterator(
+        unchecked_prototype_transitions()).Start();
+  }
+
+  // If we have an unvisited child map, return that one and advance. If we have
+  // none, return NULL and reset any destroyed FixedArray maps.
+  TraversableMap* ChildIteratorNext() {
+    IntrusiveMapTransitionIterator descriptor_iterator(instance_descriptors());
+    if (descriptor_iterator.IsIterating()) {
+      Map* next = descriptor_iterator.Next();
+      if (next != NULL) return static_cast<TraversableMap*>(next);
+    }
+    IntrusivePrototypeTransitionIterator
+        proto_iterator(unchecked_prototype_transitions());
+    if (proto_iterator.IsIterating()) {
+      Map* next = proto_iterator.Next();
+      if (next != NULL) return static_cast<TraversableMap*>(next);
+    }
+    return NULL;
+  }
+};
+
+
+// Traverse the transition tree in postorder without using the C++ stack by
+// doing pointer reversal.
+void Map::TraverseTransitionTree(TraverseCallback callback, void* data) {
+  TraversableMap* current = static_cast<TraversableMap*>(this);
+  current->ChildIteratorStart();
+  while (true) {
+    TraversableMap* child = current->ChildIteratorNext();
+    if (child != NULL) {
+      child->ChildIteratorStart();
+      child->SetParent(current);
+      current = child;
+    } else {
+      TraversableMap* parent = current->GetAndResetParent();
+      callback(current, data);
+      if (current == this) break;
+      current = parent;
+    }
   }
 }