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

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  22763@debbugs.gnu.org,  Keith David
>  Bershatsky <esq@lawlist.com>
> Date: Sun, 07 Feb 2021 18:45:51 +0100
> 
> Stefan Monnier <monnier@iro.umontreal.ca> writes:
> 
> > Why is it faster?
> >
> > Is it still always Θ(N) just with a smaller constant (if so, what makes
> > the constant smaller), or does it benefit from some kind of caching
> > (which I fail to see in the code) such that it's O(N) sometimes but much
> > faster other times (and if so, what are the cases that are sped up)?
> 
> There's no caching.  I guess find_newline is just slow compared to
> display_count_lines?

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.

Also, calling forward-line would loop in Lisp, not in C.