bug#67390: 28; shorthands-font-lock-shorthands assumes shorthand uses same separator

[Eli Zaretskii <eliz@gnu.org>]

> From: João Távora <joaotavora@gmail.com>
> Date: Sun, 26 Nov 2023 22:02:01 +0000
> Cc: Jonas Bernoulli <jonas@bernoul.li>, 67390@debbugs.gnu.org, 
> 	Adam Porter <adam@alphapapa.net>
> 
> On Sun, Nov 26, 2023 at 8:38 PM Joseph Turner <joseph@ushin.org> wrote:
> >
> > João Távora <joaotavora@gmail.com> writes:
> >
> > > On Sat, Nov 25, 2023 at 10:43 PM Joseph Turner <joseph@ushin.org> wrote:
> 
> > > So, benchmarking it will have to be, I'm afraid, because AFAIK
> > > font-locking is a very performance sensitive area of Emacs.
> >
> > Yes.  I would like to learn how to do this!
> 
> I'm CCing Eli.
> 
> In the past, ISTR, Eli suggested to benchmark such things by visiting a
> very large file in its beginning, then scrolling down by holding
> the down arrow or PgDn for some fixed time period, like 30 seconds.
> The  Emacs that scrolls the farthest is the most performant.  Not
> entirely fail-proof (other processes may interfere, etc), but not
> bad either.

I still recommend this method.  Something like the below:

  (defun scroll-up-benchmark ()
    (interactive)
    (let ((oldgc gcs-done)
	  (oldtime (float-time)))
      (condition-case nil (while t (scroll-up) (redisplay))
	(error (message "GCs: %d Elapsed time: %f seconds"
			(- gcs-done oldgc) (- (float-time) oldtime))))))

Evaluate the above, and the invoke it at the beginning of a large
file.  Then compare the timings with different font-lock arrangements.

A variant is to scroll by N lines, not by pages.  Just change the
above to call scroll-up with the argument of N, for example 1 (or any
other number, if you want).