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

[Dmitry Gutov <dgutov@yandex.ru>]

On 01/02/2023 15:10, Eli Zaretskii wrote:
>> Date: Tue, 31 Jan 2023 20:16:01 +0200
>> Cc: casouri@gmail.com, 60953@debbugs.gnu.org
>> From: Dmitry Gutov <dgutov@yandex.ru>
>>
>>> Can you describe what that function should do?  I don't think I have a
>>> clear idea of that.
>>
>> In Lisp that function could be implemented as
>>
>>     (defun buffer-substring-match (regexp &optional start end
>>                                    inhibit-modify)
>>       (string-match regexp
>>                     (buffer-substring (or start (point-min))
>>                                       (or end (point-max)))
>>                     inhibit-modify))
>>
>> Meaning, it matches the regexp against the buffer substring, with the
>> string-start and string-end anchors working.
>>
>> But it would be implemented in C, meaning we could avoid the extra
>> consing and funcall overhead.
> 
> Now I'm confused, because I thought the C functions we were
> considering all fit the above description.

Except they don't match \` and \' at START and END. Only at actual 
point-min and point-max.

I suppose the new function could be implemented with narrowing as well. 
If we decide it's the best method.

>> Anyway, it seems like it might be too late as an addition to Emacs 29.
>> And we can implement the match predicate using narrowing for this
>> release, to be updated later.
> 
> Right, this is not a catastrophe, IMO.  The work on TS-based modes is
> just beginning.

I also don't see any particular slowdown from altering BEGV and ZV in C. 
So the current method might be the fastest possible anyway.