From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760083AbYEMN7T (ORCPT ); Tue, 13 May 2008 09:59:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756206AbYEMN7M (ORCPT ); Tue, 13 May 2008 09:59:12 -0400 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:43544 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750969AbYEMN7K (ORCPT ); Tue, 13 May 2008 09:59:10 -0400 Date: Tue, 13 May 2008 14:58:39 +0100 From: Russell King To: Alexander van Heukelum Cc: Nickolay Vinogradov , linux-kernel@vger.kernel.org, Andrew Morton Subject: Re: [PATCH] asm-generic/bitops/fls64.h Message-ID: <20080513135839.GA19291@flint.arm.linux.org.uk> References: <481E076A.8060302@protei.ru> <1210677433.22341.1252875863@webmail.messagingengine.com> <48298990.7040705@protei.ru> <1210685053.13437.1252892061@webmail.messagingengine.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1210685053.13437.1252892061@webmail.messagingengine.com> User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 13, 2008 at 03:24:13PM +0200, Alexander van Heukelum wrote: > On Tue, 13 May 2008 16:29:04 +0400, "Nickolay Vinogradov" > said: > > Alexander van Heukelum пишет: > > > > > Hi Nickolay, > > > > > > The change is ok, I guess, but the cast should be a no-op (fls > > > takes an int, which is always 32 bit in linux). What is the problem > > > you are seeing? Does fls64() return a wrong value in some cases? If > > > so, what cpu? Which values? > > > > > > Why would this be a bug on big endian systems only? There is no > > > pointer magic involved, so the compiler should take care of the > > > casts in a correct way. > > > > > > Maybe you see a compiler warning? Which compiler version? > > > > > > (also note that current (development) kernels now have separate > > > versions for 32-bit and 64-bit environments.) > > > > Because fls() is a macro for asm-arm: > > > > #define fls(x) \ > > ( __builtin_constant_p(x) ? constant_fls(x) : \ > > ({ int __r; asm("clz\t%0, %1" : "=r"(__r) : "r"(x) : "cc"); > > 32-__r; }) ) > > > > We can fix it right here: No. "fls" is for finding the last set bit in an _int_. It is not supposed to have random crap passed to it, such as types longer than sizeof(int). If you're going to pass long long (64-bit) arguments to fls, and then cast them to a u32, you're truncating the value, and you'll get the wrong answer if bit 33 or greater is set. If you don't actually care about the upper bits, don't pass a 64-bit quantity to fls(). If you want to use fls with a long long, use fls64 instead. Or for top marks, use a u64 and fls64. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: