Lazily register prototype users

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <iomanip>
 #include <sstream>
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
@@ skipping 1918 matching lines @@
       MigrateFastToSlow(object, new_map, expected_additional_properties);
     }
   } else {
     // For slow-to-fast migrations JSObject::MigrateSlowToFast()
     // must be used instead.
     CHECK(new_map->is_dictionary_map());
 
     // Slow-to-slow migration is trivial.
     object->set_map(*new_map);
   }
-  if (old_map->is_prototype_map()) {
+  if (old_map->is_prototype_map() && FLAG_track_prototype_users) {
     DCHECK(new_map->is_prototype_map());
     DCHECK(object->map() == *new_map);
     new_map->set_prototype_info(old_map->prototype_info());
     old_map->set_prototype_info(Smi::FromInt(0));
     if (FLAG_trace_prototype_users) {
       PrintF("Moving prototype_info %p from map %p to map %p.\n",
              reinterpret_cast<void*>(new_map->prototype_info()),
              reinterpret_cast<void*>(*old_map),
              reinterpret_cast<void*>(*new_map));
     }
+    // If the map was registered with its prototype before, ensure that it
+    // registers with its new prototype now. This preserves the invariant that
+    // when a map on a prototype chain is registered with its prototype, then
+    // all prototypes further up the chain are also registered with their
+    // respective prototypes.
+    Object* maybe_old_prototype = old_map->prototype();
+    if (maybe_old_prototype->IsJSObject()) {
+      Handle<JSObject> old_prototype(JSObject::cast(maybe_old_prototype));
+      bool was_registered =
+          JSObject::UnregisterPrototypeUser(old_prototype, old_map);
+      if (was_registered) {
+        JSObject::LazyRegisterPrototypeUser(new_map, new_map->GetIsolate());
+      }
+    }
   }
 }
 
 
 // To migrate a fast instance to a fast map:
 // - First check whether the instance needs to be rewritten. If not, simply
 //   change the map.
 // - Otherwise, allocate a fixed array large enough to hold all fields, in
 //   addition to unused space.
 // - Copy all existing properties in, in the following order: backing store
@@ skipping 5119 matching lines @@
 void Map::ConnectTransition(Handle<Map> parent, Handle<Map> child,
                             Handle<Name> name, SimpleTransitionFlag flag) {
   parent->set_owns_descriptors(false);
   if (parent->is_prototype_map()) {
     DCHECK(child->is_prototype_map());
 #if TRACE_MAPS
     Map::TraceTransition("NoTransition", *parent, *child, *name);
 #endif
   } else {
     TransitionArray::Insert(parent, name, child, flag);
-    if (child->prototype()->IsJSObject()) {
-      Handle<JSObject> proto(JSObject::cast(child->prototype()));
-      if (!ShouldRegisterAsPrototypeUser(child, proto)) {
-        JSObject::UnregisterPrototypeUser(proto, child);
-      }
-    }
 #if TRACE_MAPS
     Map::TraceTransition("Transition", *parent, *child, *name);
 #endif
   }
 }
 
 
 Handle<Map> Map::CopyReplaceDescriptors(
     Handle<Map> map, Handle<DescriptorArray> descriptors,
     Handle<LayoutDescriptor> layout_descriptor, TransitionFlag flag,
@@ skipping 1144 matching lines @@
   if (FLAG_trace_weak_arrays) {
     PrintF("[WeakFixedArray: storing at index %d ]\n", index);
   }
   array->set_last_used_index(index);
 }
 
 
 // static
 Handle<WeakFixedArray> WeakFixedArray::Add(
     Handle<Object> maybe_array, Handle<HeapObject> value,
-    SearchForDuplicates search_for_duplicates) {
+    SearchForDuplicates search_for_duplicates, bool* was_present) {
   Handle<WeakFixedArray> array =
       (maybe_array.is_null() || !maybe_array->IsWeakFixedArray())
           ? Allocate(value->GetIsolate(), 1, Handle<WeakFixedArray>::null())
           : Handle<WeakFixedArray>::cast(maybe_array);
-
+  if (was_present != NULL) *was_present = false;
   if (search_for_duplicates == kAddIfNotFound) {
     for (int i = 0; i < array->Length(); ++i) {
-      if (array->Get(i) == *value) return array;
+      if (array->Get(i) == *value) {
+        if (was_present != NULL) *was_present = true;
+        return array;
+      }
     }
 #if 0  // Enable this if you want to check your search_for_duplicates flags.
   } else {
     for (int i = 0; i < array->Length(); ++i) {
       DCHECK_NE(*value, array->Get(i));
     }
 #endif
   }
 
