bug#18530: 24.3; Emacs unable to handle very large lines

[Alexis]

Amit Barve writes:

> Here goes the description of the bug: Whenever I open a text file
> which has a very big line(a continuous line) without a newline inside
> it (let's say first 1000000 space separated numbers) then emacs is
> just unable to handle it.  Emacs is unable to show line count of
> buffer, instead it shows 'L??'  and general commands like 'move to end
> of file', 'move to start of line' take a lot of time (something like a
> complete minute) to work.I tried by removing everything from my
> '.emacs' file but that didn't help!

i've been able to reproduce this on both 24.3.3 and 24.3.93.2 (both
manually compiled on 64-bit Debian Wheezy running on an i5), having
created the utf-8 file test.txt with:

    for i in `seq 1 1000000`; do echo -n "$i " >> test.txt; done

then loading that file into an `emacs -Q` session. Moving from one end
of the buffer takes 15-20 seconds.

If i `toggle-truncate-lines`, the line count is correctly listed as 1,
but trying to move to the end of the line only results in displaying
complete numbers up to and including '194455', even though the column
number on the mode-line says that point is at the line's end. The
highest column number at which point is still visible is 1250082; after
that, moving point further towards the end of the line makes the cursor
no longer visible.

If i call `fill-paragraph` on that line, i get a line count of 98882,
and moving from one end of the buffer to the other only takes a second
or so.