bug#22884: 25.0.92; C/l mode editing takes waaaayy too long

[Alan Mackenzie]

Hello, Eli.

On Thu, Mar 03, 2016 at 10:40:42PM +0200, Eli Zaretskii wrote:
> > Date: Thu, 3 Mar 2016 12:49:10 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: 22884@debbugs.gnu.org

> > > emacs -Q config.h
> > > M-x goto-line RET 1661 RET / /

> > > On my six-year-old desktop the second '/' takes about 10 seconds to 
> > > echo.  This sort of thing makes Emacs effectively unusable for editing 
> > > config.h.

> > The problem is in config.h.  At line 14, inside a comment, appears the
> > following string:
> >     "(at your option) any later version."
> > .  The open paren is at column zero, so the fancy code in syntax.c then
> > fails to recognise the comment as a comment.

> I think we should change syntax.c to fix this.

In particular, get syntax.c to recognise open parens in column zero as
defun starts _ONLY_ when they're not in strings or comments.  This will
clearly impact performance to some (unknown) degree.

> (Alternatively, CC mode could stop depending on it, but I doubt this is
> a viable alternative.)

This was tried for some time, by internally binding
open-paren-in-column-0-is-defun-start to nil.  It cost too much in
performance on large files (ask Martin!).

> We cannot really tell people we don't support such files, as they are
> valid C code.

I agree.

-- 
Alan Mackenzie (Nuremberg, Germany).