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

From: Stefan Monnier <[email-masked]>
Date: Thu, 14 Dec 2023 13:41:59 -0500

>>> It won't be buffer-local-if-changed, so binding it will not be costly.
>> As mentioned elsewhere in this bug-report we can make `case-fold-search`
>> into a `DEFVAR_LISP`: it would have no visible impact to ELisp and would
>> avoid the costly let-bindings.
> Wouldn't DEFVAR_LISP break major modes that do (setq case-fold-search ...)?

No, it makes no difference to Lisp.  It only changes the implementation
strategy: DEFVAR_LISP stores the value in a global C variable (which
thus requires more processing when the var is buffer-local, more
specifically updating the C var when we `set-buffer`), whereas
DEFVAR_PER_BUFFER stores the value inside the buffer object (which
thus requires more processing when the var is *not* buffer-local since
the global value is duplicated in all the buffers where it's not made
buffer-local).

DEFVAR_PER_BUFFER are thus the better option for vars which are set
buffer-locally in most buffers whereas DEFVAR_LISP is the better option
for vars which are made buffer-local only in a few buffers.

        Stefan