bug#59738: c-ts-mode is slow with large buffers.

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Wed, 7 Dec 2022 16:40:15 -0800
> Cc: Alan Mackenzie <acm@muc.de>,
>  59738@debbugs.gnu.org
> 
> > Right, but with a long series of #define lines there should be no
> > parse tree at all…
> 
> Ok, I think I know why. At the beginning of the file there is this line
> 
> #ifndef _dce_12_0_SH_MASK_HEADER
> 
> So it’s parsed into a preproc_ifdef node, which contains every #define 
> directive in the file as its immediate child. Now you have this node with a 
> tons of immediate children. And querying this node in font-lock is very slow, 
> even with a limited range. I think for the query result to be accurate, 
> tree-sitter has to query the whole node without considering the range, then 
> throw away matches that are not in the range. 
> 
> Anyway, I activated my backup backup plan, which goes down the parse tree to 
> find a sufficiently small node to query. Now scrolling the header file is 
> fast as other files.

Thanks, now c-ts-mode is twice as fast as c-mode with that file.

Great job!