bug#56682: Fix the long lines font locking related slowdowns

[Dmitry Gutov]

On 15.08.2022 21:28, Eli Zaretskii wrote:
> > Date: Mon, 15 Aug 2022 21:17:23 +0300
> > Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> > From: Dmitry Gutov <dgutov@yandex.ru>
> >
> > > > I'm pretty sure that doing all that in Lisp is a good thing, and would
> > > > be conducive to whatever improvements we might want to add later...
> > >
> > > No, because quite a few packages use jit-lock-register to run stuff
> > > that is not font-lock at all, and I don't want that to slow down
> > > redisplay when long lines are involved.  And trying various values of
> > > a variable exposed to Lisp is easy enough.
> >
> > It should also be possible to move the whole invocation of the
> > fontification-functions up to Lisp
>
> How do you do that, given that these functions should be called from
> redisplay with the region of buffer text only redisplay knows about?

Something like in the attached patch.

> > Another reason I chose font-lock-fontify-region, though, is that it's
> > called from different places. It's the common entry point for font-lock.
>
> I have no problem with doing something similar in font-lock, I'm just
> saying that it cannot be the only place.

If we apply narrowing in handle_fontified_prop (or like in the 
attachment), and also in font-lock-fontify-region, it seems like every 
significant (*) execution path will go through two repetitions of the 
same logic.

That's not ideal.

(*) Significant by the metric of having impact on Emacs's performance, 
in large files especially.

> > I've never seen fontification-functions contain anything but
> > jit-lock-function, but jit-lock-functions is used by nlinum, for
> > instance. And bug-reference-mode, and goto-address-mode. Perhaps we
> > should ask Stefan's opinion on this.
>
> Stefan was and is on the CC list for the entire discussion.
>
> > And nlinum might not appreciate being narrowed. It seems to be working
> > okay without that in my 20 MB XML file. And in 200 MB one too.
>
> Why is it important what nlinum does, when we have native line numbers
> nowadays?

Just an example of a mode which uses jit-lock-functions. With an 
implication that other, less known modes, might have similar needs and 
behavior. There aren't too many known examples.

run-fontification-functions.diff
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