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

[Eli Zaretskii]

> Cc: 60953@debbugs.gnu.org
> Date: Wed, 25 Jan 2023 05:48:13 +0200
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > We can probably match the regexp in-place, just limit the match to the 
> > range of the node.
> 
> That's what I tried to do in the patch attached to the first message: 
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=60953#5
> 
> But the effect on performance was surprisingly hard to notice. It also 
> broke the actual highlighting, but that's probably because the regexp 
> uses anchors \` and \', which don't really work for fast_looking_at 
> calls inside a buffer.

The condition for a match in that patch is not correct, AFAIU:

   if (val >= 0)
     return true;
   else
     return false;

It should be "if (val > 0)" instead, since fast_looking_at returns the
number of characters that matched (unlike fast_string_match in the
original code, which returns the _index_ of the match).

Also, fast_string_match is capable of succeeding if the match begins
not at the first character, whereas fast_looking_at does an anchored
match.  Do we expect the text to match from its beginning in this
case?  If not, I think the replacement didn't do what the original
code does, and you should have used search_buffer or maybe
search_buffer_re instead.