Oilpan: Ensure that classes with virtual trace methods always have vtables for their left-most base…

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 101 matching lines @@
 template<bool>
 class SupplementTracing;
 
 template<>
 class SupplementTracing<true> : public GarbageCollectedMixin { };
 
 template<>
 class SupplementTracing<false> {
 public:
     virtual ~SupplementTracing() { }
+    virtual void trace(Visitor*) { }
 };
 
 template<typename T, bool isGarbageCollected = false>
 class SupplementBase : public SupplementTracing<isGarbageCollected> {
 public:
 #if ENABLE(SECURITY_ASSERT)
     virtual bool isRefCountedWrapper() const { return false; }
 #endif
 
     static void provideTo(SupplementableBase<T, isGarbageCollected>& host, const char* key, typename SupplementableTraits<T, isGarbageCollected>::SupplementArgumentType supplement)
     {
         host.provideSupplement(key, supplement);
     }
 
     static SupplementBase<T, isGarbageCollected>* from(SupplementableBase<T, isGarbageCollected>& host, const char* key)
     {
         return host.requireSupplement(key);
     }
 
     static SupplementBase<T, isGarbageCollected>* from(SupplementableBase<T, isGarbageCollected>* host, const char* key)
     {
         return host ? host->requireSupplement(key) : 0;
     }
 
-    virtual void trace(Visitor*) { }
     virtual void willBeDestroyed() { }
 
     // FIXME: Oilpan: Remove this callback once PersistentHeapSupplementable is removed again.
     virtual void persistentHostHasBeenDestroyed() { }
 };
 
 template<typename T, bool>
 class SupplementableTracing;
 
-template<typename T>
-class SupplementableTracing<T, true> { };
+// We have a trace method in the specializations of SupplementableTracing 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.
 
 template<typename T>
-class SupplementableTracing<T, false> { };
+class SupplementableTracing<T, true> {

[sof, 2014/08/13 12:29:09]

What value does this templated class add now?

[zerny-chromium, 2014/08/13 13:07:01]

None really. I'll remove it. The reason it is/was there is because
SupplementableBase should be a GC mixin in the oilpan case, but that causes too
many plugin-check complications that depended on oilpan_enabled. So we removed
it again.

+public:
+    virtual void trace(Visitor*) { }
+};
+
+template<typename T>
+class SupplementableTracing<T, false> {
+public:
+    virtual void trace(Visitor*) { }
+};
 
 // 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));
@@ skipping 12 matching lines @@
         return this->m_supplements.get(key);
     }
 
     void reattachThread()
     {
 #if ENABLE(ASSERT)
         m_threadId = currentThread();
 #endif
     }
 
-    // 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("")
@@ skipping 80 matching lines @@
 };
 
 template<typename T>
 struct ThreadingTrait<blink::SupplementableBase<T, true> > {
     static const ThreadAffinity Affinity = ThreadingTrait<T>::Affinity;
 };
 
 } // namespace blink
 
 #endif // Supplementable_h