> 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.
Yep, this can apply only to anonymous namespace types where we know the derivations. So it is not a big win. > > >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? Less we have in the vtables, the better, so if you can implement this, it would be great. Honza > > Jason
