From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751331AbWAZIz7 (ORCPT ); Thu, 26 Jan 2006 03:55:59 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751332AbWAZIz7 (ORCPT ); Thu, 26 Jan 2006 03:55:59 -0500 Received: from caramon.arm.linux.org.uk ([212.18.232.186]:30729 "EHLO caramon.arm.linux.org.uk") by vger.kernel.org with ESMTP id S1751331AbWAZIz7 (ORCPT ); Thu, 26 Jan 2006 03:55:59 -0500 Date: Thu, 26 Jan 2006 08:55:41 +0000 From: Russell King To: Akinobu Mita , linux-kernel@vger.kernel.org, Ivan Kokshaysky , Ian Molton , dev-etrax@axis.com, David Howells , Yoshinori Sato , Linus Torvalds , linux-ia64@vger.kernel.org, Hirokazu Takata , linux-m68k@vger.kernel.org, Greg Ungerer , linux-mips@linux-mips.org, parisc-linux@parisc-linux.org, linuxppc-dev@ozlabs.org, linux390@de.ibm.com, linuxsh-dev@lists.sourceforge.net, linuxsh-shmedia-dev@lists.sourceforge.net, sparclinux@vger.kernel.org, ultralinux@vger.kernel.org, Miles Bader , Andi Kleen , Chris Zankel Subject: Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h Message-ID: <20060126085540.GA15377@flint.arm.linux.org.uk> Mail-Followup-To: Akinobu Mita , linux-kernel@vger.kernel.org, Ivan Kokshaysky , Ian Molton , dev-etrax@axis.com, David Howells , Yoshinori Sato , Linus Torvalds , linux-ia64@vger.kernel.org, Hirokazu Takata , linux-m68k@lists.linux-m68k.org, Greg Ungerer , linux-mips@linux-mips.org, parisc-linux@parisc-linux.org, linuxppc-dev@ozlabs.org, linux390@de.ibm.com, linuxsh-dev@lists.sourceforge.net, linuxsh-shmedia-dev@lists.sourceforge.net, sparclinux@vger.kernel.org, ultralinux@vger.kernel.org, Miles Bader , Andi Kleen , Chris Zankel References: <20060125112625.GA18584@miraclelinux.com> <20060125113206.GD18584@miraclelinux.com> <20060125200250.GA26443@flint.arm.linux.org.uk> <20060126000618.GA5592@twiddle.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20060126000618.GA5592@twiddle.net> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 25, 2006 at 04:06:18PM -0800, Richard Henderson wrote: > On Wed, Jan 25, 2006 at 08:02:50PM +0000, Russell King wrote: > > > + s = 16; if (word << 16 != 0) s = 0; b += s; word >>= s; > > > + s = 8; if (word << 24 != 0) s = 0; b += s; word >>= s; > > > + s = 4; if (word << 28 != 0) s = 0; b += s; word >>= s; > ... > > Basically, shifts which depend on a variable are more expensive than > > constant-based shifts. > > Actually, they're all constant shifts. Just written stupidly. Unfortunately that's not correct. You do not appear to have checked the compiler output like I did - this code does _not_ generate constant shifts. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core