   // Try to store the new entry if there's room. Optimize for consecutive
   // accesses.
   int first_index = array->last_used_index();
-  for (int i = first_index;;) {
-    if (array->IsEmptySlot((i))) {
-      WeakFixedArray::Set(array, i, value);
-      return array;
+  if (array->Length() > 0) {
+    for (int i = first_index;;) {
+      if (array->IsEmptySlot((i))) {
+        WeakFixedArray::Set(array, i, value);
+        return array;
+      }
+      if (FLAG_trace_weak_arrays) {
+        PrintF("[WeakFixedArray: searching for free slot]\n");
+      }
+      i = (i + 1) % array->Length();
+      if (i == first_index) break;
     }
-    if (FLAG_trace_weak_arrays) {
-      PrintF("[WeakFixedArray: searching for free slot]\n");
-    }
-    i = (i + 1) % array->Length();
-    if (i == first_index) break;
   }
 
   // No usable slot found, grow the array.
-  int new_length = array->Length() + (array->Length() >> 1) + 4;
+  int new_length =
+      array->Length() == 0 ? 1 : array->Length() + (array->Length() >> 1) + 4;
   Handle<WeakFixedArray> new_array =
       Allocate(array->GetIsolate(), new_length, array);
   if (FLAG_trace_weak_arrays) {
     PrintF("[WeakFixedArray: growing to size %d ]\n", new_length);
   }
   WeakFixedArray::Set(new_array, array->Length(), value);
   return new_array;
 }
 
 
 void WeakFixedArray::Compact() {
   FixedArray* array = FixedArray::cast(this);
   int new_length = kFirstIndex;
   for (int i = kFirstIndex; i < array->length(); i++) {
     Object* element = array->get(i);
     if (element->IsSmi()) continue;
     if (WeakCell::cast(element)->cleared()) continue;
     array->set(new_length++, element);
   }
   array->Shrink(new_length);
   set_last_used_index(0);
 }
 
 
-void WeakFixedArray::Remove(Handle<HeapObject> value) {
+bool WeakFixedArray::Remove(Handle<HeapObject> value) {
+  if (Length() == 0) return false;
   // Optimize for the most recently added element to be removed again.
   int first_index = last_used_index();
   for (int i = first_index;;) {
     if (Get(i) == *value) {
       clear(i);
       // Users of WeakFixedArray should make sure that there are no duplicates,
       // they can use Add(..., kAddIfNotFound) if necessary.
-      return;
+      return true;
     }
     i = (i + 1) % Length();
-    if (i == first_index) break;
+    if (i == first_index) return false;
   }
+  UNREACHABLE();
 }
 
 
 // static
 Handle<WeakFixedArray> WeakFixedArray::Allocate(
     Isolate* isolate, int size, Handle<WeakFixedArray> initialize_from) {
   DCHECK(0 <= size);
   Handle<FixedArray> result =
       isolate->factory()->NewUninitializedFixedArray(size + kFirstIndex);
   Handle<WeakFixedArray> casted_result = Handle<WeakFixedArray>::cast(result);
@@ skipping 1712 matching lines @@
 
 
 // static
 void JSObject::ReoptimizeIfPrototype(Handle<JSObject> object) {
   if (!object->map()->is_prototype_map()) return;
   OptimizeAsPrototype(object, FAST_PROTOTYPE);
 }
 
 
 // static
-void JSObject::RegisterPrototypeUser(Handle<JSObject> prototype,
-                                     Handle<HeapObject> user) {
+void JSObject::LazyRegisterPrototypeUser(Handle<Map> user, Isolate* isolate) {
   DCHECK(FLAG_track_prototype_users);
-  Isolate* isolate = prototype->GetIsolate();
-  if (prototype->IsJSGlobalProxy()) {
-    PrototypeIterator iter(isolate, prototype);
-    prototype = Handle<JSObject>::cast(PrototypeIterator::GetCurrent(iter));
-  }
-  Handle<PrototypeInfo> proto_info;
-  Object* maybe_proto_info = prototype->map()->prototype_info();
-  if (maybe_proto_info->IsPrototypeInfo()) {
-    proto_info = handle(PrototypeInfo::cast(maybe_proto_info), isolate);
-  } else {
-    proto_info = isolate->factory()->NewPrototypeInfo();
-    prototype->map()->set_prototype_info(*proto_info);
-  }
-  Handle<Object> maybe_registry(proto_info->prototype_users(), isolate);
-  Handle<WeakFixedArray> new_array = WeakFixedArray::Add(maybe_registry, user);
-  if (!maybe_registry.is_identical_to(new_array)) {
-    proto_info->set_prototype_users(*new_array);
-  }
-  if (FLAG_trace_prototype_users) {
-    PrintF("Registering %p as a user of prototype %p.\n",
-           reinterpret_cast<void*>(*user), reinterpret_cast<void*>(*prototype));
+  // Contract: In line with InvalidatePrototypeChains()'s requirements,
+  // leaf maps don't need to register as users, only prototypes do.
+  DCHECK(user->is_prototype_map());
+
+  Handle<Map> current_user = user;
+  for (PrototypeIterator iter(user); !iter.IsAtEnd(); iter.Advance()) {
+    Handle<Object> maybe_proto = PrototypeIterator::GetCurrent(iter);
+    if (maybe_proto->IsJSGlobalProxy()) continue;
+    // Proxies on the prototype chain are not supported.
+    if (maybe_proto->IsJSProxy()) return;
+    Handle<JSObject> proto = Handle<JSObject>::cast(maybe_proto);
+    bool just_registered =
+        RegisterPrototypeUserIfNotRegistered(proto, current_user, isolate);
+    // Walk up the prototype chain as far as links haven't been registered yet.
+    if (!just_registered) break;
+    current_user = handle(proto->map(), isolate);
   }
 }
 
