220 37480 <2111ede0-62c6-4a41-8df3-f506d07f09f2@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Myriachan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allowing std::complex's magic permission for other types Date: Mon, 26 Mar 2018 14:59:34 -0700 (PDT) Lines: 110 Approved: news@gmane.org Message-ID: <2111ede0-62c6-4a41-8df3-f506d07f09f2@isocpp.org> References: <6ec944b3-16c3-4f00-b0c9-a449a522d4ac@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_5272_970195018.1522101574846" X-Trace: blaine.gmane.org 1522101454 19929 195.159.176.226 (26 Mar 2018 21:57:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 26 Mar 2018 21:57:34 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDKLT4PURQHRBR624XKQKGQE35SDMZQ@isocpp.org Mon Mar 26 23:57:30 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f199.google.com ([209.85.217.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f0a7N-00054O-Jg for gclcip-std-proposals@m.gmane.org; Mon, 26 Mar 2018 23:57:29 +0200 Original-Received: by mail-ua0-f199.google.com with SMTP id r1sf14044287uae.6 for ; Mon, 26 Mar 2018 14:59:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=7c8dfuFeGewc33CHrlPHnLkoiOedNb6gsXHvP39DkZA=; b=O/wkubqZDZe/7ctaLEgTHEjhui9mx8MIwraD+tvnvJWs4Uv5qHTJxhbk9hqKUGwpHC k1uVx4kRu8RWIf/DkH2/QbKkoa4Nym/fNm9eoAPMXtmD686rfwYWqlmajbM1e3JG2oiJ Gfn5NPS2rQtGyslhoErOMQW4/elLmKtIy45dRsqkbDRIuCH0DfR5ikbj+lbYUQInC9qM T2BIDteDkKfCQuTvzeQTB6qQbZCTNRxQgcrNQ/QfKuz96UBeLT8+CHOXazEt2IZ8J7b9 ATwnYBZZL47pFUXN2FQ0befy5ImeBG2A9kldpssM5nTbpB91V6vaFS0/V8iX6kDij1p9 9/7A== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=7c8dfuFeGewc33CHrlPHnLkoiOedNb6gsXHvP39DkZA=; b=ZyEdgsSF5ZJPdDWmIvNKFd/k8CPlkUSYcu14rZD/0qjjZ5PfZv6qEkRzazzOKAjrXG kt9xXj5EmfdxOmBq/ZzL/U5WPwRjEMjAFxk05JK77lqCd6UbhCdAZTvWedDsQRvcH1rM NXx/PQdt52R79x006kFbIQuVO2+F+k98YQTC/yZK2v5FVzObLed7Nkiu5QwuSRjZSmiX yA4WKmjffSbTvuzj34WncWPXkyEWwuVLEiKtZne5vb93Hi+5YMr74Sq7idNUjUeK8FA0 nqz5KD24kPRlbo6Jpb8Q0K1PaqVsttdebarno+slrohmgMLlMB/+MY4jpjSNjBt2OzkP 2kfA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=7c8dfuFeGewc33CHrlPHnLkoiOedNb6gsXHvP39DkZA=; b=EqnHzJFanruD0dOZlDCI6/sE+Y+YKvHm/u5rEWErx4GpocLj3tuRJxAi7FrhhiY5+5 DrpyKWxOxU7LiHFiZuYtBTTS1rGc7TyEf/uQrgSgxX1y/f2ioMyyQMGWDjPti2HX1Dmo XIWgHpd84mUPsn1AqO/NdM2K57qN19knmqsl0zM6ZATMNFE9X1prOg62nhmRkUQSKc5g 8C9wdrKjbTfI7ANB2FuZQIexBqkZOLET9XusqeiSeZNb3t7LNOlj/fkJpT7MinRlxV8f xPNj3O8GWI3lpH+5iXF6POfjDw3Y/naZQ8tIbZg2lQsNXjaYbIl+b/qrf+mWVRX2laYd VkbA== X-Gm-Message-State: AElRT7GsNI2hFRSMQ7y/EwonmAiypv45AXypquMbU+uEJpQM2FVTAK5s 4m/uapBnTP7+Ml+C0vgcAhjIMA== X-Google-Smtp-Source: AG47ELtKbyPHLE4lWCfWn85HWscYFLTBccujA9R8vsJNGcAtx3AXsK9SKQ2AxHq39MkgNA5K0RkkEA== X-Received: by 10.31.177.1 with SMTP id a1mr13233734vkf.11.1522101576686; Mon, 26 Mar 2018 14:59:36 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.160.67 with SMTP id j64ls9443223vke.21.gmail; Mon, 26 Mar 2018 14:59:35 -0700 (PDT) X-Received: by 10.31.151.87 with SMTP id z84mr5023746vkd.5.1522101575322; Mon, 26 Mar 2018 14:59:35 -0700 (PDT) In-Reply-To: <6ec944b3-16c3-4f00-b0c9-a449a522d4ac@isocpp.org> X-Original-Sender: myriachan@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:37480 Archived-At: ------=_Part_5272_970195018.1522101574846 Content-Type: multipart/alternative; boundary="----=_Part_5273_1924448923.1522101574846" ------=_Part_5273_1924448923.1522101574846 Content-Type: text/plain; charset="UTF-8" On Monday, March 26, 2018 at 1:39:55 PM UTC-7, Nicol Bolas wrote: > > > > On Monday, March 26, 2018 at 4:26:58 PM UTC-4, Myriachan wrote: >> >> I would like to propose that C++ get a mechanism by which the magical >> ability of std::complex * to be reinterpret_cast to T * could be allowed >> for other classes. >> >> A simple case of this is a vector4 type: >> >> struct vector3 { float x; float y; float z; float w; }; >> >> The performance difference is strong enough that our project actually >> gets a much larger performance *increase* from using >> -fno-strict-aliasing versus -fstrict-aliasing with the changes required for >> compliance. >> > > What does strict aliasing have to do with this? The strict aliasing rule > is not what prevents you from being able to do this cast. Indeed, nothing > prevents you from doing the cast. Since `vector4` is standard layout, the > standard *guarantees* that its first subobject shall have the same address > as the object itself. So the cast is OK. > > What isn't OK is the pointer arithmetic that follows, > -fstrict-aliasing breaks our code, even though you're correct that it's not an aliasing problem. -fstrict-aliasing seems to enable optimizations around pointer arithmetic undefined behavior, and there's no -fallow-evil-pointer-arithmetic option. or the assumption that there is no padding between the elements. > The padding isn't a problem, because a simple static_assert can verify that your compiler isn't putting any padding. Such code would be portable to any implementation that doesn't put padding. Melissa -- 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/2111ede0-62c6-4a41-8df3-f506d07f09f2%40isocpp.org. ------=_Part_5273_1924448923.1522101574846 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Monday, March 26, 2018 at 1:39:55 PM UTC-7, Nicol Bolas= wrote:

