From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753235AbZHGPlA (ORCPT ); Fri, 7 Aug 2009 11:41:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753163AbZHGPk7 (ORCPT ); Fri, 7 Aug 2009 11:40:59 -0400 Received: from smtp.minet.uni-jena.de ([141.35.14.220]:34663 "EHLO mx1.mirz.uni-jena.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753012AbZHGPk6 (ORCPT ); Fri, 7 Aug 2009 11:40:58 -0400 X-Greylist: delayed 4199 seconds by postgrey-1.27 at vger.kernel.org; Fri, 07 Aug 2009 11:40:58 EDT Date: Thu, 6 Aug 2009 20:39:45 +0200 From: Falk Hueffner To: Matt Turner Cc: linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org Subject: Re: Bad Code generated from __{get,put}_unaligned functions Message-ID: <20090806183941.GA6854@minet.uni-jena.de> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 06, 2009 at 01:56:12PM -0400, Matt Turner wrote: > I was researching different ways of writing unaligned load/store > macros, so I checked how the kernel did it -- the most general way > possible. See include/linux/unaligned.h. As such, very bad code is > generated, for example on alpha with BWX, we can implement all these > functions with a single instruction, whereas we get stuff like this > generated from the generic functions. > > __get_unaligned_le32: > .frame $30,0,$26,0 > .prologue 0 > ldbu $0,1($16) > ldbu $1,2($16) > ldbu $2,3($16) > ldbu $3,0($16) > sll $1,16,$1 > sll $0,8,$0 > bis $0,$1,$0 > sll $2,24,$2 > bis $0,$3,$0 > bis $0,$2,$0 > addl $31,$0,$0 > ret $31,($26),1 > > 4 load byte instructions, shift, shift, or, shift, or, or, sign extend > -- or ldl_u instruction. The code is more than doubly-bad for le64. > > Do we use the generic functions for a reason I don't see? It appears > that it would be easy enough to add architecture-specific unaligned > get/put functions in arch/*/include/asm/unaligned.h There should be no need for architecture specific code for Alpha. GCC can generate the optimal code sequence for reads from unaligned struct members as in linux/unaligned/packed_struct.h, and this code should be used. So you should try to find out why it isn't. Falk