 
+// Returns true if the user was not yet registered.
 // static
-void JSObject::UnregisterPrototypeUser(Handle<JSObject> prototype,
+bool JSObject::RegisterPrototypeUserIfNotRegistered(Handle<JSObject> prototype,
+                                                    Handle<HeapObject> user,
+                                                    Isolate* isolate) {
+  Handle<PrototypeInfo> proto_info =
+      Map::GetOrCreatePrototypeInfo(prototype, isolate);
+  Handle<Object> maybe_registry(proto_info->prototype_users(), isolate);
+  bool was_present = false;
+  Handle<WeakFixedArray> new_array = WeakFixedArray::Add(
+      maybe_registry, user, WeakFixedArray::kAddIfNotFound, &was_present);
+  if (!maybe_registry.is_identical_to(new_array)) {
+    proto_info->set_prototype_users(*new_array);
+  }
+  if (FLAG_trace_prototype_users && !was_present) {
+    PrintF("Registering %p as a user of prototype %p (map=%p).\n",
+           reinterpret_cast<void*>(*user), reinterpret_cast<void*>(*prototype),
+           reinterpret_cast<void*>(prototype->map()));
+  }
+  return !was_present;
+}
+
+
+// Can be called regardless of whether |user| was actually registered with
+// |prototype|. Returns true when there was a registration.
+// static
+bool JSObject::UnregisterPrototypeUser(Handle<JSObject> prototype,
                                        Handle<HeapObject> user) {
   Isolate* isolate = prototype->GetIsolate();
   if (prototype->IsJSGlobalProxy()) {
     PrototypeIterator iter(isolate, prototype);
     prototype = Handle<JSObject>::cast(PrototypeIterator::GetCurrent(iter));
   }
   DCHECK(prototype->map()->is_prototype_map());
   Object* maybe_proto_info = prototype->map()->prototype_info();
-  if (!maybe_proto_info->IsPrototypeInfo()) return;
+  if (!maybe_proto_info->IsPrototypeInfo()) return false;
   Handle<PrototypeInfo> proto_info(PrototypeInfo::cast(maybe_proto_info),
                                    isolate);
   Object* maybe_registry = proto_info->prototype_users();
-  if (!maybe_registry->IsWeakFixedArray()) return;
-  WeakFixedArray::cast(maybe_registry)->Remove(user);
-  if (FLAG_trace_prototype_users) {
+  if (!maybe_registry->IsWeakFixedArray()) return false;
+  bool result = WeakFixedArray::cast(maybe_registry)->Remove(user);
+  if (FLAG_trace_prototype_users && result) {
     PrintF("Unregistering %p as a user of prototype %p.\n",
            reinterpret_cast<void*>(*user), reinterpret_cast<void*>(*prototype));
   }
+  return result;
 }
 
 
 static void InvalidatePrototypeChainsInternal(Map* map) {
   if (!map->is_prototype_map()) return;
+  if (FLAG_trace_prototype_users) {
+    PrintF("Invalidating prototype map %p 's cell\n",
+           reinterpret_cast<void*>(map));
+  }
   Object* maybe_proto_info = map->prototype_info();
   if (!maybe_proto_info->IsPrototypeInfo()) return;
   PrototypeInfo* proto_info = PrototypeInfo::cast(maybe_proto_info);
   Object* maybe_cell = proto_info->validity_cell();
   if (maybe_cell->IsCell()) {
     // Just set the value; the cell will be replaced lazily.
     Cell* cell = Cell::cast(maybe_cell);
     cell->set_value(Smi::FromInt(Map::kPrototypeChainInvalid));
   }
 
