bug#18558: 24.4.50; Debugger slow with long strings

[Eli Zaretskii <eliz@gnu.org>]

> From: Marcin Borkowski <mbork@mbork.pl>
> Date: Fri, 19 Feb 2016 09:41:01 +0100
> Cc: 18558@debbugs.gnu.org
> 
> On 2014-09-25, at 12:16, Helmut Eller <eller.helmut@gmail.com> wrote:
> 
> > Moving the cursor in the debugger window is very slow in the following
> > example:
> >
> > shell> cat test.el
> > (progn
> >   (defun foo (n string)
> >     (cond ((= n 0) (debug))
> >           (t (foo (1- n) string))))
> >   (foo 30 (make-string 500000 ?\\)))
> >
> > shell> emacs -Q -nw -l test.el
> >
> > Now move the cursor around to see the slowness.
> >
> > Maybe the debugger should truncate long strings as it does with long
> > lists.
> 
> Confirmed on GNU Emacs 25.1.50.2 (commit 4ccd268).
> 
> I would guess that the problem is not in long string /per se/, but with
> lon lines in the *Backtrace* buffer, i.e. probably something connected
> with the display engine.  I guess this bug should either be closed or
> merged with some other one (one of the other ones, perhaps) dealing with
> efficiency problems with long lines.

Yes, please merge it.

Thanks.