Adds a scavenge GC pass to collect unmodified references

src/global-handles.cc

Index: src/global-handles.cc
diff --git a/src/global-handles.cc b/src/global-handles.cc
index 650999f394c5cc7c2e5397afff53c74e3c186d20..9f35d0e1018b28c2a4c391cace12a631e48128d9 100644
--- a/src/global-handles.cc
+++ b/src/global-handles.cc
@@ -54,6 +54,8 @@ class GlobalHandles::Node {
                   Internals::kNodeIsIndependentShift);
     STATIC_ASSERT(static_cast<int>(IsPartiallyDependent::kShift) ==
                   Internals::kNodeIsPartiallyDependentShift);
+    STATIC_ASSERT(static_cast<int>(IsActive::kShift) ==
+                  Internals::kNodeIsActiveShift);
   }
 
 #ifdef ENABLE_HANDLE_ZAPPING
@@ -64,7 +66,12 @@ class GlobalHandles::Node {
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     index_ = 0;
     set_independent(false);
-    set_partially_dependent(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_unmodified(false);
+      set_active(false);
+    } else {
+      set_partially_dependent(false);
+    }
     set_in_new_space_list(false);
     parameter_or_next_free_.next_free = NULL;
     weak_callback_ = NULL;
@@ -77,6 +84,10 @@ class GlobalHandles::Node {
     set_state(FREE);
     set_weakness_type(NORMAL_WEAK);
     set_in_new_space_list(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_unmodified(false);
+      set_active(false);
+    }
     parameter_or_next_free_.next_free = *first_free;
     *first_free = this;
   }
@@ -86,7 +97,12 @@ class GlobalHandles::Node {
     object_ = object;
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     set_independent(false);
-    set_partially_dependent(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_unmodified(false);
+      set_active(false);
+    } else {
+      set_partially_dependent(false);
+    }
     set_state(NORMAL);
     parameter_or_next_free_.parameter = NULL;
     weak_callback_ = NULL;
@@ -106,7 +122,12 @@ class GlobalHandles::Node {
     object_ = reinterpret_cast<Object*>(kGlobalHandleZapValue);
     class_id_ = v8::HeapProfiler::kPersistentHandleNoClassId;
     set_independent(false);
-    set_partially_dependent(false);
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      set_unmodified(false);
+      set_active(false);
+    } else {
+      set_partially_dependent(false);
+    }
     weak_callback_ = NULL;
     DecreaseBlockUses();
   }
@@ -140,12 +161,26 @@ class GlobalHandles::Node {
   }
 
   bool is_partially_dependent() {
+    CHECK(!FLAG_scavenge_reclaim_unmodified_objects);
     return IsPartiallyDependent::decode(flags_);
   }
   void set_partially_dependent(bool v) {
+    CHECK(!FLAG_scavenge_reclaim_unmodified_objects);
     flags_ = IsPartiallyDependent::update(flags_, v);
   }
 
+  bool is_active() { return IsActive::decode(flags_); }
+  void set_active(bool v) { IsActive::update(flags_, v); }
+
+  bool is_unmodified() {
+    CHECK(FLAG_scavenge_reclaim_unmodified_objects);
+    return unmodified_flag;
+  }
+  void set_unmodified(bool v) {
+    CHECK(FLAG_scavenge_reclaim_unmodified_objects);
+    unmodified_flag = v;
+  }
+
   bool is_in_new_space_list() {
     return IsInNewSpaceList::decode(flags_);
   }
@@ -349,11 +384,14 @@ class GlobalHandles::Node {
   // in_new_space_list) and a State.
   class NodeState : public BitField<State, 0, 3> {};
   class IsIndependent : public BitField<bool, 3, 1> {};
+  // The following two fields are mutually exclusive
+  class IsActive : public BitField<bool, 4, 1> {};
   class IsPartiallyDependent : public BitField<bool, 4, 1> {};
   class IsInNewSpaceList : public BitField<bool, 5, 1> {};
   class NodeWeaknessType : public BitField<WeaknessType, 6, 2> {};
 
   uint8_t flags_;

[rmcilroy, 2015/10/21 13:36:51]

Maybe we could just make this a uint16_t and store Unmodified in the flag rather
than a new bool field. Or can we steal another bit from elsewhere? WDYT Jochen?

