Use map transitions when defining accessor properties.

src/objects.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // 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 4395 matching lines @@
   accessors->SetComponents(getter, setter);
 
   return SetElementCallback(index, accessors, attributes);
 }
 
 
 MaybeObject* JSObject::CreateAccessorPairFor(String* name) {
   LookupResult result(GetHeap()->isolate());
   LocalLookupRealNamedProperty(name, &result);
   if (result.IsProperty() && result.type() == CALLBACKS) {
-    ASSERT(!result.IsDontDelete());
+    // Note that the result can actually have IsDontDelete() == true when we
+    // e.g. have to fall back to the slow case while adding a setter after
+    // successfully reusing a map transition for a getter. Nevertheless, this is
+    // OK, because the assertion only holds for the whole addition of both
+    // accessors, not for the addition of each part.

[Michael Starzinger, 2012/05/03 12:23:34]

It would really be nice to preserve this assertion somehow. In my understanding
the correct place to put it would be at the beginning of
DefinePropertyAccessor(). Unfortunately we don't have a lookup result there.

Maybe you could come up with some nifty way to check that assertion there.
Otherwise a comment there might be useful.

     Object* obj = result.GetCallbackObject();
     if (obj->IsAccessorPair()) {
       return AccessorPair::cast(obj)->CopyWithoutTransitions();
     }
   }
   return GetHeap()->AllocateAccessorPair();
 }
 
 
 MaybeObject* JSObject::DefinePropertyAccessor(String* name,
                                               Object* getter,
                                               Object* setter,
                                               PropertyAttributes attributes) {
+  Heap* heap = GetHeap();
+  bool only_attribute_changes = getter->IsNull() && setter->IsNull();
+  if (HasFastProperties() && !only_attribute_changes) {
+    MaybeObject* getterOk = heap->undefined_value();
+    if (!getter->IsNull()) {
+      getterOk = DefineFastAccessor(name, ACCESSOR_GETTER, getter, attributes);
+      if (getterOk->IsFailure()) return getterOk;
+    }
+
+    MaybeObject* setterOk = heap->undefined_value();
+    if (getterOk != heap->null_value() && !setter->IsNull()) {
+      setterOk = DefineFastAccessor(name, ACCESSOR_SETTER, setter, attributes);
+      if (setterOk->IsFailure()) return setterOk;
+    }
+
+    if (getterOk != heap->null_value() && setterOk != heap->null_value()) {
+      return heap->undefined_value();
+    }
+  }
+
   AccessorPair* accessors;
   { MaybeObject* maybe_accessors = CreateAccessorPairFor(name);
     if (!maybe_accessors->To(&accessors)) return maybe_accessors;
   }
   accessors->SetComponents(getter, setter);
   return SetPropertyCallback(name, accessors, attributes);
 }
 
 
 bool JSObject::CanSetCallback(String* name) {
@@ skipping 129 matching lines @@
 
   if (!CanSetCallback(name)) return isolate->heap()->undefined_value();
 
   uint32_t index = 0;
   return name->AsArrayIndex(&index) ?
       DefineElementAccessor(index, getter, setter, attributes) :
       DefinePropertyAccessor(name, getter, setter, attributes);
 }
 
