bug#15876: 24.3.50; Highly degraded performance between rev 114715 and 115006

[Eli Zaretskii]

> Date: Wed, 11 Dec 2013 22:00:48 +0400
> From: Dmitry Antipov <dmantipov@yandex.ru>
> CC: 15876@debbugs.gnu.org, sva-news@mygooglest.com
> 
> > Anyway, what about the patch below?  With it, the problem disappears
> > even without your "threshold" based GC.
> 
> Looks good. But can we assume that FACE_FROM_ID (...) is always non-NULL?
> Some important routines, like mark_face_cache, do not rely on this.

I think you are right, and we should make sure it's non-NULL before
dereferencing.

Jan, Ken'ichi, would you please comment on this?  Are we losing
something by reusing already loaded fonts that support a given
character, as opposed to looking for the "best-match" font?