@@ skipping 19 matching lines @@
   DisallowHeapAllocation no_gc;
   if (map->IsJSGlobalProxyMap()) {
     PrototypeIterator iter(map);
     map = JSObject::cast(iter.GetCurrent())->map();
   }
   InvalidatePrototypeChainsInternal(map);
 }
 
 
 // static
+Handle<PrototypeInfo> Map::GetOrCreatePrototypeInfo(Handle<JSObject> prototype,
+                                                    Isolate* isolate) {
+  Object* maybe_proto_info = prototype->map()->prototype_info();
+  if (maybe_proto_info->IsPrototypeInfo()) {
+    return handle(PrototypeInfo::cast(maybe_proto_info), isolate);
+  }
+  Handle<PrototypeInfo> proto_info = isolate->factory()->NewPrototypeInfo();
+  prototype->map()->set_prototype_info(*proto_info);
+  return proto_info;
+}
+
+
+// static
 Handle<Cell> Map::GetOrCreatePrototypeChainValidityCell(Handle<Map> map,
                                                         Isolate* isolate) {
   Handle<Object> maybe_prototype(map->prototype(), isolate);
   if (!maybe_prototype->IsJSObject()) return Handle<Cell>::null();
   Handle<JSObject> prototype = Handle<JSObject>::cast(maybe_prototype);
   if (prototype->IsJSGlobalProxy()) {
     PrototypeIterator iter(isolate, prototype);
     prototype = Handle<JSObject>::cast(PrototypeIterator::GetCurrent(iter));
   }
-  Handle<PrototypeInfo> proto_info(
-      PrototypeInfo::cast(prototype->map()->prototype_info()), isolate);
+  // Ensure the prototype is registered with its own prototypes so its cell
+  // will be invalidated when necessary.
+  JSObject::LazyRegisterPrototypeUser(handle(prototype->map(), isolate),
+                                      isolate);
+  Handle<PrototypeInfo> proto_info =
+      GetOrCreatePrototypeInfo(prototype, isolate);
   Object* maybe_cell = proto_info->validity_cell();
   // Return existing cell if it's still valid.
   if (maybe_cell->IsCell()) {
     Handle<Cell> cell(Cell::cast(maybe_cell), isolate);
     if (cell->value() == Smi::FromInt(Map::kPrototypeChainValid)) {
-      return handle(Cell::cast(maybe_cell), isolate);
+      return cell;
     }
   }
   // Otherwise create a new cell.
   Handle<Cell> cell = isolate->factory()->NewCell(
       handle(Smi::FromInt(Map::kPrototypeChainValid), isolate));
   proto_info->set_validity_cell(*cell);
   return cell;
 }
 
 
+// static
 void Map::SetPrototype(Handle<Map> map, Handle<Object> prototype,
                        PrototypeOptimizationMode proto_mode) {
-  if (map->prototype()->IsJSObject() && FLAG_track_prototype_users) {
-    Handle<JSObject> old_prototype(JSObject::cast(map->prototype()));
-    JSObject::UnregisterPrototypeUser(old_prototype, map);
-  }
   if (prototype->IsJSObject()) {
     Handle<JSObject> prototype_jsobj = Handle<JSObject>::cast(prototype);
     JSObject::OptimizeAsPrototype(prototype_jsobj, proto_mode);
-    if (ShouldRegisterAsPrototypeUser(map, prototype_jsobj)) {
-      JSObject::RegisterPrototypeUser(prototype_jsobj, map);
-    }
   }
   WriteBarrierMode wb_mode =
       prototype->IsNull() ? SKIP_WRITE_BARRIER : UPDATE_WRITE_BARRIER;
   map->set_prototype(*prototype, wb_mode);
 }
 
 
-// static
-bool Map::ShouldRegisterAsPrototypeUser(Handle<Map> map,
-                                        Handle<JSObject> prototype) {
-  if (!FLAG_track_prototype_users) return false;
-  if (map->is_prototype_map()) return true;
-  Object* back = map->GetBackPointer();
-  if (!back->IsMap()) return true;
-  if (Map::cast(back)->prototype() != *prototype) return true;
-  return false;
-}
-
-
 Handle<Object> CacheInitialJSArrayMaps(
     Handle<Context> native_context, Handle<Map> initial_map) {
   // Replace all of the cached initial array maps in the native context with
   // the appropriate transitioned elements kind maps.
   Factory* factory = native_context->GetIsolate()->factory();
   Handle<FixedArray> maps = factory->NewFixedArrayWithHoles(
       kElementsKindCount, TENURED);
 
   Handle<Map> current_map = initial_map;
   ElementsKind kind = current_map->elements_kind();
@@ skipping 6910 matching lines @@
 void PropertyCell::SetValueWithInvalidation(Handle<PropertyCell> cell,
                                             Handle<Object> new_value) {
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 } }  // namespace v8::internal