[Bug libstdc++/51078] [PATCH] performance improvement of std::count algorithm

[grygoriy.fuchedzhy at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51078

--- Comment #14 from Grygoriy Fuchedzhy <grygoriy.fuchedzhy at gmail dot com> 2011-11-12 10:09:52 UTC ---
(In reply to comment #13)
> I would say, the next step, is analyzing why: std::count seems a very simple
> algorithm, no aliasing issues for example, compiler should be able to unroll,
> maybe vectorize too, and everything else, without having to fiddle with the
> code in an ad-hoc way.

I completely agree that analizing why is necessary, but this probably should be
done in separate bug?(should I open it?) As regarding current bug: it seems
that automatic loop unrolling is far from being considered "stable" or
"recommended" optimization, so this patch may improve performance until this
optimization become commonly used. By the way, manually unrolled std::find, one
of old algorithms you mentioned also performs significantly better than version
without loop unrolling.