bug#66117: 30.0.50; `find-buffer-visiting' is slow when opening large number of buffers

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, dmitry@gutov.dev,
>  66117@debbugs.gnu.org
> Date: Tue, 12 Dec 2023 16:33:05 +0000
> 
> - Then, `set_default_internal', for any variable with positive
>   buffer_local_flags loops over all the live buffers to overwrite the
>   buffer-local value.
> 
>   This loop is the bottleneck, making let-binding
>   of `case-fold-search' (and any other variable marked in buffer.c:4691
>   block) scale with the number of open buffers.
> 
> - What I propose is to avoid this loop in `do_specbind' altogether.
>   Instead of having to loop through all the buffers to set temporary
>   buffer-local value, I propose to introduce special buffer object
>   `buffer_overrides' that will hold such temporary bindings.
>   Then, we can change BVAR to something like
> 
>   #define BVAR(buf, field) (buffer_overrides->field ## _ == UNSET ?\
>                               (buf)->field ## _ :\
>                              buffer_overrides->field ## _)
> 
>   and replace the loop with simply setting buffer_overrides slot.

So for starters, we make each BVAR more expensive, i.e. make Emacs
uniformly slower (because we call BVAR all over the place).  And then
I'm not sure I understand how buffer_overrides will get the values of
those temporary bindings without having to use the same loops when we
have let-binding within another one.  Also, what about threads?