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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 10 Aug 2022 20:02:20 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, stephen.berman@gmx.net,
>  monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> It's much worse. In the aforementioned dictionary.json, with 
> fully-featured font-lock (narrowing disabled in handle_fontified_props) 
> the buffer is perfectly responsive to edit as long as I (setq 
> bidi-inhibit-bpa nil). Even with show-paren-mode enabled.
> 
> Without that, however, simple navigation commands take a significant 
> amount of time. These measurements are with show-paren-mode off:
> 
> Near BOB:
> 
> (benchmark 1 '(next-line 1)) =>
> Elapsed time: 0.246551s
> Elapsed time: 0.247237s
> Elapsed time: 0.247392s
> 
> Near EOB:
> 
> (benchmark 1 '(next-line 1)) =>
> Elapsed time: 0.059583s
> Elapsed time: 0.040372s
> Elapsed time: 0.059508s

That's what I meant: it isn't a catastrophe.  1/4th of a second for
C-n is barely perceptible.

"Catastrophe" in my book is when it takes more than a couple of
seconds, like 10 sec or more.

> > not
> > like we've seen with the files we used for the long-lines speedups.
> > And if someone does think it's a catastrophe, they can always disable
> > the BPA, perhaps even globally,
> 
> That reminds me of "the user can always disable font-lock" and your 
> dismissal of Alan's advice to change font-lock-maximum-decoration to 2, 
> saying that we should have good editing performance OOTB.

No, because font-lock is much more important than the effect of the
BPA.  (Do you even understand what the BPA does, and did you ever see
it in action?  Without that, this part of the discussion is just waste
of time.)

> > if they know they will never need to
> > look closely at bidirectional text: the effects are hardly visible
> > unless you actually read the text.
> 
> We could add (setq bidi-inhibit-bpa nil) to prog-mode, I suppose.

Not to prog-mode, that I won't agree.  But maybe for JSON files, if we
think they are unlikely to suffer.

> It already has a bidi-paragraph-direction setting anyway.

That's not even similar.  That's just saving Emacs to figure that out
itself, wasting CPU cycles on what is known in advance.  Because a PL
buffer _always_ has its paragraphs left-to-right.

> > This is impossible without a complete redesign of how the bidi display
> > works: it saves no information in the buffer object, none whatsoever.
> > The information is recomputed every time the display code is called,
> > and only for the portion of the buffer that needs to be traversed or
> > displayed; then it is discarded.  There are no caches of any kind that
> > keep the information after redisplay has done its job.
> 
> Perhaps a cache similar to syntax-ppss one could do the job? Not sure 
> what would be the trafeoffs, though. Like, how that would change the 
> performance in the simple case.

There are no caches for a good reason: the Unicode Bidirectional
Algorithm that we implement results in very far-reaching non-local
effects of small changes.  Even inserting or deleting a single
character can dramatically change how the buffer looks on display very
far from the locus of the change.  Figuring out which changes
invalidate which caches is extremely non-trivial, and in many cases
requires the same amount of work as would take to just recompute
everything from scratch.