From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 17799 invoked by alias); 4 Sep 2014 17:23:42 -0000 Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-help-owner@gcc.gnu.org Received: (qmail 17789 invoked by uid 89); 4 Sep 2014 17:23:39 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-4.7 required=5.0 tests=AWL,BAYES_00,RP_MATCHES_RCVD,SPF_HELO_PASS autolearn=ham version=3.3.2 X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Thu, 04 Sep 2014 17:23:38 +0000 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id s84HNYCJ014079 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 4 Sep 2014 13:23:34 -0400 Received: from zebedee.pink ([10.3.113.10]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id s84HNX92031048; Thu, 4 Sep 2014 13:23:33 -0400 Message-ID: <5408A015.5040106@redhat.com> Date: Thu, 04 Sep 2014 17:23:00 -0000 From: Andrew Haley User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.7.0 MIME-Version: 1.0 To: Andy Webber CC: gcc-help@gcc.gnu.org Subject: Re: is portable aliasing possible in C++? References: <5408988E.2060301@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-IsSubscribed: yes X-SW-Source: 2014-09/txt/msg00033.txt.bz2 On 09/04/2014 06:18 PM, Andy Webber wrote: > On 9/4/14, Andrew Haley wrote: >> On 09/04/2014 05:11 PM, Andy Webber wrote: Regrettably, >>> Our goal is to avoid bugs caused by strict aliasing in our networking >>> libraries. My question is how to guarantee that we're not violating >>> the aliasing rules while also getting the most optimization. I've >>> read through a ton of information about this online and in some gcc >>> discussions, but I don't see a consensus. >>> >>> Memcpy always works, but is dependent on optimization to avoid copies. >>> The union of values is guaranteed to work by C++11, but may involve >>> copies. >> >> Is this a real worry? IME it makes copies when it needs to. >> >>> Each test works when built with -O3 on gcc-4.8.3, but I would like to >>> standardize across compilers and versions. The optimization >>> information generated by -fdump-tree-all is interesting here as it >>> shows slightly different optimization for each case though >>> reinterpret_cast and placement new generate identical code in the end. >> >> The "union trick" has always worked with GCC, and is now hallowed by >> the standard. It's also easy to understand. It generates code as >> efficient as all the other ways of doing it, AFAIAA. It's what we >> have always recommended. >> >> Your test is nice. I suppose we could argue that this is a missed >> optimization: >> >> union_copy(): >> movl $2, %eax >> cmpw $2, %ax >> jne .L13 >> >> I don't know why we only generate code for one of the tests. > > Thanks for responding. I appreciate any clarity that the gcc devs and > standards experts can give here. > > I'm especially interested in the validity of the placement new > approach. Your recommendation of going through unions causes some > difficulty for us in terms of type abstraction. Specifically, > receiving network bytes directly into a union with all possible > message types present in the union is somewhat less flexible than > determining the correct message type and doing a placement new to > create essentially a memory overlay. Is placement new a suitable > substitute for __may_alias__ in this specific example? I regret that the exact legality of placement new in this context is beyond me. I think it's OK as long as you only do it with POD-types, but I'd have bounce this off someone like Jason Merrill. Andrew.