From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 8690 invoked by alias); 4 Sep 2014 17:47:39 -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 8679 invoked by uid 89); 4 Sep 2014 17:47:38 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.8 required=5.0 tests=AWL,BAYES_00,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.2 X-HELO: mail-vc0-f179.google.com Received: from mail-vc0-f179.google.com (HELO mail-vc0-f179.google.com) (209.85.220.179) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES128-SHA encrypted) ESMTPS; Thu, 04 Sep 2014 17:47:37 +0000 Received: by mail-vc0-f179.google.com with SMTP id hy4so10852151vcb.24 for ; Thu, 04 Sep 2014 10:47:35 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:cc:content-type; bh=IQS6fofwKzppM5RVAmkWsk6O+9eexpExN87dfXhizoE=; b=RfbuyJ+fX30otMgjIb8yOtik2VuLdkyirWfHamfUqi1UtQeDvSMVm3SuzEKZVEZKAb HT1ZG6KGT97nqVwDR9yB1wDJ3F6Y7NTa4E+gd/H1ybnwZgSDFKw2r6lvF3g0jbrFRAkw qg1yv8fEZOGd7APnQ6ZGN1yymGtwXiRbeEEn1ld0dU+fvc0KvQHun4FbARoA8F6aC8M9 weLMmk3vtSOr41HTvBPSbDLJtAyhDgjG6gg2lesMQCscZtoAr/qcrkNFFg/2TJP1SjQd FKbqIIuSZSurYoD1tJ/RuzX5ZYVejVaLHNK9YusdHbso9ayg2ROYsx7E57nFU1HKHGT6 XgDg== X-Gm-Message-State: ALoCoQlzwdC0Y27eANfMcoHWs+/7QFA1kWS12oH+eEuZ8gHGts1mhBTi1BpxvWmvSq3ZDT1z478G MIME-Version: 1.0 X-Received: by 10.221.6.201 with SMTP id ol9mr5848832vcb.2.1409852855381; Thu, 04 Sep 2014 10:47:35 -0700 (PDT) Received: by 10.52.94.37 with HTTP; Thu, 4 Sep 2014 10:47:35 -0700 (PDT) In-Reply-To: References: <5408988E.2060301@redhat.com> <5408A015.5040106@redhat.com> Date: Thu, 04 Sep 2014 17:47:00 -0000 Message-ID: Subject: Re: is portable aliasing possible in C++? From: Andy Webber To: Andrew Haley Cc: gcc-help@gcc.gnu.org Content-Type: text/plain; charset=UTF-8 X-SW-Source: 2014-09/txt/msg00035.txt.bz2 On 9/4/14, Andy Webber wrote: > Know of any way to ask Jason Merrill or Richard Biener to weigh in? > They seem to be very knowledgable in this area. > > On 9/4/14, Andrew Haley wrote: >> 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. >> >> >> > Sorry, didn't mean to top post the last reply. Know of any way to ask Jason Merrill or Richard Biener to weigh in? They seem to be very knowledgable in this area.