bug#12867: 24.3.50; easy-to-repro crash involving mode line

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 12 Nov 2012 18:31:53 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: Drew Adams <drew.adams@oracle.com>, 12867@debbugs.gnu.org
> 
>  > The need to redisplay the mode line on every move of point is a killer
>  > of many redisplay optimizations.  It is also potentially expensive by
>  > itself, because it requires Emacs to count lines, something that is
>  > not an easy operation in Emacs, which sees the buffer text as a linear
>  > array of characters, not a series of lines.  So the display engine
>  > tries very hard to avoid redisplaying the mode line if it decides that
>  > the line number does not need to appear.
> 
> I think it would make sense to use a cache for maintaining line numbers.

We already do, but maybe not as extensively as you may have in mind.
See the base_line_pos member of 'struct window'.

There's also region-cache.c, which we could use for that.  But before
we do, Someone(TM) should time the thing and see if it's worth our
while to make that cache.