Re: "Final" version of tty child frames

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: emacs-devel@gnu.org
> Date: Tue, 22 Oct 2024 12:23:04 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> >> Cc: emacs-devel@gnu.org
> >> Date: Tue, 22 Oct 2024 10:21:43 +0200
> >> 
> >> Thanks for taking a look.
> >
> > Can you explain this comment and the code change:
> >
> >   +         /* Struct frame can move with igc, and so on.  But we need
> >   +            something that takes different frames into account. Use the
> >   +            face_cache pointer for that which is malloc'd. */
> >   +         if (glyph->frame && glyph->frame != f)
> >   +           face_id += (ptrdiff_t) glyph->frame->face_cache;
> >
> > Why do we need to take the frame into account here?  Is this relevant
> > to the current GC used on master?
> 
> I can try.
> 
> A combined frame matrix's glyph contents may come from different frames,
> either the root frame one of its descendants (children, grandchildren
> and so on). See copy_child_glyphs. glyph->frame is th eframe from where
> the glyph stems. It is filled out when producing glyphs.
> 
> Face ids are valid only in frame's face cache. A Face with a given id in
> one frame may be different from the same id another frame. So that's why
> I'm taking the face cache into account here. Otherwise it could happen
> that row hashes are the same although the contents are different. Not a
> big deal, but one can avoid it, so I did.

Why not simply include the frame pointer in the hash?  Adding a
pointer to a number looks kludgey, and might even make the hash
weaker.