Oilpan:Allow custom handling of classes that contain weak pointers that are embedded in collections.

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 Google Inc. 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
@@ skipping 218 matching lines @@
     }
 
     // Member version of the one-argument templated trace method.
     template<typename T>
     void trace(const Member<T>& t)
     {
         t.verifyTypeIsGarbageCollected();
         mark(t.get());
     }
 
+    template<typename T>
+    void trace(Member<T>& t)

[haraken, 2014/06/06 08:03:02]

Just help me understand: Why do we have both const version and non-const version
for trace(T*) and trace(Member<T>&), whereas we have only const version for
trace(WeakMember<T>&), trace(T&) etc?

[Erik Corry, 2014/06/06 08:47:35]

We have to have a const and non-const version for T* because if we only have the
non-const version it doesn't work when the pointer is const at the call site,
and if we only have the const version then the compiler will pick the T& version
rather than the const T* version when given a non-const T*.

For Member<T>& we don't need the non-const version, so I deleted it.

+    {
+        t.verifyTypeIsGarbageCollected();
+        mark(t.get());
+    }
+
     // Fallback method used only when we need to trace raw pointers of T.
     // This is the case when a member is a union where we do not support members.
     template<typename T>
+    void trace(const T* t)
+    {
+        mark(const_cast<T*>(t));
+    }
+
+    template<typename T>
     void trace(T* t)
     {
         mark(t);
     }
 
     // WeakMember version of the templated trace method. It doesn't keep
     // the traced thing alive, but will write null to the WeakMember later
-    // if the pointed-to object is dead.
+    // if the pointed-to object is dead. It's lying for this to be const,
+    // but the overloading resolver prioritizes constness too high when
+    // picking the correct overload, so all these trace methods have to have
+    // the same constness on their argument to allow the type to decide.

[haraken, 2014/06/06 08:03:02]

I agree that we need a const version of trace(const WeakMember<T>&) but won't we
need a non-const version as well? We have both const version and non-const
version for trace(T*) and trace(Member<T>&).

[Erik Corry, 2014/06/06 08:47:35]

If you have only const methods to choose between then the best match for a
non-const WeakMember<T>& is to convert it to a const WeakMember<T>& and call
that.  We have to be sure that there is not a non-const T& method, because that
would be a better match for WeakMember<T>& than the method with const
WeakMember<T>& signature.

     template<typename T>
     void trace(const WeakMember<T>& t)
     {
-        registerWeakCell(t.cell());
+        registerWeakCell(const_cast<WeakMember<T>&>(t).cell());
     }
 
-    // Fallback trace method for part objects to allow individual
-    // trace methods to trace through a part object with
-    // visitor->trace(m_partObject).
+    template<typename T>
+    void traceInCollection(T& t, ShouldWeakPointersBeMarkedStrongly strongify)
+    {
+        HashTraits<T>::traceInCollection(this, t, strongify);
+    }
+
+    // Fallback trace method for part objects to allow individual trace methods
+    // to trace through a part object with visitor->trace(m_partObject). This
+    // takes a const argument, because otherwise it will match too eagerly: a
+    // non-const argument would match a non-const Vector<T>& argument better
+    // than the specialization that takes const Vector<T>&. For a similar reason,
+    // the other specializations take a const argument even though they are
+    // usually used with non-const arguments, otherwise this function would match
+    // too well.
     template<typename T>
     void trace(const T& t)
     {
         const_cast<T&>(t).trace(this);
     }
 
     // The following trace methods are for off-heap collections.
     template<typename T, size_t inlineCapacity>
-    void trace(const Vector<T, inlineCapacity, WTF::DefaultAllocator>& vector)
+    void trace(const Vector<T, inlineCapacity>& vector)
     {
         OffHeapCollectionTraceTrait<Vector<T, inlineCapacity, WTF::DefaultAllocator> >::trace(this, vector);
     }
 
     template<typename T, typename U, typename V>
-    void trace(const HashSet<T, U, V, WTF::DefaultAllocator>& hashSet)
+    void trace(const HashSet<T, U, V>& hashSet)
     {
-        OffHeapCollectionTraceTrait<HashSet<T, U, V, WTF::DefaultAllocator> >::trace(this, hashSet);
+        OffHeapCollectionTraceTrait<HashSet<T, U, V> >::trace(this, hashSet);
     }
 
     template<typename T, size_t inlineCapacity, typename U>
     void trace(const ListHashSet<T, inlineCapacity, U>& hashSet)
     {
         OffHeapCollectionTraceTrait<ListHashSet<T, inlineCapacity, U> >::trace(this, hashSet);
     }
 
     template<typename T, typename U>
     void trace(const LinkedHashSet<T, U>& hashSet)
@@ skipping 55 matching lines @@
     // Remove this once we remove WeakPtrWillBeMember.
     template<typename T>
     void trace(const WeakPtr<T>&)
     {
 #if ENABLE(OILPAN)
         // WeakPtrs should never be traced.
         ASSERT_NOT_REACHED();
 #endif
     }
 
+    template<typename T>
+    void trace(WeakPtr<T>&)
+    {
+#if ENABLE(OILPAN)
+        // WeakPtrs should never be traced.
+        ASSERT_NOT_REACHED();
+#endif
+    }
+
     // This method marks an object and adds it to the set of objects
     // that should have their trace method called. Since not all
     // objects have vtables we have to have the callback as an
     // explicit argument, but we can use the templated one-argument
     // mark method above to automatically provide the callback
     // function.
     virtual void mark(const void*, TraceCallback) = 0;
     virtual void markNoTracing(const void* pointer) { mark(pointer, reinterpret_cast<TraceCallback>(0)); }
 
     // Used to mark objects during conservative scanning.
@@ skipping 100 matching lines @@
 template<typename T, typename HashFunctions, typename Traits>
 struct OffHeapCollectionTraceTrait<WTF::HashSet<T, HashFunctions, Traits, WTF::DefaultAllocator> > {
     typedef WTF::HashSet<T, HashFunctions, Traits, WTF::DefaultAllocator> HashSet;
 
     static void trace(Visitor* visitor, const HashSet& set)
     {
         if (set.isEmpty())
             return;
         if (WTF::ShouldBeTraced<Traits>::value) {
             HashSet& iterSet = const_cast<HashSet&>(set);
-            for (typename HashSet::iterator it = iterSet.begin(), end = iterSet.end(); it != end; ++it)
-                CollectionBackingTraceTrait<WTF::ShouldBeTraced<Traits>::value, Traits::isWeak, WeakPointersActWeak, T, Traits>::trace(visitor, *it);
+            for (typename HashSet::iterator it = iterSet.begin(), end = iterSet.end(); it != end; ++it) {
+                const T& t = *it;
+                CollectionBackingTraceTrait<WTF::ShouldBeTraced<Traits>::value, Traits::isWeak, WeakPointersActWeak, T, Traits>::trace(visitor, const_cast<T&>(t));
+            }
         }
         COMPILE_ASSERT(!Traits::isWeak, WeakOffHeapCollectionsConsideredDangerous0);
     }
 };
 
 template<typename T, size_t inlineCapacity, typename HashFunctions>
 struct OffHeapCollectionTraceTrait<ListHashSet<T, inlineCapacity, HashFunctions> > {
     typedef WTF::ListHashSet<T, inlineCapacity, HashFunctions> ListHashSet;
 
     static void trace(Visitor* visitor, const ListHashSet& set)
@@ skipping 241 matching lines @@
 struct GCInfoTrait {
     static const GCInfo* get()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::get();
     }
 };
 
 }
 
 #endif