bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> Date: Fri, 5 Aug 2022 22:01:24 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > First, we need to establish that indeed parse-partial-sexp is called
> > in that manner in the relevant major modes (not just one of them), or
> > by font-lock itself regardless of the mode.
> 
> It is called by font-lock itself, which ends up calling syntax-ppss, 
> which does its job with parse-partial-sexp.

In all modes, regardless of what the mode wants to highlight?

> > Second, we need to establish that indeed this takes a large portion of
> > the time in the slow operations.  Not just one particular operation,
> > but most or all of them.
> 
> To establish that, I have described the experiment in the grandparent 
> email (with scenarios 1,2a;1,2b;1,2a,2b), and performed it myself as well.
> 
> But I'm talking about the slowdown observed when doing 'M->'. Not about 
> any operations one might try to perform. Having said that, after the 
> initial 'M->' most of navigation operations look snappy to me. So that's 
> the slowdown I decided to investigate.

We need to look at more than just M->.  C-n/C-p, C-v/M-v, C-l are also
important, as are the time it takes from typing M-x or M-: until you
see the prompt in the minibuffer, and the time to update the display
after inserting or deleting a single character.

> All major modes we can currently use for JSON (the built-in js-mode and 
> the two json-mode's in ELPA) inherit the value of 
> syntax-propertize-function from js-mode. But there's no need for it: 
> JSON doesn't have division, or regexps, or preprocessor directives, or 
> embedded JSX structures.
> 
> Setting syntax-propertize-function to nil speeds up parse-partial-sexp 
> significantly.

Thanks, so I guess we may have a solution for JSON files, if disabling
syntax-propertize-function doesn't have any downsides.  What about
other modes that we see in files with long lines, like XML?

And how scalable is the solution you propose, i.e. how it behaves in
JSON files with a much longer lines?