bug#69385: 30.0.50; Long lines with bidi text slow down Emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: 69385@debbugs.gnu.org
> Date: Mon, 26 Feb 2024 15:13:38 +0100
> 
> On Mon, 26 Feb 2024 14:33:18 +0200 Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > The CPU is fast.  My HarfBuzz version is much older, but I doubt that
> > HarfBuzz became so much slower for Arabic.  So it's very strange.  But
> > maybe it's worth to try downgrading to an earlier version, just to be
> > sure this is not the culprit?
> 
> That's not so straightforward on this system, where I build system
> packages from source, so downgrading HarfBuzz may require also
> rebuilding packages that depend on it.  I have a somewhat older system
> on this machine with HarfBuzz 7.1.0, so I can easily try that.  What
> version of HarfBuzz are you using?

I have 2.4.0 here.

> > If, after starting Emacs, you set all the entries of Arabic characters
> > in composition-function-table to nil (e.g., using
> > set-char-table-range), do you see significant speedup?
> 
> Yes, there is a noticeable speedup when M-> and C-v, M-v, and even
> holding down C-n and C-p is faster, though they still freeze after a few
> seconds, but the freeze doesn't last as long as when it happens with the
> shaped Arabic script.  Also, probably unsurprisingly, the speed after
> disabling shaping is still much slower than keeping shaping enabled but
> setting bidi-display-reordering to nil.

Is this with text from HELLO, or with some other Arabic text?  The
prominent effect of bidi-display-reordering is actually surprising,
and not what I see here, with or without shaping.  Very strange.