bug#42490: Emacs is very slow when navigating into a specific C++ file

[Olivier Scalbert]

Hello Alan,

Ping reply !
;-)

I was sure I had already answered. Sorry for that. With a fresh compile 
of Emacs 27.1, it is effectively faster than before. So, for me, you can 
close the bug. Many thanks ! Regards, Olivier



On 9/21/20 11:06 AM, Alan Mackenzie wrote:
> Hello, Olivier.
>
> Ping?
>
> On Fri, Jul 24, 2020 at 21:34:51 +0200, Olivier Scalbert wrote:
> > Hi,
> > Thanks for your answer.
> > Unfortunately, I am out of my home, with no PC, for the week-end.
> > If I'll survive, I will test it next Monday. Very sorry !
> > ;-)
> Emacs 27.1 was released just a few weeks ago.  Maybe you're already
> using it.
>
> I think CC Mode processes the test file fast enough on 27.1, so it's
> probably time to close this bug.  What do you say?
>
> > Regards,
> > Olivier
>
>
>
>
>
>
> > On 7/24/20 9:24 PM, Alan Mackenzie wrote:
> > > Hello, Mattias and Olivier.
> > > Firstly Olivier, thanks for taking the trouble to report the bug.
> > > On Fri, Jul 24, 2020 at 18:46:45 +0200, Mattias Engdegård wrote:
> > > > Hello Olivier,
> > > > Thanks for the report! Could you try Emacs 27 (or git master), building
> > > > from source if necessary? Those versions should be slightly faster,
> > > > although the response time is probably well below acceptable.
> > > > If we distill the essentials of your file to some sort of benchmark, we
> > > > might end up with:
> > > > (with-temp-buffer
> > > >     (c++-mode)
> > > >     (dotimes (_ 1000)
> > > >       (insert "OP(ed,b0) { ldir(); } /* LDIR */\n"))
> > > >     (garbage-collect)
> > > >     (let ((t0 (current-time)))
> > > >       (font-lock-ensure (point-min) (point-max))
> > > >       (time-to-seconds (time-since t0))))
> > > > Emacs 26.3 runs it in 11.9 s on this old lappy, but Emacs 27 does it in
> > > > 3.3 s. This is a clear improvement but we should be able to do better.
> > > > Alan may have a feeling for where the cycles are spent.
> > > I've bisected CC Mode to find the critical change, and it is:
> > > commit cc80eeb4a43d2079963de3d181002a6a6b56560d
> > > Author: Alan Mackenzie <acm@muc.de>
> > > Date:   Fri Apr 12 20:07:03 2019 +0000
> > >       Analyze C++ method with & or && ref-qualifier as defun, not brace list
> > >       Also firm up detection of beginning of brace list in
> > >       c-looking-at-or-maybe-in-bracelist.
> > > I have a simple benchmark which scrolls through a file, fontifying it,
> > > and my results from this benchmark are:
> > > (i) Before applying that patch: 53.022s.
> > > (ii) After applying that patch:  7.039s.
> > > I don't understand at the moment why that patch sped up scrolling in your
> > > (Olivier's) file, but it would seem the patch is most desirable.
> > > Unfortunately, the patch won't apply cleanly to the Emacs 26.3 sources.
> > > It might be possible to find a sequence of patches which would do the
> > > job.  I think (though I haven't checked) the patch will have been
> > > included in the upcoming Emacs 27.1 release.