Add a weak property type to the virtual machine.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index 55199b460c2b87da057836677eb93cdf68f6f4a2..12fe96ec2ffe79e22d4a58c53a32c13792e8e71a 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -4,6 +4,8 @@
 
 #include "vm/gc_marker.h"
 
+#include <map>

[turnidge, 2012/08/09 18:34:34]

Do we get to use stl now?  That's helpful.

+
 #include "vm/allocation.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
@@ -137,6 +139,29 @@ class MarkingVisitor : public ObjectPointerVisitor {
     }
   }
 
+  void DelayWeakProperty(RawWeakProperty* raw_weak) {
+    RawObject* raw_key = raw_weak->ptr()->key_;
+    DelaySet::iterator it = delay_set_.find(raw_key);
+    if (it != delay_set_.end()) {
+      ASSERT(raw_key->IsWatched());
+      WeakProperty::Push(raw_weak, &it->second);
+    } else {
+      ASSERT(!raw_key->IsWatched());
+      raw_key->SetWatchedBit();
+      delay_set_[raw_key] = raw_weak;
+    }

[turnidge, 2012/08/09 18:34:34]

So what we are implementing here is a map from Key to a list of RawWeakProperty
objects (where the list is implemented using the next_ field), right?  We make
sure that we only set the mark on the key once, but then we push all of the
associated WeakProperties onto a stack.

Did you consider dropping the next_ field from RawWeakProperty and just making
it an explicit map from key to list?  It would save a word per RawWeakProperty,
since the next_ field is only used during GC.  This may just be premature
optimization, since there won't be so many RawWeakProperties kicking around, but
wanted to discuss it.

[cshapiro, 2012/08/14 04:58:18]

I am now using a multimap instead of a map.  As such, the next_ field and the
Push and Pop methods are gone.  Done!

+  }
+
+  void Finalize() {
+    DelaySet::iterator it = delay_set_.begin();
+    for (; it != delay_set_.end(); ++it) {
+      while (it->second != Object::null()) {
+        RawWeakProperty* head = WeakProperty::Pop(&it->second);
+        WeakProperty::Clear(head);
+      }
+    }
+  }
+
   void set_update_store_buffers(bool val) { update_store_buffers_ = val; }
 
  private:
@@ -148,6 +173,19 @@ class MarkingVisitor : public ObjectPointerVisitor {
     ASSERT(!raw_obj->IsMarked());
     RawClass* raw_class = isolate()->class_table()->At(raw_obj->GetClassId());
     raw_obj->SetMarkBit();
+    if (raw_obj->IsWatched()) {
+        DelaySet::iterator it = delay_set_.find(raw_obj);
+        ASSERT(it != delay_set_.end());
+        RawWeakProperty* raw_weak = it->second;
+        while (raw_weak != WeakProperty::null()) {

[turnidge, 2012/08/09 18:34:34]

Is there are reason that you use WeakProperty::Pop during Finalize but not here?

[cshapiro, 2012/08/14 04:58:18]

That was an oversight.  This has been superseded by the use of a multimap.

+          raw_weak->VisitPointers(this);
+          RawWeakProperty* next = raw_weak->ptr()->next_;
+          raw_weak->ptr()->next_ = WeakProperty::null();
+          raw_weak = next;
+        }
+        delay_set_.erase(it);
+        raw_obj->ClearWatchedBit();
+    }
     marking_stack_->Push(raw_obj);
 
     // Update the number of used bytes on this page for fast accounting.
@@ -184,6 +222,8 @@ class MarkingVisitor : public ObjectPointerVisitor {
   Heap* vm_heap_;
   PageSpace* page_space_;
   MarkingStack* marking_stack_;
+  typedef std::map<RawObject*, RawWeakProperty*> DelaySet;
+  DelaySet delay_set_;
   bool update_store_buffers_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(MarkingVisitor);
@@ -318,12 +358,31 @@ void GCMarker::DrainMarkingStack(Isolate* isolate,
   visitor->set_update_store_buffers(true);
   while (!visitor->marking_stack()->IsEmpty()) {
     RawObject* raw_obj = visitor->marking_stack()->Pop();
-    raw_obj->VisitPointers(visitor);
+    if (raw_obj->GetClassId() != kWeakProperty) {
+      raw_obj->VisitPointers(visitor);
+    } else {
+      RawWeakProperty* raw_weak = reinterpret_cast<RawWeakProperty*>(raw_obj);
+      ProcessWeakProperty(raw_weak, visitor);
+    }
   }
   visitor->set_update_store_buffers(false);
 }
 
 
+void GCMarker::ProcessWeakProperty(RawWeakProperty* raw_weak,
+                                   MarkingVisitor* visitor) {
+  // The fate of the weak property is determined by its key.
+  RawObject* raw_key = raw_weak->ptr()->key_;
+  if (!raw_key->IsMarked()) {
+    // Key is white.  Delay the weak property.
+    visitor->DelayWeakProperty(raw_weak);
+  } else {
+    // Key is gray or black.  Make the weak property black.
+    raw_weak->VisitPointers(visitor);
+  }
+}
+
+
 void GCMarker::MarkObjects(Isolate* isolate,
                            PageSpace* page_space,
                            bool invoke_api_callbacks) {
@@ -335,6 +394,7 @@ void GCMarker::MarkObjects(Isolate* isolate,
   IterateWeakReferences(isolate, &mark);
   MarkingWeakVisitor mark_weak;
   IterateWeakRoots(isolate, &mark_weak, invoke_api_callbacks);
+  mark.Finalize();
   Epilogue(isolate, invoke_api_callbacks);
 }