Throughout hotspot code, except for [s390], the subject methods are called as Klass::vtable_start_offset() and Klass::vtable_length_offset(), whereas [s390] calls them via subclass InstanceKlass. This is not a bug, but a source of questions ("Why do they do that?"). There does not exist any specialized method in InstanceKlass for any supported architecture.

Direct all calls to Klass::vtable_start_offset() and Klass::vtable_length_offset().