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

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From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 22763 <at> debbugs.gnu.org, esq <at> lawlist.com, monnier <at> iro.umontreal.ca
Subject: bug#22763: 25.1.50; Feature Request -- A faster method to obtain line number at position.
Date: Sun, 07 Feb 2021 22:58:12 +0100

Lars Ingebrigtsen <larsi <at> gnus.org> writes:

> It seems it tests one longword_ptr at a time?  So that's 8 bytes on
> 64-bit CPUs, I think.  Doesn't look like any SSE/AVX support, though.

Never mind -- there are even more optimised versions using SSE2 etc,
too:

https://github.com/lattera/glibc/blob/master/sysdeps/x86_64/memchr.S

I've done some further benchmarking -- with a perhaps more typical
file than the ChangeLog:

(with-temp-buffer
  (dotimes (_ 100)
    (insert-file-contents "~/src/emacs/trunk/lisp/printing.el")
    (goto-char (point-max)))
  (benchmark-run 10
    (dotimes (i 100)
      (goto-char (* (/ (buffer-size) 100) i))
      (line-number-at-pos (point)))))

The cached version is only 5x slower than the non-cached one.

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

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