220 36410 article Path: news.gmane.org!.POSTED!not-for-mail From: Antony Polukhin Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Sub-objects and their copy ellision Date: Thu, 28 Dec 2017 10:01:21 +0300 Lines: 85 Approved: news@gmane.org Message-ID: References: <1c6eff77-d71f-4dd7-8c13-f609460a03f0@isocpp.org> <3ec0bd22-b228-4026-bb49-aec5102918c7@isocpp.org> <924751e5-c880-42aa-95ab-99ef1050be2e@isocpp.org> <80bbb75a-2a62-537a-3733-898225325a41@technion.ac.il> <83d8a425-fd2f-41b6-9941-2a5deb27806d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="94eb2c1c00f4670c1e056161151b" X-Trace: blaine.gmane.org 1514444373 30319 195.159.176.226 (28 Dec 2017 06:59:33 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 28 Dec 2017 06:59:33 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDR7DZXHY4MRBQ5NSLJAKGQEQSXIJEY@isocpp.org Thu Dec 28 07:59:29 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f71.google.com ([209.85.215.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eUS9x-0007E8-J5 for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id h23sor5739816lji.79.2017.12.27.23.01.22 for (Google Transport Security); Wed, 27 Dec 2017 23:01:22 -0800 (PST) Received-SPF: pass (google.com: domain of antoshkka@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.46.25.85 with SMTP id p82mr18856600lje.29.1514444482348; Wed, 27 Dec 2017 23:01:22 -0800 (PST) Original-Received: by 10.25.215.210 with HTTP; Wed, 27 Dec 2017 23:01:21 -0800 (PST) In-Reply-To: <83d8a425-fd2f-41b6-9941-2a5deb27806d@isocpp.org> X-Original-Sender: antoshkka@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=XnUAUCA1; spf=pass (google.com: domain of antoshkka@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=antoshkka@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com 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:36410 Archived-At: --94eb2c1c00f4670c1e056161151b Content-Type: text/plain; charset="UTF-8" 2017-12-26 18:25 GMT+03:00 Nicol Bolas : > On Tuesday, December 26, 2017 at 4:05:52 AM UTC-5, Eyal Rozenberg wrote: >> >> On 12/25/17 8:15 PM, Nicol Bolas wrote: >> > On Monday, December 25, 2017 at 1:12:19 PM UTC-5, Antony Polukhin >> wrote: >> > Why is this circumstance important enough to be /worth/ all of this >> hassle? >> >> Because right now compilers are entirely-or-partially prevented from >> applying a useful optimization in some relatively-common cases. >> > > I see little evidence that these cases are indeed "relatively-common" > enough to bother with this. > I've added an example where moving out std::string produces much worse assembly than with move elision of subobjects. Is that example convincing? It's in the "The problem is not that big" concern http://apolukhin.github.io/papers/ultimate_copy_elision.html -- Best regards, Antony Polukhin -- 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/CAKqmYPYTBDjbzhuaNKZd54XqEx7W%3Dx4HrmP390CqMODNP7E0Rg%40mail.gmail.com. --94eb2c1c00f4670c1e056161151b Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


2017-12-26 18:25 GMT+03:00 Nicol Bolas <jmckesson@gmail.com><= /span>:
On Tuesday, December 26, 2017 at 4:05:52 AM UTC-5= , Eyal Rozenberg wrote:
On 12/25/17 8:15 PM, Nicol Bolas wrote:
> On Monday, December 25, 2017 at 1:12:19 PM UTC-5, Antony Polukhin = wrote:
> Why is this circumstance important e= nough to be /worth/ all of this hassle?

Because right now compilers are entirely-or-partially prevented from
applying a useful optimization in some relatively-common cases.

I see little evidence that these cases = are indeed "relatively-common" enough to bother with this.
<= /div>

I've added an example where movin= g out std::string produces much worse assembly than with move elision of su= bobjects. Is that example convincing? It's in the "The problem is = not that big" concern http://apolukhin.github.io/papers/ultimate_copy_el= ision.html

--
Best regards,
Antony Polukhin

--
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