On Monday, March 26, 2018 at 4:26:58 PM UTC-4, Myriachan wrote:
I would like to propose that= C++ get a mechanism by which the magical ability of std::complex<T> = * to be reinterpret_cast to T * could be allowed for other classes.

= A simple case of this is a vector4 type:

struct vector3 { float x; f= loat y; float z; float w; };

The performance difference is strong en= ough that our project actually gets a much larger performance increase from using -fno-strict-aliasing versus -fstrict-aliasing with the change= s required for compliance.

What does strict = aliasing have to do with this? The strict aliasing rule is not what prevent= s you from being able to do this cast. Indeed, nothing prevents you from do= ing the cast. Since `vector4` is standard layout, the standard *guarantees*= that its first subobject shall have the same address as the object itself.= So the cast is OK.

What isn't OK is the pointer arithmetic that= follows,

-fstrict-aliasing breaks our co= de, even though you're correct that it's not an aliasing problem.= =C2=A0 -fstrict-aliasing seems to enable optimizations around pointer arith= metic undefined behavior, and there's no -fallow-evil-pointer-arithmeti= c option.

or the assumption that there is no padding between the eleme= nts.

The padding isn't a problem, = because a simple static_assert can verify that your compiler isn't putt= ing any padding.=C2=A0 Such code would be portable to any implementation th= at doesn't put padding.

Melissa

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/2111ede0-62c6-4a41-8df3-f506d07f09f2= %40isocpp.org.
------=_Part_5273_1924448923.1522101574846-- ------=_Part_5272_970195018.1522101574846-- .