Re: Major modes using `widen' is a good, even essential, programming practice.

[Eli Zaretskii]

> Date: Sun, 7 Aug 2022 19:20:44 +0000
> Cc: gregory@heytings.org, emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > > > Either in the cache or in the buffer: the previous chunk was
> > > > fontified, so its end has the font-lock-comment-face.  So you know.
> 
> > > No, you don't.  The buffer might be being opened by desktop in a large
> > > comment in the middle of the file.
> 
> > You've changed the scenario, yes?
> 
> Yes.  We've got to deal with all scenarios, preferably without
> special-caseing special cases.

No one said that all the scenarios must have the same solution.

> > > What jit-lock/font-lock actually do at the moment is to widen, then use
> > > syntax-ppss, i.e. in effect scan from BOB.
> 
> > Yes, and that's SLOOOWWWW!
> 
> On my machine, with an optimised build, it takes just under 20 ms to
> parse-partial-sexp over xdisp.c (not counting any redisplay at the end).
> I don't understand any more than Dmitry does, why your unoptimised build
> is taking 25 times as long.

It doesn't help to know that some very fast machine can do this stuff
quickly enough to remain below the annoyance threshold.  20 ms is a
very long time by the current CPU speed measure: just calculate the
number of CPU cycles in that time and you will see it.

> > > That "needing to go too far" is an instantaneous jump, not a scanning.
> 
> > Please tell that to someone who doesn't edit C sources as frequently
> > as I do.
> 
> Are you saying that long strings and long comments cause a particular
> slowdown in C Mode, not seen when strings and comments are all short?

I don't know what makes it slow, but it feels sluggish in even the
simplest editing operations, and font-lock updates are slow as well.

> > > The string start will be in a parse-partial-sexp result somewhere.
> > > Sometimes people write long strings.  They certainly write long comments.
> 
> > Why do I have top suffer every day just because someone, somewhere,
> > might do that?  I'd rather we "punish" those few people who do it
> > (rarely).
> 
> I don't think we should punish people who write comments.  I'm thinking
> of Gerd M., who was likely the writer of the comment at the beginning of
> xdisp.c.

We are still talking about long lines, yes?  There are no long lines
in that commentary at the beginning of xdisp.c.