bug#60953: The :match predicate with large regexp in tree-sitter font-lock seems inefficient

[Eli Zaretskii]

> Date: Thu, 26 Jan 2023 21:35:55 +0200
> Cc: casouri@gmail.com, 60953@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > If you are saying that GC is responsible, then running the benchmark
> > with gc-cons-threshold set to most-positive-fixnum should produce a
> > more interesting profile and perhaps a more interesting comparison.
> 
> That really helps:
> 
> (benchmark-run 1000 (progn (font-lock-mode -1) (font-lock-mode 1) (let 
> (treesit--font-lock-fast-mode) (font-lock-ensure))))
> 
> => (16.078430587 251 5.784299419999996)
> 
> (let ((gc-cons-threshold most-positive-fixnum)) (benchmark-run 1000 
> (progn (font-lock-mode -1) (font-lock-mode 1) (let 
> (treesit--font-lock-fast-mode) (font-lock-ensure)))))
> 
> => (10.369389725 0 0.0)
> 
> Do you want a perf profile for the latter? It might not be very useful.

I'd be interested in comparing the profiles of the two techniques, the
:pred and the :match, with GC disabled like that.

> > (But I thought you concluded that GC alone cannot explain the
> > difference in performance?)
> 
> I'm inclined to think the difference is related to copying of the regexp 
> string, but whether the time is spent in actually copying it, or 
> scanning its copies for garbage later, it was harder to say. Seems like 
> it's the latter, though.

If we can avoid the copying, I think it's desirable in any case.  They
are constant regexps, aren't they?