On 03/11/2014 05:08 PM, Jan Hubicka wrote:
Jason, I was looking into this and I think I have patch that works. I would just like to verify I inderstnad things right. First thing I implemented is to consistently skip dtors of abstract classes as per the comment in abstract_class_dtor_p there is no way to call those by virtual table pointer. Unlike your patch it will i.e. enable better unreachable code removal since they will not appear in possible target lists of polymorphic calls.
Makes sense.
The second change I did is to move methods that are reachable only via abstract class into the part of list that is in construction, since obviously we do not have instances of these classes.
I'm not sure how you would tell that a method that is reachable only via abstract class; a derived class doesn't have to override methods other than the destructor, so we could get the abstract class method for an object of a derived class.
What I would like to verify with you shtat I also changed walk when looking for destructors to not consider types in construction. I believe there is no way to get destructor call via construction vtable as we always know the type. Is that right?
I guess it would be possible to get the abstract destructor via construction vtable if someone deletes the object while it's being constructed. But surely that's undefined behavior anyway.
also if abstract_class_dtor_p functions are never called via vtables, is there reason for C++ FE to put them there? I understand that there is a slot in vtable initializer for them, but things would go smoother if it was initialized to NULL or some other marker different from cxa_pure_virtual. Then gimple-fold will already substitute it for builtin_unreachable and they will get ignored during the ipa-devirt's walks.
Hmm, interesting idea. Shall I implement that? Jason
