Re: Use ODR for canonical types construction in LTO

[Jason Merrill <jason at redhat dot com>]

On Wed, Jun 19, 2019 at 2:47 PM Nathan Sidwell <nathan@acm.org> wrote:
>
> On 6/19/19 1:53 PM, Jan Hubicka wrote:
> >>>> -    ctype = CLASSTYPE_AS_BASE (ctype);
> >>>> +    {
> >>>> +      if (!tree_int_cst_equal (TYPE_SIZE (ctype),
> >>>> +                         TYPE_SIZE (CLASSTYPE_AS_BASE (ctype))))
> >>>> +        ctype = CLASSTYPE_AS_BASE (ctype);
> >>>> +    }
> >>>>      tree clobber = build_clobber (ctype);
> >>
> >> I have noticed we build a distinct as-base type in rather more cases than
> >> strictly necessary.  For instance when there's a member of reference type or
> >> we have a non-trivial dtor. (CLASSTYPE_NON_LAYOUT_POD_P gets set by a bunch
> >> of things that don't affect ABI layout)
> >
> > Avoiding the extra copies at first place would be great. In my
> > understanding the types differ by virtual bases and also by their size
> > since the fake types are not padded to multiply of their alignment.
> > I guess this can be tested ahead of producing the copy and saving some
> > memory...
> >
> > I am not sure if my C++ FE abilities are on par to implement this tough.
>
> I don't think it's simple to fix there, just unfortunate.  your
> understanding is correct, and I think your workaround will work.
> However, remember it's possible for T == CLASSTYPE_AS_BASE (T), so might
> be worth checking that before doing the size comparison?
>
> It'd be great to comment on why you're not just using classtype_as_base
> there.  I suppose I'm serializing this stuff too, with the same
> inefficiencies ...

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?

Attachment: as-base.txt
Description: Text document