220 36063 article Path: news.gmane.org!.POSTED!not-for-mail From: Eyal Rozenberg Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Sub-objects and their copy ellision Date: Tue, 19 Dec 2017 18:34:31 +0100 Lines: 49 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1513704761 22166 195.159.176.226 (19 Dec 2017 17:32:41 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 19 Dec 2017 17:32:41 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:57.0) Gecko/20100101 Thunderbird/57.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDHKZN7LR4ARBLM34XIQKGQETSGJQII@isocpp.org Tue Dec 19 18:32:37 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f70.google.com ([74.125.82.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eRLkq-0005Lg-Pb for gclcip-std-proposals@m.gmane.org; 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[192.16.191.27]) by mx.google.com with ESMTPS id z34si1224754edz.7.2017.12.19.09.34.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 19 Dec 2017 09:34:37 -0800 (PST) Received-SPF: pass (google.com: domain of srs0=kaojp=dp=technion.ac.il=eyalroz@cwi.nl designates 192.16.191.27 as permitted sender) client-ip=192.16.191.27; Original-Received: from fester.cwi.nl (fester.cwi.nl [192.16.191.27]) by fester.cwi.nl (8.14.4/8.12.3) with ESMTP id vBJHYb9i024544 for ; Tue, 19 Dec 2017 18:34:37 +0100 Original-Received: from [192.168.2.1] (ip565d4370.direct-adsl.nl [86.93.67.112] (may be forged)) (authenticated bits=0) by fester.cwi.nl (8.14.4/8.12.3) with ESMTP id vBJHYagj024542 (version=TLSv1/SSLv3 cipher=AES128-SHA bits=128 verify=NO) for ; Tue, 19 Dec 2017 18:34:37 +0100 Openpgp: preference=signencrypt Content-Language: en-US X-Original-Sender: eyalroz@technion.ac.il X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of srs0=kaojp=dp=technion.ac.il=eyalroz@cwi.nl designates 192.16.191.27 as permitted sender) smtp.mailfrom=SRS0=KAOJP=DP=technion.ac.il=eyalroz@cwi.nl; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=technion.ac.il Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:36063 Archived-At: Hello list, I've recently gone over Antony Polukhin's proposal (or proposal draft) regarding subobject copy elision: https://apolukhin.github.io/papers/subobjects_copy_elision.html where he describes how, at the moment, if your function returns fields in an object, compilers do not construct them where the output needs to be placed, and consequently generate much longer and slower code. Supposedly, this is due to the standard's provision regarding copy elision, [class.copy.elision], which says that elision is permitted: "... in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object etc. etc." As I was reading the paper, it made me wonder: Why does it matter that these are subobjects? Aren't subobjects just another kind of object? So I looked it up in the current standard draft: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4700.pdf and, indeed, in Section [intro.object], we find the definition of a subobject: "Objects can contain other objects, called \emph{subobjects}" that is to say, subobjects _are_ objects. And if an object has automatic storage duration, then so do its (non-static) subobjects. Well, then, why is it the case that copy elision doesn't occur for subobjects? And is this proposal really necessary, or is it just an unimaginative interpretation of the wording of [class.copy.elision] by compiler implementers? I'm hoping some language lawyers or compiler people can shed some light on this point. Eyal -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/caca241d-3498-b79b-fbf3-e57d8b7c3518%40technion.ac.il. .