Re: Reliable after-change-functions (via: Using incremental parsing in Emacs)

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org, casouri@gmail.com, monnier@iro.umontreal.ca,
>  akrl@sdf.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 2 Apr 2020 00:48:20 +0300
> 
> >> No 'goto-char'. As we've established, it only affects the time taken by
> >> redisplay, and I can't measure that. So I'm not profiling it either,
> >> otherwise I'd be comparing apples to oranges.
> > 
> > See the second profile below.
> 
> Comparing both, looks like redisplay (when at eob, at least) takes 
> approx. the same amount of time?

About 55% taken by redisplay (almost all of it due to fontification),
and the other 45% are the C mode "preprocessing" when the mode is
turned on in a buffer.

> >> Yes. The numbers can be different, but there is definitely some up-front
> >> computation there. One that's not present with e.g. js-mode.
> > 
> > So you are saying that we should do that up-front computation just
> > because CC mode currently does it?  That we shouldn't try to eliminate
> > such preprocessing?  I don't think so.
> 
> AFAIU CC Mode could actually eliminate it, but that would require a 
> significant rework of its internals.

Are we still talking about integrating a completely different parsing
engine into CC Mode?  Then redesign is a must, right?

> I'm just pointing out that apparently you didn't even notice an even 
> larger delay (1.7s), and were fine with it until now.

I didn't "didn't notice", I actually filed several bug reports and
complaints about the various slow aspects of CC mode, because the
slowdown in CC mode over the years annoys me quite a lot.  Some of the
problems were fixed, some weren't (due to limitations of the current
design, I was told).  I'm not at all complacent about this.

> I'm not saying that nobody should try to explore how to decrease the 
> delay, and what tradeoffs come with that. But for now, I think, we 
> should encourage our kind volunteers to just implement integration the 
> way TreeSitter's authors expect it. And try, on our side, to provide the 
> best tools for it. Then we can see how well it does or doesn't work, and 
> what are the biggest annoyances that the users have with it.

I cannot tell the volunteers what to do and where to invest their
resources.  But I can provide feedback on the design ideas, based on
what I know and on my experience, and I can suggest how to design and
implement this to achieve good and scalable performance.  In
particular, I think that it is useful to know what we have tried in
the past and what were the lessons we learned from that.  I hope what
I say is of some help, and I hope we will soon have such engine
available to Emacs.