220 15617 <0098656E-0F82-4B50-A59A-F55630245AB5@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Library support for relaxed constexpr Date: Tue, 13 Jan 2015 13:20:12 +0800 Lines: 135 Approved: news@gmane.org Message-ID: <0098656E-0F82-4B50-A59A-F55630245AB5@gmail.com> References: <5ccd3d6e-859c-4b66-b61c-317b704647f2@isocpp.org> <14072581.09eDfPqFOs@tjmaciei-mobl4> <9F9D7F25-F862-4358-B074-8F8C87255E2A@gmail.com> <1c00c8fd-140a-42f6-b73a-628cbde1737b@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.1 \(1993\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_AC2654A7-ED28-4CEC-9B30-D2F8139AD82B" X-Trace: ger.gmane.org 1421126436 24290 80.91.229.3 (13 Jan 2015 05:20:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 13 Jan 2015 05:20:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBF6W2KSQKGQEBA2ZGFQ@isocpp.org Tue Jan 13 06:20:28 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f199.google.com ([209.85.213.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YAttx-00045q-JC for gclcip-std-proposals@m.gmane.org; Tue, 13 Jan 2015 06:20:25 +0100 Original-Received: by mail-ig0-f199.google.com with SMTP id l13sf8521670iga.2 for ; Mon, 12 Jan 2015 21:20:24 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-type:message-id:mime-version :subject:date:references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=dCTX4aXWZOWRiaWJSlX5wfVHhPHWTG8dXENFwUTGxG8=; b=QaDV4jUL4xTAgOa1xWfaQygsEcUoAB26ufZmIiGhoirNDQeeIairY46yByVcOp7Wqt TIwfTyq553W7EffdmQHR6UtoHd8KIUsQSTWcbqIla0TyyvpIomZ22uCFU/P4u4A+55R1 yufR6IOuqbTuUeG73dfVRWN2DEDSdIYCZJzlLWxlWEp/IRMKZ8kp7Dw1OWy6arbK+4xe D5pacPDlTI2/P4xDUxHoHediZJAb4/CgZ+ShEmAJVBQih7iG5j0nM4GNXmu8vvo7jZPj bQjE1CepfLiM6CkNu69F68us3sFhK2MmTR4LDlhJHnCbaHCIrJsdJJZTgK2jN4Rroh/z Bcsg== X-Gm-Message-State: ALoCoQmGbG4LbfjHSezn9R8eppLPjJOR1MQdkfgdwiXS/PiMJ9JFo22bbMRfM0ST2a700UN3mfqe X-Received: by 10.182.29.101 with SMTP id j5mr12464159obh.32.1421126424520; Mon, 12 Jan 2015 21:20:24 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.158.132 with SMTP id h126ls72576ioe.28.gmail; Mon, 12 Jan 2015 21:20:23 -0800 (PST) X-Received: by 10.68.196.231 with SMTP id ip7mr48487594pbc.156.1421126423606; Mon, 12 Jan 2015 21:20:23 -0800 (PST) Original-Received: from mail-pa0-x22c.google.com (mail-pa0-x22c.google.com. [2607:f8b0:400e:c03::22c]) by mx.google.com with ESMTPS id g1si20054506pde.233.2015.01.12.21.20.23 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 12 Jan 2015 21:20:23 -0800 (PST) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22c as permitted sender) client-ip=2607:f8b0:400e:c03::22c; Original-Received: by mail-pa0-f44.google.com with SMTP id et14so1454746pad.3 for ; Mon, 12 Jan 2015 21:20:23 -0800 (PST) X-Received: by 10.70.131.78 with SMTP id ok14mr49050044pdb.112.1421126423281; Mon, 12 Jan 2015 21:20:23 -0800 (PST) Original-Received: from [172.20.10.2] ([121.54.44.91]) by mx.google.com with ESMTPSA id i7sm15815586pdp.3.2015.01.12.21.20.21 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 12 Jan 2015 21:20:22 -0800 (PST) In-Reply-To: <1c00c8fd-140a-42f6-b73a-628cbde1737b@isocpp.org> X-Mailer: Apple Mail (2.1993) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22c as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:15617 Archived-At: --Apple-Mail=_AC2654A7-ED28-4CEC-9B30-D2F8139AD82B Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 2015=E2=80=9301=E2=80=9313, at 11:54 AM, Myriachan wrote: >=20 > Is it still the case that implementations aren't allowed to declare stand= ard functions as constexpr that are not explicitly constexpr in the Standar= d? What this thread is about is otherwise confusing me=E2=80=A6sorry. Yes, it=E2=80=99s still the case. My recollection is that the restriction w= as added by a DR which was resolved after C++11, but it might have been bef= ore. Considering the potential for language fragmentation, the decision is unlik= ely to be reversed. > On 2015=E2=80=9301=E2=80=9313, at 11:47 AM, Richard Smith wrote: >=20 > One consideration, though: C supports controlling the floating point envi= ronment in various ways, and C++ pays lipservice to this by providing the <= cfenv> header. If we ever start also supporting "#pragma STDC FENV_ACCESS" = and following the C rules here (which seems like a very bad idea, but we're= already half-way there) then that will have "interesting" interactions wit= h compile-time evaluation of floating-point expressions. Nothing that #pragma STDC FP_CONTRACT doesn=E2=80=99t already do. Such inte= ractions are acknowledged and dealt with simply. C99/C11 =C2=A76.5/8: > Otherwise, whether and how expressions are contracted is implementation-d= efined. * >=20 > * This license is specifically intended to allow implementations to explo= it fast machine instructions that combine multiple C operators. As contract= ions potentially undermine predictability, and can even decrease accuracy f= or containing expressions, their use needs to be well-defined and clearly d= ocumented. The difference is that FP_CONTRACT can be turned off but constexpr can=E2= =80=99t. If we set aside the C/C++ rivalry, would it be too unreasonable to= let be non-constexpr when FP_CONTRACT OFF is in effect in the scop= e enclosing the call? (Despite being a pragma, it is scoped.) --=20 ---=20 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 e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --Apple-Mail=_AC2654A7-ED28-4CEC-9B30-D2F8139AD82B Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 2015=E2=80=9301= =E2=80=9313, at 11:54 AM, Myriachan <myriachan@gmail.com> wrote:

Is it still the case that implementations aren't allowed to declare s= tandard functions as constexpr that are not explicitly constexpr in the Standard?&nbs= p; What this thread is about is otherwise confusing me=E2=80=A6sorry.

Yes,= it=E2=80=99s still the case. My recollection is that the restriction was a= dded by a DR which was resolved after C++11, but it might have been before.=

Considering the potential for language= fragmentation, the decision is unlikely to be reversed.


On 2015=E2=80=9301=E2=80=9313, at 11:47 AM, Richard S= mith <richard@metafo= o.co.uk> wrote:

=

Nothing that #pragma STDC FP_CONTRA= CT doesn=E2=80=99t already do. Such interactions are acknowledged an= d dealt with simply. C99/C11 =C2=A76.5/8:

=
Otherwise, whether and how expressions are contracted is impl= ementation-defined. *

* This license is specifically intended to allow implement= ations to exploit fast machine instructions that combine multiple C op= erators. As contractions potentially undermine predictability, and can= even decrease accuracy for containing expressions, their use needs to= be well-defined and clearly documented.

The difference is that FP_CONTRACT can be turned off but constexpr can=E2=80=99t. If we set aside th= e C/C++ rivalry, would it be too unreasonable to let <cmath> be non-constexpr when FP_CONTRACT OFF is in effect in the scope enclosing th= e call? (Despite being a pragma, it is scoped.)

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