Allow collection of DOM objects in minor GC cycles.

src/global-handles.cc

 // Copyright 2009 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 50 matching lines @@
 
   Node() {}
 
 #ifdef DEBUG
   ~Node() {
     // TODO(1428): if it's a weak handle we should have invoked its callback.
     // Zap the values for eager trapping.
     object_ = NULL;
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     index_ = 0;
     independent_ = false;

[Michael Starzinger, 2012/10/26 14:16:48]

We should be defensive an reset the externally_unreachble_ flag here.

[haraken, 2012/10/30 08:51:17]

Done.

     in_new_space_list_ = false;
     parameter_or_next_free_.next_free = NULL;
     callback_ = NULL;
   }
 #endif
 
   void Initialize(int index, Node** first_free) {
     index_ = static_cast<uint8_t>(index);
     ASSERT(static_cast<int>(index_) == index);
     state_ = FREE;
     in_new_space_list_ = false;
     parameter_or_next_free_.next_free = *first_free;
     *first_free = this;
   }
 
   void Acquire(Object* object, GlobalHandles* global_handles) {
     ASSERT(state_ == FREE);
     object_ = object;
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     independent_ = false;

[Michael Starzinger, 2012/10/26 14:16:48]

The externally_unreachable_ flag needs to be initialized here.

[haraken, 2012/10/30 08:51:17]

Done.

[haraken, 2012/10/30 08:51:17]

Done.

     state_  = NORMAL;
     parameter_or_next_free_.parameter = NULL;
     callback_ = NULL;
     IncreaseBlockUses(global_handles);
   }
 
   void Release(GlobalHandles* global_handles) {
     ASSERT(state_ != FREE);
     if (IsWeakRetainer()) {
       global_handles->number_of_weak_handles_--;
@@ skipping 45 matching lines @@
     state_ = PENDING;
   }
 
   // Independent flag accessors.
   void MarkIndependent() {
     ASSERT(state_ != FREE);
     independent_ = true;
   }
   bool is_independent() const { return independent_; }
 
+  void MarkExternallyUnreachable(GlobalHandles* global_handles) {
+    ASSERT(state_ != FREE);
+    if (global_handles->isolate()->heap()->InNewSpace(object_))
+      externally_unreachable_ = true;
+  }
+  bool is_externally_unreachable() const { return externally_unreachable_; }
+  bool clear_externally_unreachable() { externally_unreachable_ = false; }
+
   // In-new-space-list flag accessors.
   void set_in_new_space_list(bool v) { in_new_space_list_ = v; }
   bool is_in_new_space_list() const { return in_new_space_list_; }
 
   // Callback accessor.
   WeakReferenceCallback callback() { return callback_; }
 
   // Callback parameter accessors.
   void set_parameter(void* parameter) {
     ASSERT(state_ != FREE);
@@ skipping 86 matching lines @@
   // Wrapper class ID.
   uint16_t class_id_;
 
   // Index in the containing handle block.
   uint8_t index_;
 
   // Need one more bit for MSVC as it treats enums as signed.
   State state_ : 4;
 
   bool independent_ : 1;
+  bool externally_unreachable_ : 1;
   bool in_new_space_list_ : 1;
 
   // Handle specific callback.
   WeakReferenceCallback callback_;
 
   // Provided data for callback.  In FREE state, this is used for
   // the free list link.
   union {
     void* parameter;
     Node* next_free;
@@ skipping 168 matching lines @@
 void GlobalHandles::ClearWeakness(Object** location) {
   Node::FromLocation(location)->ClearWeakness(this);
 }
 
 
 void GlobalHandles::MarkIndependent(Object** location) {
   Node::FromLocation(location)->MarkIndependent();
 }
 
 
+void GlobalHandles::MarkExternallyUnreachable(Object** location) {
+  Node::FromLocation(location)->MarkExternallyUnreachable(this);
+}
+
+
 bool GlobalHandles::IsIndependent(Object** location) {
   return Node::FromLocation(location)->is_independent();
 }
 
 
 bool GlobalHandles::IsNearDeath(Object** location) {
   return Node::FromLocation(location)->IsNearDeath();
 }
 
