bug#22763: 25.1.50; Feature Request -- A faster method to obtain line number at position.

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

>> find_newline does the same as display_count_lines: it calls memchr.
>> But it also maintains a newline cache.  If you disable that cache (by
>> turning of cache-long-scans), you might see a different speedup.
>
> Shouldn't the cache speed things up?

No, you're right -- if I set `cache-long-scans' to nil and
`line-number-at-pos' in Emacs 27 becomes ~8x faster in my test, which is
currently

(with-temp-buffer
  (setq cache-long-scans nil)
  (dotimes (_ 1000)
    (insert-file-contents "~/src/emacs/trunk/src/ChangeLog.11")
    (goto-char (point-max)))
  (benchmark-run 1
    (dotimes (i 1000)
      (goto-char (/ (buffer-size) 1000))
      (line-number-at-pos (point)))))

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no