Re: Major modes using `widen' is a good, even essential, programming practice.

[Dmitry Gutov]

On 07.08.2022 21:00, Eli Zaretskii wrote:
> > Date: Sun, 7 Aug 2022 20:53:58 +0300
> > Cc:gregory@heytings.org,emacs-devel@gnu.org
> > From: Dmitry Gutov<dgutov@yandex.ru>
> >
> > On 07.08.2022 20:23, Eli Zaretskii wrote:
> > > > What jit-lock/font-lock actually do at the moment is to widen, then use
> > > > syntax-ppss, i.e. in effect scan from BOB.
> > > Yes, and that's SLOOOWWWW!
> > It's not slow on files of reasonable size. E.g. even if we take xdisp.c
> > (the file I've seen referred to in complaints on CC Mode's speed),
> >
> >     (benchmark 1 '(save-excursion (parse-partial-sexp (point-min)
> > (point-max))))
> >
> > reports 20-50 ms on my machine.
> It takes 0.5 sec here.

Interesting. That would mean that scrolling to the end of xdisp.c will 
at least take this long the first time around.

The more than 10x difference in performance is weird, though. Perhaps 
you have any ideas why parse-partial-sexp's implementation might behave 
poorly on your un-optimized build? Some tweak in the code might make a 
lot of difference.

This is an area I can't be of much help with, unfortunately.