 
@@ skipping 33 matching lines @@
       it.node()->MarkPending();
     }
   }
 }
 
 
 void GlobalHandles::IterateNewSpaceStrongAndDependentRoots(ObjectVisitor* v) {
   for (int i = 0; i < new_space_nodes_.length(); ++i) {
     Node* node = new_space_nodes_[i];
     if (node->IsStrongRetainer() ||
-        (node->IsWeakRetainer() && !node->is_independent())) {
-      v->VisitPointer(node->location());
+        (node->IsWeakRetainer() && !node->is_independent() &&
+         !node->is_externally_unreachable())) {
+        v->VisitPointer(node->location());
     }
   }
 }
 
 
 void GlobalHandles::IdentifyNewSpaceWeakIndependentHandles(
     WeakSlotCallbackWithHeap f) {
   for (int i = 0; i < new_space_nodes_.length(); ++i) {
     Node* node = new_space_nodes_[i];
     ASSERT(node->is_in_new_space_list());
-    if (node->is_independent() && node->IsWeak() &&
-        f(isolate_->heap(), node->location())) {
+    if ((node->is_independent() || node->is_externally_unreachable()) &&
+        node->IsWeak() && f(isolate_->heap(), node->location())) {
       node->MarkPending();
     }
   }
 }
 
 
 void GlobalHandles::IterateNewSpaceWeakIndependentRoots(ObjectVisitor* v) {
   for (int i = 0; i < new_space_nodes_.length(); ++i) {
     Node* node = new_space_nodes_[i];
     ASSERT(node->is_in_new_space_list());
-    if (node->is_independent() && node->IsWeakRetainer()) {
+    if ((node->is_independent() || node->is_externally_unreachable()) &&
+        node->IsWeakRetainer()) {
       v->VisitPointer(node->location());
     }
   }
 }
 
 
 bool GlobalHandles::PostGarbageCollectionProcessing(
     GarbageCollector collector) {
   // Process weak global handle callbacks. This must be done after the
   // GC is completely done, because the callbacks may invoke arbitrary
   // API functions.
   ASSERT(isolate_->heap()->gc_state() == Heap::NOT_IN_GC);
   const int initial_post_gc_processing_count = ++post_gc_processing_count_;
   bool next_gc_likely_to_collect_more = false;
   if (collector == SCAVENGER) {
     for (int i = 0; i < new_space_nodes_.length(); ++i) {
       Node* node = new_space_nodes_[i];
       ASSERT(node->is_in_new_space_list());
       // Skip dependent handles. Their weak callbacks might expect to be
       // called between two global garbage collection callbacks which
       // are not called for minor collections.
-      if (!node->is_independent()) continue;
+      if (!node->is_independent() && !node->is_externally_unreachable()) continue;

[Michael Starzinger, 2012/10/26 14:16:48]

More than 80 characters, move the continue into the next line and add curly
brackets around it.

[haraken, 2012/10/30 08:51:17]

Done.

+      node->clear_externally_unreachable();
       if (node->PostGarbageCollectionProcessing(isolate_, this)) {
         if (initial_post_gc_processing_count != post_gc_processing_count_) {
           // Weak callback triggered another GC and another round of
           // PostGarbageCollection processing.  The current node might
           // have been deleted in that round, so we need to bail out (or
           // restart the processing).
           return next_gc_likely_to_collect_more;
         }
       }
       if (!node->IsRetainer()) {
         next_gc_likely_to_collect_more = true;
       }
     }
   } else {
     for (NodeIterator it(this); !it.done(); it.Advance()) {
+      it.node()->clear_externally_unreachable();
       if (it.node()->PostGarbageCollectionProcessing(isolate_, this)) {
         if (initial_post_gc_processing_count != post_gc_processing_count_) {
           // See the comment above.
           return next_gc_likely_to_collect_more;
         }
       }
       if (!it.node()->IsRetainer()) {
         next_gc_likely_to_collect_more = true;
       }
     }
@@ skipping 147 matching lines @@
   implicit_ref_groups_.Clear();
 }
 
 
 void GlobalHandles::TearDown() {
   // TODO(1428): invoke weak callbacks.
 }
 
 
 } }  // namespace v8::internal