bug#63040: 30.0.50; Performance of buf_bytepos_to_charpos when a buffer has large number of markers

[Eli Zaretskii <eliz@gnu.org>]

> From: Ihor Radchenko <yantar92@posteo.net>
> Date: Sun, 23 Apr 2023 19:41:40 +0000
> 
> When investigating `re-search-forward' performance in
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=58558 (bug#58558), I
> noticed that buf_bytepos_to_charpos is taking most of the CPU time,
> according to perf stats.
> 
> This was partially caused by `parse-sexp-lookup-properties', but even
> after working around the text property issue, buf_bytepos_to_charpos
> still shows up on top of the perf profile.
> 
> Since one of the apparent bottlenecks in buf_bytepos_to_charpos is
> 
> for (tail = BUF_MARKERS (b); tail; tail = tail->next)
> 
> which obviously scales with the number of markers in buffer, I decided
> to add a cut-off parameter, as in the attached patch (number 50 has no
> particular motivation underneath).
> 
> Surprisingly, this simple change reduced my Org agenda generation times
> from 20 seconds down to 3-4 seconds!
> 
> I am sure that my dumb approach is not the best way to improve the
> performance, but this place in buf_bytepos_to_charpos is clearly
> something that can be optimized.

Interesting.  Would it be possible to show the effect of different
values of the cut-off on the performance, so we could decide which
value to use?

Thanks.