bug#48734: 28.0.50; Performance regression in `string-width`?

[Eli Zaretskii <eliz@gnu.org>]

> From: Imran Khan <contact@imrankhan.live>
> Date: Sun, 30 May 2021 02:45:57 +0600
> 
> A package I use (deft-mode) has been hanging for minutes with high cpu
> use recently. Profiler says most time is spent in `string-width`, and
> upon looking it seems to happen in files that have multibyte characters
> in them.
> 
> I reproduced the problem by creating a file that has both single and
> multi byte characters:
> 
> with open("/tmp/test", "w") as f:
>     for i in range(50_000):
>         print("1", file=f, end="")
>     print("α", file=f, end="")
> 
> And now:
> 
> (benchmark-run 1
>   (let ((str))
>     (with-temp-buffer
>       (insert-file-contents-literally "/tmp/test")
>       (setq str (buffer-string)))
>     (string-width str)))
> 
> This takes 20 seconds in my machine (if string is exclusively full of
> either single or multibyte characters, weirdly it seems to finish
> instantly).

Since you use insert-file-contents-literally, why don't you also make
the temporary buffer unibyte?  That is:

  (benchmark-run 1
    (let ((str))
      (with-temp-buffer
	(set-buffer-multibyte nil)  ; <<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	(insert-file-contents-literally "/tmp/test")
	(setq str (buffer-string)))
      (string-width str)))

Or maybe I don't understand your real-life use case?  Because if you
treat the file as a raw bytestream, why do you need to compute the
width of its text?