bug#64735: 29.0.92; find invocations are ~15x slower because of ignores

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: Michael Albinus <michael.albinus@gmx.de>, dmitry@gutov.dev,
>  64735@debbugs.gnu.org, sbaugh@janestreet.com
> Date: Fri, 21 Jul 2023 16:15:41 +0000
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > The figures provided in this thread indicate speedups that are modest
> > at best, so I'm not sure they justify measures which could cause
> > problems (if that indeed could happen).
> 
> Not that modest. Basically, it all depends on how frequently Emacs file API is
> being used. If we take `find-lisp-find-files', which triggers more file
> handler lookup, the difference becomes more significant:
> 
> (benchmark-run-compiled 1 (find-lisp-find-files "/home/yantar92/.data" ""))
> ;; (3.853305824 4 0.9142656910000007)
> (let (file-name-handler-alist) (benchmark-run-compiled 1 
> (find-lisp-find-files "/home/yantar92/.data" "")))
> ;; (1.545292093 4 0.9098995830000014)

The above just means that find-lisp is not a good way of emulating
Find in Emacs.  It is no accident that it is not used too much.

> In particular, `expand-file-name' is commonly used in the wild to ensure
> that a given path is full. For a single file, it may not add much
> overheads, but it is so common that I believe that it would be worth it
> to make even relatively small improvements in performance.

The Right Way of avoiding unnecessary calls to expand-file-name is to
program dedicated primitives that perform more specialized jobs,
instead of calling existing primitives in some higher-level code.
Then you can avoid these calls altogether once you know that the input
file names are already in absolute form.

IOW, if a specific job, when implemented in Lisp, is not performant
enough, it means implementing it that way is not a good idea.

Disabling file-name-handlers is the wrong way to solve these
performance problems.

> I am pretty sure that file name handlers are checked behind the scenes
> by many other common operations.

I'm pretty sure they aren't.  But every file-related primitive calls
expand-file-name (it must, by virtue of the Emacs paradigm whereby
each buffer "lives" in a different directory), and that's what you
see, by and large.