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

[Dmitry Gutov]

On 01.08.2022 01:23, Gregory Heytings wrote:
> > > Yes, I wasn't clear enough, I should have written "when the file is 
> > > large enough (say 1 GB), even without long lines".
> >
> > What kind of scenario are you thinking of that would exhibit this 
> > slowness in a 1 GB file?
> >
> > If we're talking about syntax-ppss only, regular editing operations 
> > (typing and deleting code) limited to, say, one screen should trigger 
> > only a rescan of a limited area inside such buffer.
>
> That's true, but with such big files, the initial scan is slow.  So the 
> scenario is simple: you open a big enough file, type M->, and C-p.  M-> 
> will be instantaneous, and C-p will take a while, because of syntax-ppss.

Yeah, ok. If you are going to visit EOB, a single full scan seems 
unavoidable. I don't think 'M->' should be instantaneous, though: to 
display the last page, you need to fontify it, and font-lock depends on 
syntax-ppss.

But one big slow scan (and how slow it is actually depends on a 
particular major mode) followed by responsive editing sounds much better 
than what we've had before.

So I would recommend against trying to solve this part right now. And 
yes, some faster approximations of syntax-propertize-rules are possible, 
especially if we ask individual language modes to provide "simpler" 
syntax rules.