Cache multiple ElementsKind map transition per map.

src/objects.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 2004 matching lines @@
     cache->Update(descriptors, name, number);
   }
   if (number != DescriptorArray::kNotFound) {
     result->DescriptorResult(holder, descriptors->GetDetails(number), number);
   } else {
     result->NotFound();
   }
 }
 
 
+static Map* GetElementsTransitionMapFromDescriptor(Object* descriptor_contents,
+                                                   ElementsKind elements_kind) {
+  if (descriptor_contents->IsMap()) {
+    Map* map = Map::cast(descriptor_contents);
+    if (map->elements_kind() == elements_kind) {
+      return map;
+    }
+    return NULL;
+  }
+
+  FixedArray* map_array = FixedArray::cast(descriptor_contents);
+  for (int i = 0; i < map_array->length(); ++i) {
+    Map* current_map = Map::cast(map_array->get(i));
+    if (current_map->elements_kind() == elements_kind) {
+      return current_map;
+    }
+  }
+
+  return NULL;
+}
+
+
+static MaybeObject* AddElementsTransitionMapToDescriptor(
+    Object* descriptor_contents,
+    Map* new_map) {
+  // Nothing was in the descriptor for an ELEMENTS_TRANSITION,
+  // simply add the map.
+  if (descriptor_contents == NULL) {
+    return new_map;
+  }
+
+  // There was already a map in the descriptor, create a 2-element FixedArray
+  // to contain the existing map plus the new one.
+  FixedArray* new_array;
+  Heap* heap = new_map->GetHeap();
+  if (descriptor_contents->IsMap()) {
+    MaybeObject* maybe_new_array = heap->AllocateFixedArray(2);
+    if (!maybe_new_array->To<FixedArray>(&new_array)) {
+      return maybe_new_array;
+    }
+    new_array->set(0, descriptor_contents);
+    new_array->set(1, new_map);
+    return new_array;
+  }
+
+  // The descriptor already contained a list of maps for different ElementKinds
+  // of ELEMENTS_TRANSITION, create a FixedArray to hold the existing maps plus
+  // the new one and fill it it.
+  FixedArray* array = FixedArray::cast(descriptor_contents);
+  MaybeObject* maybe_new_array =
+      heap->AllocateFixedArray(array->length() + 1);
+  if (!maybe_new_array->To<FixedArray>(&new_array)) {
+    return maybe_new_array;
+  }
+  int i = 0;
+  while (i < array->length()) {
+    new_array->set(i, FixedArray::cast(descriptor_contents)->get(i));

[Jakob Kummerow, 2011/09/23 12:12:28]

Simplification: s/FixedArray::cast(descriptor_contents)/array/

[danno, 2011/09/23 13:33:45]

Done.

+    ++i;
+  }
+  new_array->set(i, new_map);
+  return new_array;
+}
+
+
 MaybeObject* JSObject::GetElementsTransitionMap(ElementsKind elements_kind) {
   Heap* current_heap = GetHeap();
   Map* current_map = map();
   DescriptorArray* descriptors = current_map->instance_descriptors();
   String* elements_transition_sentinel_name = current_heap->empty_symbol();
 
   if (current_map->elements_kind() == elements_kind) return current_map;
 
   // Only objects with FastProperties can have DescriptorArrays and can track
   // element-related maps. Also don't add descriptors to maps that are shared.
   bool safe_to_add_transition = HasFastProperties() &&
       !current_map->IsUndefined() &&
       !current_map->is_shared();
 
   // Prevent long chains of DICTIONARY -> FAST_ELEMENTS maps cause by objects
   // with elements that switch back and forth between dictionary and fast
   // element mode.
   if ((current_map->elements_kind() == DICTIONARY_ELEMENTS &&
        elements_kind == FAST_ELEMENTS)) {
     safe_to_add_transition = false;
   }
 
+  Object* descriptor_contents = NULL;
   if (safe_to_add_transition) {
     // It's only safe to manipulate the descriptor array if it would be
     // safe to add a transition.
 
     // Check if the elements transition already exists.
     DescriptorLookupCache* cache =
         current_heap->isolate()->descriptor_lookup_cache();
     int index = cache->Lookup(descriptors, elements_transition_sentinel_name);
     if (index == DescriptorLookupCache::kAbsent) {
       index = descriptors->Search(elements_transition_sentinel_name);
       cache->Update(descriptors,
                     elements_transition_sentinel_name,
                     index);
     }
 
     // If the transition already exists, check the type. If there is a match,
     // return it.
     if (index != DescriptorArray::kNotFound) {
       PropertyDetails details(PropertyDetails(descriptors->GetDetails(index)));
-      if (details.type() == ELEMENTS_TRANSITION &&
-          details.elements_kind() == elements_kind) {
-        return descriptors->GetValue(index);
-      } else {
-        safe_to_add_transition = false;
+      if (details.type() == ELEMENTS_TRANSITION) {
+        descriptor_contents = descriptors->GetValue(index);
+        Map* maybe_transition_map =
+            GetElementsTransitionMapFromDescriptor(descriptor_contents,
+                                                   elements_kind);
+        if (maybe_transition_map) return maybe_transition_map;
       }
+      safe_to_add_transition = false;
     }
   }
 
   // No transition to an existing map for the given ElementsKind. Make a new
   // one.
   Object* obj;
   { MaybeObject* maybe_map = current_map->CopyDropTransitions();
     if (!maybe_map->ToObject(&obj)) return maybe_map;
   }
   Map* new_map = Map::cast(obj);
 
   new_map->set_elements_kind(elements_kind);
 
   // Only remember the map transition if the object's map is NOT equal to the
   // global object_function's map and there is not an already existing
   // non-matching element transition.
-  bool allow_map_transition =
-      safe_to_add_transition &&
-      (GetIsolate()->context()->global_context()->object_function()->map() !=
-       map());
+  bool allow_map_transition = safe_to_add_transition;
+  //&&
+  //  (GetIsolate()->context()->global_context()->object_function()->map() !=
+  //   map());
   if (allow_map_transition) {
-    // Allocate new instance descriptors for the old map with map transition.
+    MaybeObject* maybe_new_contents =
+        AddElementsTransitionMapToDescriptor(descriptor_contents, new_map);
+    Object* new_contents;
+    if (!maybe_new_contents->To<Object>(&new_contents)) {

[Jakob Kummerow, 2011/09/23 12:12:28]

Simplification: s/To<Object>/ToObject/

[danno, 2011/09/23 13:33:45]

Done.

+      return maybe_new_contents;
+    }
+
     ElementsTransitionDescriptor desc(elements_transition_sentinel_name,
-                                      Map::cast(new_map),
-                                      elements_kind);
+                                      new_contents);
     Object* new_descriptors;
     MaybeObject* maybe_new_descriptors = descriptors->CopyInsert(
         &desc,
         KEEP_TRANSITIONS);
     if (!maybe_new_descriptors->ToObject(&new_descriptors)) {
       return maybe_new_descriptors;
     }
     descriptors = DescriptorArray::cast(new_descriptors);
     current_map->set_instance_descriptors(descriptors);
   }
@@ skipping 9777 matching lines @@
   if (break_point_objects()->IsUndefined()) return 0;
   // Single break point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 #endif
 
 
 } }  // namespace v8::internal