Majority of java methods do not have generic signature. For those methods, ConstMethod::generic_signature_index is always 0. ConstMethod::generic_signature_index can be optional and only allocated for methods with generic signature.
The Method class has a 2-byte unused space due to padding. Method::_max_stack can be moved to ConstMethod after changing ConstMethod::generic_signature_index to be optionally allocated. This removes an u2 field in Method, and can avoid wasting 2-byte unused space due to padding.
Together, above would save 4-bytes for method without generic signature.