Re: emacs rendering comparisson between emacs23 and emacs26.3

[Alan Mackenzie]

Hello, Eli.

On Sat, Mar 28, 2020 at 10:09:59 +0300, Eli Zaretskii wrote:
> > From: Richard Stallman <address@hidden>
> > Cc: address@hidden, address@hidden, address@hidden
> > Date: Fri, 27 Mar 2020 22:48:38 -0400

[ .... ]

> I expect an average twofold slowdown due to bidirectional support.
> The OP indicates less than that (comparing Emacs 23 with Emacs 26), so
> it's reasonable, IMO, and probably means the CC Mode contribution is
> relatively minor.

> The tricky part is testing the bidirectional part and the CC Mode part
> separately, by loading the old Emacs-23 CC Mode files into later
> versions of Emacs.  Maybe Alan could help by providing a Lisp file
> which, when loaded, would load CC Mode from a given directory -- this
> should be run before visiting any C/C++ file.

Here you go:

(defvar loaded-cc-version nil)
(defun load-cc-version (dir)
  "Load the version of CC Mode contained in directory DIR."
  (interactive "D")
  (or (string-match "/$" dir)
      (setq dir (concat dir "/")))
  (let (fname)
    ;; `c-fallback-style' gets stuff pushed onto it by loading cc-vars.  So
    ;; clear it out first, after a quick and dirty sanity check.
    (if (file-exists-p (concat dir "cc-vars.elc"))
        (setq c-fallback-style nil))
    (mapc (lambda (f)
            (condition-case err
                (load-file (setq fname (concat dir "cc-" f ".elc")))
              (error (message "Couldn't load %s" fname))))
          '("defs" "vars" "fix" "engine" "cmds" "menus"
            "align" "styles" "awk" "fonts" "mode"
            "subword" "bytecomp" "compat" "guess"))
    (setq loaded-cc-version dir)))

You can actually run load-cc-version repeatedly within an Emacs session
(I frequently do), together with M-x c-mode (etc.) on the pertinent
buffers.  Very occasionally, there're problems with defvars when their
value have changed between versions, since they don't get reinitialised.

-- 
Alan Mackenzie (Nuremberg, Germany).