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

[Dmitry Gutov]

On 02.04.2020 17:23, Eli Zaretskii wrote:

> > 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.

So, all in all, when xdisp.c is opened at eob, it will be displayed 
after ~2.5 seconds, I guess.

> > > 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?

No, that's without TreeSitter.

> > 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.

Still, compare that with 0.15 sec, which is the current estimate of 
parsing xdisp.c. It could probably be improved still by supporting a 
no-copy buffer-string in modules.

> 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.

We shouldn't, however, create an impression that unless they follow our 
ideas to a T we won't help them realize their own preferred approach 
(e.g. by improving the module API).

> 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.

I'm fairly confident that implementing deferred/on-demand parsing in 
emacs-tree-sitter can be done later without requiring a major redesign. 
It will require, however, an extra layer of complexity either way.