bug#38407: 27.0.50; infinite loop with display of large file without newlines

[Eli Zaretskii]

> Cc: Eli Zaretskii <eliz@gnu.org>, 38407@debbugs.gnu.org
> From: Phil Sainty <psainty@orcon.net.nz>
> Date: Sun, 1 Dec 2019 23:37:46 +1300
> 
> On 1/12/19 8:23 PM, Pieter van Oostrum wrote:
> > Here it is, compressed. I am loading the uncompressed file,
> > of course.
> 
> Without bidi-inhibit-bpa, visual-line-mode is certainly brutal
> with this file (I've been waiting several minutes for Emacs 27
> to respond to `end-of-buffer').

8.5 minutes here, with an unoptimized build.

> However, for me, bidi-inhibit-bpa comprehensively deals to that.

Down to 1 min here.

One other observation is that the first time you do that in a session
after visiting the file, it takes much longer than the subsequent
times.  Here, the second M-> takes 5 min with bidi-inhibit-bpa nil and
just 17 sec with it non-nil.

So I think some of this time is spent doing something we do once,
maybe loading fonts or something (this file has a lot of different
scripts).

It would be interesting to run perf and compare its output with
bidi-inhibit-bpa nil and non-nil.  Maybe the performance hit due to
visual-line-mode is because of some code that could be optimized, or
maybe (gasp!) even a bug.  The difference between the first and
subsequent moves is also interesting to time.

I don't have access to a GUI system where I can run perf, so could
someone please do these measurements and post the resulting profiles?