[webkit-dev] When to use FastAllocBase?

For general reference, when is it appropriate to use FastAllocBase? If you subclass RefCounted<T> or Noncopyable, which is very common, you pick up FastAllocBase. So, my naive guess is that any class/struct which doesn't pick up FastAllocBase through its inheritance chain should subclass it directly. Is that a reasonable guideline? Thanks, -Tony