Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753919AbXFKXP0 (ORCPT ); Mon, 11 Jun 2007 19:15:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754285AbXFKXPM (ORCPT ); Mon, 11 Jun 2007 19:15:12 -0400 Received: from terminus.zytor.com ([192.83.249.54]:47831 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753662AbXFKXPK (ORCPT ); Mon, 11 Jun 2007 19:15:10 -0400 Message-ID: <466DD696.8080901@zytor.com> Date: Mon, 11 Jun 2007 16:11:18 -0700 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.0 (X11/20070419) MIME-Version: 1.0 To: "Robert P. J. Day" CC: Jan Engelhardt , Nick Piggin , 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. References: <45BF3954.8050106@yahoo.com.au> In-Reply-To: X-Enigmail-Version: 0.95.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1162 Lines: 29 Robert P. J. Day wrote: > > seriously, though, there is the potential of breaking something with > this change since you can see how there is some inconsistency in how > it's done *now* just for powerpc which, in some places, defines its > own versions of this: > > ./arch/ppc/mm/pgtable.c: > #define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0)) > ./arch/ppc/syslib/ppc85xx_rio.c: > #define is_power_of_2(x) (((x) & ((x) - 1)) == 0) > ./arch/powerpc/mm/pgtable_32.c: > #define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0)) > > note how the first and third macros *won't* consider zero a power of > two, while the second one *will*. hence the need for a single > standard for all of this, just to play it safe. > I suspect the reason the test for zero was omitted is because the author didn't want the extra cost (the test for zero needs an extra branch on a lot of architectures.) -hpa - 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/