Re: C++ PATCH for c++/61687 (extra errors with -O2)

[Jan Hubicka <hubicka at ucw dot cz>]

``
> >Given my experience about numbers of functions that become reachable when you stream all virtuals into LTO,
> >I wonder if we don't want to use possible_polymorphic_call_targets within the front-end to avoid instantiating
> >those that can't be called?
> 
> Yes, I think we need to do that.  I'll look into it.
> 
> >I think it should not be too hard - all we need is to populate the type inheritance graph from FE and then
> >for each polymorphic call produce the list to mark possible targets are reachable.
> 
> Hmm, do you think it's reasonable to call
> build_type_inheritance_graph from the FE?  The FE doesn't currently
> track all types derived from a particular base.

ipa-deivrt has code to do the tracking for you. All is needed is to cal get_odr_type
for all polymorphic type that we care about.  build_type_inheritance_graph just
walks all virtual methods to register all polymorphic types that matters (i.e.
have methods associated with them) into the datastructure.  I think within C++
FE we can just reigster all polymoprhic types we produce.

I see that during the reachability walk new types may be instantiated. This may
extend target lists earlier visited, I guess we need to add some way to make
to track this?

Honza
> 
> Jason