From owner-freebsd-current@FreeBSD.ORG  Wed Feb  7 10:25:29 2007
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
X-Original-To: current@freebsd.org
Delivered-To: freebsd-current@FreeBSD.ORG
Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52])
	by hub.freebsd.org (Postfix) with ESMTP id 85B5A16A401
	for <current@freebsd.org>; Wed,  7 Feb 2007 10:25:29 +0000 (UTC)
	(envelope-from bakul@bitblocks.com)
Received: from mail.bitblocks.com (ns1.bitblocks.com [64.142.15.60])
	by mx1.freebsd.org (Postfix) with ESMTP id 7145F13C4AC
	for <current@freebsd.org>; Wed,  7 Feb 2007 10:25:29 +0000 (UTC)
	(envelope-from bakul@bitblocks.com)
Received: from bitblocks.com (localhost.bitblocks.com [127.0.0.1])
	by mail.bitblocks.com (Postfix) with ESMTP id 211C95B2E;
	Wed,  7 Feb 2007 02:01:01 -0800 (PST)
To: Luigi Rizzo <rizzo@icir.org>
In-reply-to: Your message of "Wed, 07 Feb 2007 00:41:31 PST."
	<20070207004131.A62183@xorpc.icir.org> 
Date: Wed, 07 Feb 2007 02:01:01 -0800
From: Bakul Shah <bakul@bitblocks.com>
Message-Id: <20070207100101.211C95B2E@mail.bitblocks.com>
Cc: current@freebsd.org
Subject: Re: C macro to find the next power of 2 ? 
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 07 Feb 2007 10:25:30 -0000

> 	#define b2(x)   (   (x) | (   (x) >> 1) )
> 	#define b4(x)   ( b2(x) | ( b2(x) >> 2) )
> 	#define b8(x)   ( b4(x) | ( b4(x) >> 4) )
> 	#define b16(x)  ( b8(x) | ( b8(x) >> 8) )
> 	#define b32(x)  (b16(x) | (b16(x) >>16) )
> 	#define next_power_of_2(x)      (b32(x-1) + 1)
...
> But i was wondering if there is some simpler code which
> can still be computed as a compile-time constant ?

If this works, why bother?  let the compiler work harder!