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Re: How the long-lines "optimisation" breaks font locking.

From: Alan Mackenzie
Subject: Re: How the long-lines "optimisation" breaks font locking.
Date: Thu, 4 Aug 2022 14:44:05 +0000

Hello, Eli.

On Thu, Aug 04, 2022 at 15:54:34 +0300, Eli Zaretskii wrote:
> > Date: Thu, 4 Aug 2022 10:44:05 +0000
> > Cc: emacs-devel@gnu.org, Gregory Heytings <gregory@heytings.org>
> > From: Alan Mackenzie <acm@muc.de>

> > Hello, Lars.

> > On Wed, Aug 03, 2022 at 21:03:56 +0200, Lars Ingebrigtsen wrote:
> > > Alan Mackenzie <acm@muc.de> writes:

> > > > I don't think it is improbable that a C++ hacker will create a long line
> > > > in a raw string.

> > > If you don't think that's improbable, then you should like the new
> > > optimisations a lot -- ....

> > These "optimisations" break CC Mode.

> They don't "break" it, they cause it occasionally produce inaccurate
> fontifications.  And only when otherwise Emacs becomes unusable.

> > > .... because if you, instead of a 10K line in that C++ file, inserted
> > > a 1M line, then Emacs would previously hang indefinitely, but with the
> > > optimisation, it doesn't.

> > Well I tried CC Mode with a 1,000,000 character raw string.  It was
> > indeed a bit sluggish but "hang indefinitely" is an exaggeration.

> Try it with a 20MB raw string, then.  And, for good measure, in an
> unoptimized build.  These are the cases we are trying to make
> workable.

No matter what criterion one deems "workable", there will be some size of
buffer, or line, which renders Emacs "unworkable".

> If all you are saying is that the default value of long-line-threshold
> is too low, we can definitely discuss a better value.

> > Having loaded the file in C++ Mode (without the spiking of
> > narrow-to-region and widen), it took 90 seconds for M-> (first time).

> And you consider that reasonable?

No, I don't.  I was just correcting Lars's mistaken assertion that such a
line would make Emacs "hang indefinitely".

[ .... ]

> > Doing these things in the current master branch indeed appeared to be
> > fast, but at one point an error in an after-change-function caused
> > after-change-functions to get set to nil, crashing CC Mode.

> If you can show a recipe for this problem, we will fix it.  This code
> is WIP, so some problems definitely remain and should be reported and
> fixed.

I'll see if I can recreate it.

Alan Mackenzie (Nuremberg, Germany).

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