The comment in memory/allocation.hpp says:

// Base class for objects used as value objects.
// Calling new or delete will result in fatal error.
//
// Portability note: Certain compilers (e.g. gcc) will
// always make classes bigger if it has a superclass, even
// if the superclass does not have any virtual methods or
// instance fields. The HotSpot implementation relies on this
// not to happen. So never make a ValueObj class a direct subclass
// of this object, but use the VALUE_OBJ_CLASS_SPEC class instead, e.g.,
// like this:
//
// class A VALUE_OBJ_CLASS_SPEC {
// ...
// }
//
// With gcc and possible other compilers the VALUE_OBJ_CLASS_SPEC can
// be defined as a an empty string "".
//

For modern GCCs, this might no longer be true. Should also check with LLVM.