Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965340AbXA3M5c (ORCPT ); Tue, 30 Jan 2007 07:57:32 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S965341AbXA3M5c (ORCPT ); Tue, 30 Jan 2007 07:57:32 -0500 Received: from nic.NetDirect.CA ([216.16.235.2]:53462 "EHLO rubicon.netdirect.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965340AbXA3M5b (ORCPT ); Tue, 30 Jan 2007 07:57:31 -0500 X-Originating-Ip: 74.109.98.130 Date: Tue, 30 Jan 2007 07:56:20 -0500 (EST) From: "Robert P. J. Day" X-X-Sender: rpjday@CPE00045a9c397f-CM001225dbafb6 To: Nick Piggin cc: Linux kernel mailing list , Andrew Morton , Paul Mackerras , dhowells@redhat.com, galak@kernel.crashing.org Subject: Re: [PATCH] Add "is_power_of_2" checking to log2.h. In-Reply-To: <45BF3954.8050106@yahoo.com.au> Message-ID: References: <45BF3954.8050106@yahoo.com.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Net-Direct-Inc-MailScanner-Information: Please contact the ISP for more information X-Net-Direct-Inc-MailScanner: Found to be clean X-Net-Direct-Inc-MailScanner-SpamCheck: not spam, SpamAssassin (not cached, score=-16.8, required 5, autolearn=not spam, ALL_TRUSTED -1.80, BAYES_00 -15.00) X-Net-Direct-Inc-MailScanner-From: rpjday@mindspring.com Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1351 Lines: 41 On Tue, 30 Jan 2007, Nick Piggin wrote: > Robert P. J. Day wrote: > > Add the inline function "is_power_of_2()" to log2.h, where the value > > zero is *not* considered to be a power of two. > > > > > Signed-off-by: Robert P. J. Day > > > > /* > > + * Determine whether some value is a power of two, where zero is > > + * *not* considered a power of two. > > + */ > > Why the qualifier? Zero *is* not a power of 2, is it? no, but it bears repeating since some developers might think it *is*. if you peruse the current kernel code, you'll find some tests of the simpler form: ((n & (n - 1)) == 0)) which is clearly testing for "power of twoness" but which will return true for a value of zero. that's wrong, and it's why it's emphasized in the comment. rday -- ======================================================================== Robert P. J. Day Linux Consulting, Training and Annoying Kernel Pedantry Waterloo, Ontario, CANADA http://www.fsdev.dreamhosters.com/wiki/index.php?title=Main_Page ======================================================================== - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/