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

[Eli Zaretskii]

> Date: Mon, 30 Jan 2023 20:20:46 +0200
> Cc: casouri@gmail.com, 60953@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> On 30/01/2023 19:49, Eli Zaretskii wrote:
> >> Date: Mon, 30 Jan 2023 19:15:07 +0200
> >> Cc: casouri@gmail.com, 60953@debbugs.gnu.org
> >> From: Dmitry Gutov <dgutov@yandex.ru>
> >>
> >>> fast_looking_at already does an anchored match, so I'm not sure I
> >>> follow.  I don't even understand why you need th \` part, when the
> >>> match will either always start from the first position or fail.
> >>
> >> The regexp might include the anchors, or it might not.
> >>
> >> It might also use a different anchor like ^ or $ or \b.
> > 
> > OK, but it always goes only forward, so narrowing to the beginning
> > shouldn't be necessary.  Right? 
> 
> Are you saying that fast_looking_at ("\\`", ...) will always succeed?
> 
> And fast_looking_at ("^", ...), etc.

For example, for "^", if you hint that it must look back to make sure
there's a newline there, then your narrowing will also prevent it from
doing that, right?

> >> One possible alternative, I suppose, would be to create a raw pointer to
> >> a part of the buffer text and call re_search directly specifying the
> >> known length of the node in bytes. If buffer text is one contiguous
> >> region in memory, that is.
> > 
> > It isn't, though: there's the gap.  Which is why doing this is not
> > recommended; instead, use something like search_buffer_re, which
> > already handles this complication for you.  (Except that
> > search_buffer_re is a static function, so only code in search.c can
> > use it.  So you'd need to make it non-static.)
> 
> Interesting. Does search_buffer_re match the \` anchor at POS and \' at 
> LIM? IOW, does in treat the rest of the buffer as non-existing? Or could it?

That is the low-level subroutine called by re-search-forward, so you
know the answers already, I think?  IOW, that function behaves exactly
like re-search-forward in those situations.