From owner-freebsd-hackers@FreeBSD.ORG  Thu Jun 11 14:52:55 2015
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115])
 (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id CBD10456
 for <freebsd-hackers@freebsd.org>; Thu, 11 Jun 2015 14:52:55 +0000 (UTC)
 (envelope-from ian@freebsd.org)
Received: from pmta2.delivery4.ore.mailhop.org
 (pmta2.delivery4.ore.mailhop.org [54.200.247.200])
 by mx1.freebsd.org (Postfix) with SMTP id AB097151D
 for <freebsd-hackers@freebsd.org>; Thu, 11 Jun 2015 14:52:55 +0000 (UTC)
 (envelope-from ian@freebsd.org)
Received: from ilsoft.org (unknown [73.34.117.227])
 by outbound1.ore.mailhop.org (Halon Mail Gateway) with ESMTPSA;
 Thu, 11 Jun 2015 14:52:54 +0000 (UTC)
Received: from revolution.hippie.lan (revolution.hippie.lan [172.22.42.240])
 by ilsoft.org (8.14.9/8.14.9) with ESMTP id t5BEqrVX006682;
 Thu, 11 Jun 2015 08:52:53 -0600 (MDT) (envelope-from ian@freebsd.org)
Message-ID: <1434034373.1415.3.camel@freebsd.org>
Subject: Re: allow ffs & co. a binary search
From: Ian Lepore <ian@freebsd.org>
To: Richard Yao <ryao@gentoo.org>
Cc: Erich Dollansky <erichsfreebsdlist@alogt.com>,
 "freebsd-hackers@freebsd.org" <freebsd-hackers@FreeBSD.org>
Date: Thu, 11 Jun 2015 08:52:53 -0600
In-Reply-To: <5579A016.4040800@gentoo.org>
References: <20150607081315.7c0f09fb@B85M-HD3-0.alogt.com>
 <5579A016.4040800@gentoo.org>
Content-Type: text/plain; charset="us-ascii"
X-Mailer: Evolution 3.12.10 FreeBSD GNOME Team Port 
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.20
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
 <freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-hackers>, 
 <mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers/>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
 <mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 11 Jun 2015 14:52:55 -0000

On Thu, 2015-06-11 at 10:49 -0400, Richard Yao wrote:
> On 06/06/2015 08:13 PM, Erich Dollansky wrote:
[...]
> 
> The fastest portable way of calculating highest bit set is to use a
> debrujin sequence:
> 
> https://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
> 
> The binary approach is inferior, especially on Pentium 4 processors.

And of course it's crucial that we optimize for Pentium 4 processors in
2015.

-- Ian