Performance under bidi-paragraph-direction 'left-to-right vs bidi-display-reordering nil

[Phil Sainty]

As discussed in
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00294.html
setting bidi-display-reordering nil isn't a supported/tested scenario,
and in so-long.el we changed that to set bidi-paragraph-direction to
'left-to-right instead.

I've just traced what seemed like a performance regression to this
change -- it's now apparent to me that there's a noticeable effect on
the redisplay performance having bidi-display-reordering enabled and
bidi-paragraph-direction set (at least under long-line situations)
which isn't experienced when bidi-display-reordering is disabled.

(Which doesn't seem entirely unexpected when stated like that.  I
think I'd simply been under a misapprehension that the change we'd
made would have more similar performance characteristics than is
actually the case.  I failed to notice anything at the time, because
I'd forgotten that I'd (setq-default bidi-display-reordering nil) in
my own config after bug 23801 -- believing back then that this was
a supported setting.)


As before, the JSON file from https://emacs.stackexchange.com/q/598
serves as an example:

wget
https://github.com/Wilfred/ReVo-utilities/blob/a4bdc40dd2656c496defc461fc19c403c8306d9f/revo-export/dictionary.json?raw=true
-O one_line.json

With global-so-long-mode enabled visiting one_line.json does not hang
Emacs; but for me there is a very noticeable 'lag' to interactive
movements around the early parts of that buffer which disappears
completely if I set bidi-display-reordering to nil.

I do imagine that the code already tries hard to be efficient, but if
there's any chance that with only bidi-paragraph-direction set we can
achieve performance nearer to that with bidi-display-reordering
disabled, that would seemingly be beneficial for files with extremely
long lines.

Failing that, it makes me wonder whether a nil bidi-display-reordering
should be under consideration as a supported state for end users, for
the performance benefits.


-Phil