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

Source/platform/Supplementable.h

Index: Source/platform/Supplementable.h
diff --git a/Source/platform/Supplementable.h b/Source/platform/Supplementable.h
index 650a204bd57cbbee12b7ebf1ad25f73e6e3b5791..31e776fa79c035b7c0273b23436d0a7eb6f60234 100644
--- a/Source/platform/Supplementable.h
+++ b/Source/platform/Supplementable.h
@@ -119,6 +119,7 @@ template<>
 class SupplementTracing<false> {
 public:
     virtual ~SupplementTracing() { }
+    virtual void trace(Visitor*) { }
 };
 
 template<typename T, bool isGarbageCollected = false>
@@ -143,7 +144,6 @@ public:
         return host ? host->requireSupplement(key) : 0;
     }
 
-    virtual void trace(Visitor*) { }
     virtual void willBeDestroyed() { }
 
     // FIXME: Oilpan: Remove this callback once PersistentHeapSupplementable is removed again.
@@ -153,11 +153,22 @@ public:
 template<typename T, bool>
 class SupplementableTracing;
 
+// 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, true> { };
+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> { };
+class SupplementableTracing<T, false> {
+public:
+    virtual void trace(Visitor*) { }
+};
 
 // Helper class for implementing Supplementable, HeapSupplementable, and
 // PersistentHeapSupplementable.
@@ -190,12 +201,6 @@ public:
 #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;