 
+static MaybeObject* CreateFreshAccessor(JSObject* obj,
+                                        String* name,
+                                        AccessorComponent component,
+                                        Object* accessor,
+                                        PropertyAttributes attributes) {
+  // step 1: create a new getter/setter pair with only the accessor in it
+  Heap* heap = obj->GetHeap();
+  AccessorPair* accessors2;
+  { MaybeObject* maybe_accessors2 = heap->AllocateAccessorPair();
+    if (!maybe_accessors2->To(&accessors2)) return maybe_accessors2;
+  }
+  accessors2->set(component, accessor);
+
+  // step 2: create a copy of the descriptors, incl. the new getter/setter pair
+  Map* map1 = obj->map();
+  CallbacksDescriptor callbacks_descr2(name, accessors2, attributes);
+  DescriptorArray* descriptors2;
+  { MaybeObject* maybe_descriptors2 =
+        map1->instance_descriptors()->CopyInsert(&callbacks_descr2,
+                                                 REMOVE_TRANSITIONS);
+    if (!maybe_descriptors2->To(&descriptors2)) return maybe_descriptors2;
+  }
+
+  // step 3: create a new map with the new descriptors
+  Map* map2;
+  { MaybeObject* maybe_map2 = map1->CopyDropDescriptors();
+    if (!maybe_map2->To(&map2)) return maybe_map2;
+  }
+  map2->set_instance_descriptors(descriptors2);
+
+  // step 4: create a new getter/setter pair with a transition to the new map
+  AccessorPair* accessors1;
+  { MaybeObject* maybe_accessors1 = heap->AllocateAccessorPair();
+    if (!maybe_accessors1->To(&accessors1)) return maybe_accessors1;
+  }
+  accessors1->set(component, map2);
+
+  // step 5: create a copy of the descriptors, incl. the new getter/setter pair
+  // with the transition
+  CallbacksDescriptor callbacks_descr1(name, accessors1, attributes);
+  DescriptorArray* descriptors1;
+  { MaybeObject* maybe_descriptors1 =
+        map1->instance_descriptors()->CopyInsert(&callbacks_descr1,
+                                                 KEEP_TRANSITIONS);
+    if (!maybe_descriptors1->To(&descriptors1)) return maybe_descriptors1;
+  }
+
+  // step 6: everything went well so far, so we make our changes visible
+  obj->set_map(map2);
+  map1->set_instance_descriptors(descriptors1);
+  return obj;
+}
+
+static bool TransitionToSameAccessor(Object* map,
+                                     String* name,
+                                     AccessorComponent component,
+                                     Object* accessor,
+                                     PropertyAttributes attributes ) {
+  DescriptorArray* descs = Map::cast(map)->instance_descriptors();
+  int number = descs->SearchWithCache(name);
+  ASSERT(number != DescriptorArray::kNotFound);
+  Object* target_accessor =
+      AccessorPair::cast(descs->GetCallbacksObject(number))->get(component);
+  PropertyAttributes target_attributes = descs->GetDetails(number).attributes();
+  return target_accessor == accessor && target_attributes == attributes;
+}
+
+
+static MaybeObject* NewCallbackTransition(JSObject* obj,
+                                          String* name,
+                                          AccessorComponent component,
+                                          Object* accessor,
+                                          PropertyAttributes attributes,
+                                          AccessorPair* accessors2) {
+  // step 1: copy the old getter/setter pair and set the new accessor
+  AccessorPair* accessors3;
+  { MaybeObject* maybe_accessors3 = accessors2->CopyWithoutTransitions();
+    if (!maybe_accessors3->To(&accessors3)) return maybe_accessors3;
+  }
+  accessors3->set(component, accessor);
+
+  // step 2: create a copy of the descriptors, incl. the new getter/setter pair
+  Map* map2 = obj->map();
+  CallbacksDescriptor callbacks_descr3(name, accessors3, attributes);
+  DescriptorArray* descriptors3;
+  { MaybeObject* maybe_descriptors3 =
+        map2->instance_descriptors()->CopyInsert(&callbacks_descr3,
+                                                 REMOVE_TRANSITIONS);
+    if (!maybe_descriptors3->To(&descriptors3)) return maybe_descriptors3;
+  }
+
+  // step 3: create a new map with the new descriptors
+  Map* map3;
+  { MaybeObject* maybe_map3 = map2->CopyDropDescriptors();
+    if (!maybe_map3->To(&map3)) return maybe_map3;
+  }
+  map3->set_instance_descriptors(descriptors3);
+
+  // step 4: everything went well so far, so we make our changes visible
+  obj->set_map(map3);
+  accessors2->set(component, map3);
+  return obj;
+}
+
+
+MaybeObject* JSObject::DefineFastAccessor(String* name,
+                                          AccessorComponent component,
+                                          Object* accessor,
+                                          PropertyAttributes attributes) {
+  ASSERT(accessor->IsSpecFunction() || accessor->IsUndefined());
+  LookupResult result(GetIsolate());
+  LocalLookup(name, &result);
+
+  // If we have a new property, create a fresh accessor plus a transition to it.
+  if (!result.IsFound()) {
+    return CreateFreshAccessor(this, name, component, accessor, attributes);
+  }
+
+  // If the property is not a JavaScript accessor, fall back to the slow case.
+  if (result.type() != CALLBACKS) return GetHeap()->null_value();
+  Object* callback_value = result.GetValue();
+  if (!callback_value->IsAccessorPair()) return GetHeap()->null_value();
+  AccessorPair* accessors = AccessorPair::cast(callback_value);
+
+  // Follow a callback transition, if there is a fitting one.
+  Object* entry = accessors->get(component);
+  if (entry->IsMap() &&
+      TransitionToSameAccessor(entry, name, component, accessor, attributes)) {
+    set_map(Map::cast(entry));
+    return this;
+  }
+
+  // When we re-add the same accessor again, there is nothing to do.
+  if (entry == accessor && result.GetAttributes() == attributes) return this;
+
+  // Only the other accessor has been set so far, create a new transition.
+  if (entry->IsTheHole()) {
+    return NewCallbackTransition(this,
+                                 name,
+                                 component,
+                                 accessor,
+                                 attributes,
+                                 accessors);
+  }
+
+  // Nothing from the above worked, so we have to fall back to the slow case.
+  return GetHeap()->null_value();
+}
+
+
 MaybeObject* JSObject::DefineAccessor(AccessorInfo* info) {
   Isolate* isolate = GetIsolate();
   String* name = String::cast(info->name());
   // Check access rights if needed.
   if (IsAccessCheckNeeded() &&
       !isolate->MayNamedAccess(this, name, v8::ACCESS_SET)) {
     isolate->ReportFailedAccessCheck(this, v8::ACCESS_SET);
     return isolate->heap()->undefined_value();
   }
 
@@ skipping 1343 matching lines @@
   { MaybeObject* maybe_copy = heap->AllocateAccessorPair();
     if (!maybe_copy->To(&copy)) return maybe_copy;
   }
   copy->set_getter(getter()->IsMap() ? heap->the_hole_value() : getter());
   copy->set_setter(setter()->IsMap() ? heap->the_hole_value() : setter());
   return copy;
 }
 
 
 Object* AccessorPair::GetComponent(AccessorComponent component) {
-    Object* accessor = (component == ACCESSOR_GETTER) ? getter() : setter();
-    return accessor->IsTheHole() ? GetHeap()->undefined_value() : accessor;
+  Object* accessor = get(component);
+  return accessor->IsTheHole() ? GetHeap()->undefined_value() : accessor;
 }
 
 
 MaybeObject* DeoptimizationInputData::Allocate(int deopt_entry_count,
                                                PretenureFlag pretenure) {
   ASSERT(deopt_entry_count > 0);
   return HEAP->AllocateFixedArray(LengthFor(deopt_entry_count),
                                   pretenure);
 }
 
@@ skipping 7013 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal