bug#11545: 24.0.96-mac-2.92; Strange speed problem scrolling in C++ code

[Alan Mackenzie]

Hello, John.

On Mon, May 28, 2012 at 06:05:15PM -0500, John Wiegley wrote:
> >>>>> John Wiegley <jwiegley@gmail.com> writes:

> >>>>>> YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp> writes:
> >> With the Time Profiler in Instrument.app, I found that fontification
> >> of CC Mode in Emacs 24 is much heavier and causes frequent GCs than
> >> that in Emacs 23.  Please try the following:

> >>   1. Start Emacs 24 Mac port with -Q (alternatively, pressing the
> >>      shift key.)
> >>   2. M-x load-file PREFIX/share/emacs/23.4/lisp/progmodes/cc-fonts.elc
> >>      RTE.
> >>   3. Replay scrolling a large C++ file.

> >> I'm not sure if this slowdown is intended or expected.

> > Indeed, this makes the speed situation much better on Emacs 24.0.97.

> I can now confirm that loading CC-Mode 5.32.3 into Emacs 23.4 causes
> the identical speed issues that I was seeing with Emacs 24.0.97, so
> this is a performance issue in the latest CC-Mode, not a bug in Emacs
> 24 or with Mac-Port Emacs.

> To the CC-Mode maintainers: is there a way to disable the slower, "more
> correct" mode in the latest CC-Mode, and go back to the entirely
> sufficient (for me) mode of previous versions?

Not as such, no.  The only workaround at the moment is to use a
"pre-correct" version of CC Mode in place of an up to date one.

I've just done a binary chop on CC Mode versions, and it seems the latest
version before (?the first of) these enhancements was the one created by
this (mercurial) changeset (the repository can be downloaded from
<http://cc-mode.sourceforge.net>):

    changeset:   5109:981fa4f0270c
    parent:      5107:bd4013c5633b
    user:        acmacm
    date:        Wed Sep 15 17:47:52 2010 +0000
    files:       cc-engine.el
    description:
    (c-forward-<>-arglist-recur): Fix an infinite recursion.

To undo these changes would be difficult, since several later
enhancements and bug fixes are based on the new code.  I'll see if I can
find some way of optimising the offending code - most of the time it's
doing expensive checks and finding nothing.

Anyhow, I've got the problem flagged as a bug now.  Thanks again for
reporting it.

> Thanks,
>   John

-- 
Alan Mackenzie (Nuremberg, Germany).