bug#26079: 26.0.50; Performance regression in delete-trailing-whitespace

[Michal Nazarewicz <mina86@mina86.com>]

On Mon, Mar 13 2017, npostavs wrote:
> tags 26079 confirmed
> quit
>
> Sho Takemori <stakemorii@gmail.com> writes:
>
>> Dear developers,
>>
>> delete-trailing-whitespace in Emacs 26 for large files is very slow.
>>
>> For example, it took about 1.6s for this file (https://raw.githubusercontent.com/stakemori/e8theta_degree3/master/results/wt18_17_5/wt18_17_5.org).
>> But in Emacs 25, it took about 0.003s.
>> A similar code to the following is used in delete-trailing-whitespace. And it is slow for large files.
>>
>> (save-excursion
>> (let ((end-marker nil))
>> (goto-char (point-min))
>> (with-syntax-table (make-syntax-table (syntax-table))
>> (modify-syntax-entry ?\f "_")
>> (modify-syntax-entry ?\n "_")
>> (re-search-forward "\\s-+$" end-marker t))))
>
> It seems that this regex causes a lot of backtracking when \n is not
> whitespace.  It was introduced in [1: 7c6317a049]; restoring the
> strategy from before seems make it fast again.  Michal, do you think
> that's the best way to fix this?

I wish Emacs’ RE was O(n). :(

Yeah, I think reverting my commit is the best course of action.

> ---   i/lisp/simple.el
> +++   i/lisp/simple.el
> @@ -632,12 +632,11 @@ delete-trailing-whitespace
>          (goto-char (or start (point-min)))
>          (with-syntax-table (make-syntax-table (syntax-table))
>            ;; Don't delete formfeeds, even if they are considered whitespace.
>            (modify-syntax-entry ?\f "_")
> -          ;; Treating \n as non-whitespace makes things easier.
> -          (modify-syntax-entry ?\n "_")
> -          (while (re-search-forward "\\s-+$" end-marker t)
> -            (let ((b (match-beginning 0)) (e (match-end 0)))
> +          (while (re-search-forward "\\s-$" end-marker t)
> +            (skip-syntax-backward "-" (line-beginning-position))
> +            (let ((b (point)) (e (match-end 0)))
>                (when (region-modifiable-p b e)
>                  (delete-region b e)))))
>          (if end
>              (set-marker end-marker nil)
>
>
> 1: 2016-07-04 23:44:06 +0200 7c6317a0498b6690ea668909ac012cb45e6f809b
>   Simplify ‘delete-trailing-whitespace’ by not treating \n as whitespace

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»