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: dmitry@gutov.dev, 66117@debbugs.gnu.org
> Date: Tue, 12 Dec 2023 14:24:35 +0000
> 
> Ihor Radchenko <yantar92@posteo.net> writes:
> 
> > Ihor Radchenko <yantar92@posteo.net> writes:
> >
> >> The main contributor is set_default_internal. I _suspect_ (because we
> >> have non-linear scaling with the number of buffers - 3x increase in the
> >> number of files lead to ~6x increase in the run time) that the main
> >> contributor here is let-binding `case-fold-search' - special per-buffer
> >> variable with no default value.
> >
> > My suspicion appears to be correct - removing let-binding for
> > case-fold-search from `abbreviate-file-name' and
> > `inhibit-local-variables-p' made them disappear from the top of the
> > reverse call tree.
> >
> > Then, considering the ubiquity of let-binding case-fold-search across
> > Elisp code, speeding up let-binding of special per-buffer variables may
> > give us performance boost across Emacs.
> 
> What do you think about introducing something like
> 
> struct buffer buffer_overrides;
> 
> It will be similar to buffer_defaults  in buffer.c, but will hold the
> overriding values of buffer-local variables that are to be used instead
> of proper buffer-local variables. That way, we can speed up let-binding
> of case-fold-search.

I'm sorry, I don't think I understand how this will work and avoid the
problems we have with let-binding.  So I added Stefan, in the hope
that he might understand what I am evidently missing.