+  bool unmodified_flag;
 
   // Handle specific callback - might be a weak reference in disguise.
   WeakCallback weak_callback_;
@@ -646,10 +684,18 @@ void GlobalHandles::IdentifyWeakHandles(WeakSlotCallback f) {
 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() &&
-         !node->is_partially_dependent())) {
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      if (node->IsStrongRetainer() ||
+          (node->IsWeakRetainer() && !node->is_independent() &&
+           !node->is_unmodified())) {
+        v->VisitPointer(node->location());
+      }
+    } else {
+      if (node->IsStrongRetainer() ||
+          (node->IsWeakRetainer() && !node->is_independent() &&
+           !node->is_partially_dependent())) {
         v->VisitPointer(node->location());
+      }
     }
   }
 }
@@ -687,6 +733,51 @@ void GlobalHandles::IterateNewSpaceWeakIndependentRoots(ObjectVisitor* v) {
 }
 
 
+void GlobalHandles::IdentifyWeakUnmodifiedObjects(
+    WeakSlotCallback is_unmodified) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    if (!node->is_active() && node->IsWeak() &&
+        is_unmodified(node->location())) {
+      node->set_unmodified(true);
+    }
+    node->set_active(false);
+  }
+}
+
+
+void GlobalHandles::MarkNewSpaceWeakUnmodifiedObjectsPending(
+    WeakSlotCallbackWithHeap is_unscavenged) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    DCHECK(node->is_in_new_space_list());
+    if ((node->is_independent() || node->is_unmodified()) && node->IsWeak() &&
+        is_unscavenged(isolate_->heap(), node->location())) {
+      node->MarkPending();
+    }
+  }
+}
+
+
+void GlobalHandles::IterateNewSpaceWeakUnmodifiedRoots(ObjectVisitor* v) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    DCHECK(node->is_in_new_space_list());
+    if ((node->is_independent() || node->is_unmodified()) &&
+        node->IsWeakRetainer()) {
+      // Pending weak phantom handles die immediately. Everything else survives.
+      if (node->state() == Node::PENDING &&
+          node->weakness_type() != NORMAL_WEAK) {
+        node->CollectPhantomCallbackData(isolate(),
+                                         &pending_phantom_callbacks_);
+      } else {
+        v->VisitPointer(node->location());
+      }
+    }
+  }
+}
+
+
 bool GlobalHandles::IterateObjectGroups(ObjectVisitor* v,
                                         WeakSlotCallbackWithHeap can_skip) {
   ComputeObjectGroupsAndImplicitReferences();
@@ -757,13 +848,23 @@ int GlobalHandles::PostScavengeProcessing(
       // the freed_nodes.
       continue;
     }
-    // Skip dependent handles. Their weak callbacks might expect to be
+    // Skip dependent or unmodified 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() && !node->is_partially_dependent()) {
-      continue;
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      if (!node->is_independent() && !node->is_unmodified()) {
+        continue;
+      }
+      node->set_active(false);
+      node->set_unmodified(false);
+    } else {
+      if (!node->is_independent() && !node->is_partially_dependent()) {
+        continue;
+      }
+      node->clear_partially_dependent();
     }
-    node->clear_partially_dependent();
+
     if (node->PostGarbageCollectionProcessing(isolate_)) {
       if (initial_post_gc_processing_count != post_gc_processing_count_) {
         // Weak callback triggered another GC and another round of
@@ -790,7 +891,12 @@ int GlobalHandles::PostMarkSweepProcessing(
       // the freed_nodes.
       continue;
     }
-    it.node()->clear_partially_dependent();
+    if (FLAG_scavenge_reclaim_unmodified_objects) {
+      it.node()->set_unmodified(false);
+      it.node()->set_active(false);
+    } else {
+      it.node()->clear_partially_dependent();
+    }
     if (it.node()->PostGarbageCollectionProcessing(isolate_)) {
       if (initial_post_gc_processing_count != post_gc_processing_count_) {
         // See the comment above.
@@ -955,6 +1061,16 @@ void GlobalHandles::IterateAllRootsInNewSpaceWithClassIds(ObjectVisitor* v) {
 }
 
 
+void GlobalHandles::IterateWeakRootsInNewSpaceWithClassIds(ObjectVisitor* v) {
+  for (int i = 0; i < new_space_nodes_.length(); ++i) {
+    Node* node = new_space_nodes_[i];
+    if (node->has_wrapper_class_id() && node->IsWeak()) {
+      v->VisitEmbedderReference(node->location(), node->wrapper_class_id());
+    }
+  }
+}
+
+
 int GlobalHandles::NumberOfWeakHandles() {
   int count = 0;
   for (NodeIterator it(this); !it.done(); it.Advance()) {