[oilpan]: Remove support for tracing off-heap hashmaps.

Source/platform/Supplementable.h

 /*
  * Copyright (C) 2012 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 141 matching lines @@
 
 template<typename T, bool>
 class SupplementableTracing;
 
 template<typename T>
 class SupplementableTracing<T, true> { };
 
 template<typename T>
 class SupplementableTracing<T, false> { };
 
+// Helper class for implementing Supplementable, HeapSupplementable, and
+// PersistentHeapSupplementable.
 template<typename T, bool isGarbageCollected = false>
 class SupplementableBase : public SupplementableTracing<T, isGarbageCollected> {
 public:
     void provideSupplement(const char* key, typename SupplementableTraits<T, isGarbageCollected>::SupplementArgumentType supplement)
     {
         ASSERT(m_threadId == currentThread());
         ASSERT(!this->m_supplements.get(key));
         this->m_supplements.set(key, supplement);
     }
 
     void removeSupplement(const char* key)
     {
         ASSERT(m_threadId == currentThread());
         this->m_supplements.remove(key);
     }
 
     SupplementBase<T, isGarbageCollected>* requireSupplement(const char* key)
     {
         ASSERT(m_threadId == currentThread());
         return this->m_supplements.get(key);
     }
 
     void reattachThread()
     {
 #if ENABLE(ASSERT)
         m_threadId = currentThread();
 #endif
     }
 
-    virtual void trace(Visitor* visitor) { visitor->trace(m_supplements); }
+    // We have a trace method in the SupplementableBase class to ensure we have
+    // the vtable at the first word of the object. However we don't trace the
+    // m_supplements here, but in the partially specialized template subclasses
+    // since we only want to trace it for garbage collected classes.
+    virtual void trace(Visitor*) { }
 
     void willBeDestroyed()
     {
         typedef typename SupplementableTraits<T, isGarbageCollected>::SupplementMap::iterator SupplementIterator;
         for (SupplementIterator it = m_supplements.begin(); it != m_supplements.end(); ++it)
             it->value->willBeDestroyed();
     }
 
     // FIXME: Oilpan: Make private and remove this ignore once PersistentHeapSupplementable is removed again.
 protected:
     GC_PLUGIN_IGNORE("")
     typename SupplementableTraits<T, isGarbageCollected>::SupplementMap m_supplements;
 
 #if ENABLE(ASSERT)
 protected:
     SupplementableBase() : m_threadId(currentThread()) { }
 
 private:
     ThreadIdentifier m_threadId;
 #endif
 };
 
+// This class is used to make an on-heap class supplementable. Its supplements
+// must be HeapSupplement.
 template<typename T>
 class HeapSupplement : public SupplementBase<T, true> { };
 
 // FIXME: Oilpan: Move GarbageCollectedMixin to SupplementableBase<T, true> once PersistentHeapSupplementable is removed again.
 template<typename T>
 class GC_PLUGIN_IGNORE("http://crbug.com/395036") HeapSupplementable : public SupplementableBase<T, true>, public GarbageCollectedMixin {
 public:
-    virtual void trace(Visitor* visitor) { SupplementableBase<T, true>::trace(visitor); }
+    virtual void trace(Visitor* visitor) OVERRIDE
+    {
+        visitor->trace(this->m_supplements);
+        SupplementableBase<T, true>::trace(visitor);
+    }
 };
 
+// This class is used to make an off-heap class supplementable with supplements
+// that are on-heap, aka. HeapSupplements.
 template<typename T>
-class PersistentHeapSupplementable : public SupplementableBase<T, true> {
+class GC_PLUGIN_IGNORE("http://crbug.com/395036") PersistentHeapSupplementable : public SupplementableBase<T, true> {
 public:
     PersistentHeapSupplementable() : m_root(this) { }
     virtual ~PersistentHeapSupplementable()
     {
         typedef typename SupplementableTraits<T, true>::SupplementMap::iterator SupplementIterator;
         for (SupplementIterator it = this->m_supplements.begin(); it != this->m_supplements.end(); ++it)
             it->value->persistentHostHasBeenDestroyed();
     }
+
+    virtual void trace(Visitor* visitor)
+    {
+        visitor->trace(this->m_supplements);
+        SupplementableBase<T, true>::trace(visitor);
+    }
+
 private:
     class TraceDelegate : PersistentBase<ThreadLocalPersistents<AnyThread>, TraceDelegate> {
     public:
         TraceDelegate(PersistentHeapSupplementable* owner) : m_owner(owner) { }
         void trace(Visitor* visitor) { m_owner->trace(visitor); }
     private:
         PersistentHeapSupplementable* m_owner;
     };
 
     TraceDelegate m_root;
 };
 
 template<typename T>
 class Supplement : public SupplementBase<T, false> { };
 
+// This class is used to make an off-heap class supplementable with off-heap
+// supplements (Supplement).
 template<typename T>
-class Supplementable : public SupplementableBase<T, false> { };
+class GC_PLUGIN_IGNORE("http://crbug.com/395036") Supplementable : public SupplementableBase<T, false> {
+public:
+    virtual void trace(Visitor* visitor)
+    {
+        // No tracing of off-heap supplements. We should not have any Supplementable
+        // object on the heap. Either the object is HeapSupplementable or if it is
+        // off heap it should use PersistentHeapSupplementable to trace any on-heap
+        // supplements.
+        COMPILE_ASSERT(!IsGarbageCollectedType<T>::value, GarbageCollectedObjectMustBeHeapSupplementable);
+        SupplementableBase<T, false>::trace(visitor);
+    }
+};
 
 template<typename T>
 struct ThreadingTrait<blink::SupplementBase<T, true> > {
     static const ThreadAffinity Affinity = ThreadingTrait<T>::Affinity;
 };
 
 template<typename T>
 struct ThreadingTrait<blink::SupplementableBase<T, true> > {
     static const ThreadAffinity Affinity = ThreadingTrait<T>::Affinity;
 };
 
 } // namespace blink
 
 #endif // Supplementable_h