bug#55729: 27.1; Emacs 28.1 `string-match' is five times slower than Emacs 27.1

[Po Lu]

Chen Bin <chenbin.sh@gmail.com> writes:

> Run below code in both Emacs 28.1 and Emacs 27.1
>
> ;; Run "base64 /dev/urandom | head -c 3000000000 > 3g.txt" to create 3g.txt
> ;; Then run below code in Emacs,
> (setq content
>       (with-temp-buffer
>         (let ((coding-system-for-read 'utf-8-unix))
>           (insert-file-contents "3g.txt"))
>         (buffer-string)))
>
> (message "content length=%s" (length content))
> (let* ((gc-cons-threshold most-positive-fixnum))
>   (message "%s vs %s"
>            (benchmark-run-compiled 1
>              (string-match "aaaaa" content))
>            (benchmark-run-compiled 1
>              (string-match "bbbbb" content))))
>
> Using same machine, same emacs setup, 28 is alwasy much slower than 27.
>
> Emacs 28 enables native compilation. But string-match is C API, so I'm not
> sure native compilation matters.

Did you disable compiler optimizations when building Emacs?