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

[Dmitry Gutov]

On 25/01/2023 05:13, Yuan Fu wrote:

> FYI the predicates are not processed by tree-sitter, but by us. For example, 
> the #equal predicate is handled by treesit_predicate_equal. For #match, right 
> now we create a string with Fbuffer_substring and pass it to fast_string_match, 
> so it definitely causes a lot of gc’s, as you observed.

Right.

> 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.

I also experimented with replacing the current 
buffer-substring+string-match-p scheme with looking-at. No difference in 
performance.

Reducing the size of the regexp, however, made a lot of difference. 
ruby-ts--builtin-methods is 721 characters long.

So my current hypothesis is that the extra GC is caused by copying the 
regexp string back and forth. Which seems a bit more difficult to avoid. 
But could be done if we replace that value with some indirection.