> > This simple (untested) patch doesn't avoid creating the unnecessary
> > as-base types, but it should avoid using them in a way that causes
> > them to be streamed, and should let them be discarded by GC.
> > Thoughts?
>
> Looks better than Honzas patch fixing a single place.
I wonder if we can go ahead with Jason's patch to handle the common
case.
>
> I've spent some thoughts on this and I wonder whether we can
> re-implement classtype-as-base with fake inheritance (which would
> also solve the TBAA alias set issue in a natural way). That is,
> we'd lay out structs as-base and make instances of it use a
>
> class as-instance { as-base b; X pad1; Y pad2; };
>
> with either explicit padding fields or with implicit ones
> (I didn't check how we trick stor-layout to not pad the as-base
> type to its natural alignment...).
>
> I realize that this impacts all code building component-refs ontop
> of as-instance typed objects so this might rule out this approach
> completely - but maybe that's reasonably well abstracted into common
> code so only few places need adjustments.
Modulo the empty virtual bases which I have no understnading to I
suppose this should work.
One issue is that we will need to introduce view_convert_exprs at some
times.
As
class a var;
class b:a {} *bptr;
var.foo;
Expanding this as var.as_base_a.foo would make access path oracle to
disambiguate it from bptr->as_base_b->as_base_a.foo which is wrong with
gimple memory moel where we allow placement new replacing var by
instance of b.
So the way to generate it would be to first view convert expr *bptr to
as_base_b and continue from that. This would probably force us to not
give up at view converts in the access path disambiguation :)
Honza
