bug#19846: 25.0.50; Problem with auto-fill-mode and C mode

[Alan Mackenzie]

Hello, Lars.

On Sun, May 01, 2022 at 12:08:58 +0200, Lars Ingebrigtsen wrote:
> martin rudalics <rudalics@gmx.at> writes:

> > Here the impact is felt immediately, probably because of the way I build
> > Emacs.  Inserting 72 spaces doesn't make much sense anyway, so this is
> > not what's bothering me.  But here inserting _any_ character with auto
> > filling turned on is a pain.  I'm usually typing 2 to 3 characters ahead
> > of what I see on screen.

> (I'm going through old bug reports that unfortunately weren't resolved
> at the time.)

> I tried reproducing this, but inserting characters normally in that doc
> string seems to be quite speedy.  It's not until I have about two
> hundred spaces at the start of the line that I start seeing hangs.

You've got a fast machine, perhaps.  :-)

> Are you still seeing these issues in recent Emacs versions?

I still see them.  What's more, I know exactly what's causing the hangs.

In forward-paragraph, a regular expression is formed by concatenating
the major mode's paragraph-start and paragraph-separate with a few other
things.  In CC Mode (at least), this gives rise to ill-formed regexps
with adjacent sub-expressions which each can match the empty string, or
arbitrary numbers of spaces.

When there fails to be a match, the regexp engine tries in every
possible way to find a match, and thus failing to find one takes a long
time.

I started work on a library to fix ill-formed regexps in 2015, and after
a long gap, recently re-started work on it.  Although there is still
quite a bit to do on it, I can see my way to finishing it.  This would
fix the cause of the bug properly.  Perhaps I may be able to make the
current state available on a git branch under scratch "soon".

So, can you leave this bug open, please?

> -- 
> (domestic pets only, the antidote for overdose, milk.)
>    bloggy blog: http://lars.ingebrigtsen.no

-- 
Alan Mackenzie (Nuremberg, Germany).