From mboxrd@z Thu Jan 1 00:00:00 1970 From: Segher Boessenkool Subject: Re: [PATCH] make atomic_t volatile on all architectures Date: Sun, 12 Aug 2007 07:53:58 +0200 Message-ID: References: <20070808230733.GA17270@shell.boston.redhat.com> <46BAC2BE.1090106@redhat.com> <46BB508B.7050601@redhat.com> <1186683646.9669.20.camel@localhost> Mime-Version: 1.0 (Apple Message framework v623) Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Cc: wjiang@resilience.com, Linus Torvalds , wensong@linux-vs.org, heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, ak@suse.de, cfriesen@nortel.com, netdev@vger.kernel.org, horms@verge.net.au, akpm@linux-foundation.org, Chuck Ebbert , davem@davemloft.net, zlynx@acm.org, Chris Snook To: schwidefsky@de.ibm.com Return-path: Received: from gate.crashing.org ([63.228.1.57]:42304 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752793AbXHLFzF (ORCPT ); Sun, 12 Aug 2007 01:55:05 -0400 In-Reply-To: <1186683646.9669.20.camel@localhost> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org >> You'd have to use "+m". > > Yes, though I would use "=m" on the output list and "m" on the input > list. The reason is that I've seen gcc fall on its face with an ICE on > s390 due to "+m". The explanation I've got from our compiler people was > quite esoteric, as far as I remember gcc splits "+m" to an input > operand > and an output operand. Now it can happen that the compiler chooses two > different registers to access the same memory location. "+m" requires > that the two memory references are identical which causes the ICE if > they are not. The problem is very nicely described here, last paragraph: It's not a problem anymore in (very) recent GCC, although that of course won't help you in the kernel (yet). > I do not know if the current compilers still do this. Has > anyone else seen this happen ? In recent GCC, it's actually documented: The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character `+' to indicate such an operand and list it with the output operands. You should only use read-write operands when the constraints for the operand (or the operand in which only some of the bits are to be changed) allow a register. Note that last line. Segher