Re: Redisplay slower in Emacs 28 than Emacs 27

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: ghe@sdf.org,  emacs-devel@gnu.org
> Date: Tue, 08 Dec 2020 18:29:24 +0100
> 
> > I don't think I follow: since an image spec can (and usually does)
> > include a file name, how can we use EQ?  That would mean identical
> > file names will cause us to think an image isn't cached, right?  Or
> > what am I missing?
> 
> I'm not sure -- I don't quite understand what you mean here.
> 
> Here's a spec:
> 
> (setq spec '(image :type "xpm" :file "/tmp/foo.xpm"))
> 
> Every time spec `spec' is redisplayed, EQ (spec, spec) will be true.

Not if the list is re-assembled again, right?

> We'd be going by "object identity" of the spec, and not looking at the
> contents at all.
> 
> I.e., we'd start treating image specs as (immutable) objects (to make
> redisplay more efficient).

It may make redisplay faster, but it will produce confusing effects if
an identical spec is constructed from the same or identical elements:
Emacs will think the image is not in the cache.  Sounds like a lot to
pay for a minor speedup, to say nothing of backward compatibility.