220 15615 article Path: news.gmane.org!not-for-mail From: Richard Smith Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Library support for relaxed constexpr Date: Mon, 12 Jan 2015 19:47:05 -0800 Lines: 130 Approved: news@gmane.org Message-ID: References: <5ccd3d6e-859c-4b66-b61c-317b704647f2@isocpp.org> <14072581.09eDfPqFOs@tjmaciei-mobl4> <9F9D7F25-F862-4358-B074-8F8C87255E2A@gmail.com> <2233213.JephozMv50@tjmaciei-mobl4> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=089e01294836f6f1e3050c807998 X-Trace: ger.gmane.org 1421120832 14415 80.91.229.3 (13 Jan 2015 03:47:12 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 13 Jan 2015 03:47:12 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDVNBJG4YAIBBONK2KSQKGQEQCMBURA@isocpp.org Tue Jan 13 04:47:07 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f197.google.com ([209.85.214.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YAsRe-0003iV-V8 for gclcip-std-proposals@m.gmane.org; Tue, 13 Jan 2015 04:47:07 +0100 Original-Received: by mail-ob0-f197.google.com with SMTP id nt9sf4420971obb.0 for ; Mon, 12 Jan 2015 19:47:06 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:date :message-id:subject:from:to:content-type: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=cotqtyHc2yjQ9xU36wyVojVE9YwGyqc/qcdaQBzWz+g=; b=e98Jo7fxeiHCFf7VcgOPrkLS7R7H4aIBwEWHdS1XNMZTZ38m3D8Bmvh1kNil9Ay8Kt IM4kOdI9NtgtksZ/rrNNOQ2DE+593Ggs4EXL2A1XzYVkrv9MaoF12nzRiBNR+xf1y3r0 Yegbcuc+HPm6uUmXti4CtrbAgPZbS59L9EaNxfgFBtr5EXpAvz9bJfi//m6y5Ib12YQk DT629QJ6SrLLHan9QqwrgUvGewiQ9znM1DF9Cs2Z+EqwUlwB1Qj5tVT4DjHblt52pJHC e7pqLc6o/Ab7gFEmhwf0heNy364h37O+f8CqRVccKAhz1sb9pKouMjPhRwwtJy//++DO tz2A== X-Gm-Message-State: ALoCoQlkm1jtQqX9w4D0IDEf6iLLAAnifzY2NHumqCCbnp4EbMHwz4q+bGfyZZJbU/0JoUvINDec X-Received: by 10.43.99.202 with SMTP id ct10mr27168203icc.29.1421120825881; Mon, 12 Jan 2015 19:47:05 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.61.137 with SMTP id p9ls982101igr.38.canary; Mon, 12 Jan 2015 19:47:05 -0800 (PST) X-Received: by 10.42.209.196 with SMTP id gh4mr27462871icb.10.1421120825319; Mon, 12 Jan 2015 19:47:05 -0800 (PST) Original-Received: from mail-ig0-x22b.google.com (mail-ig0-x22b.google.com. [2607:f8b0:4001:c05::22b]) by mx.google.com with ESMTPS id 15si13095602ioj.14.2015.01.12.19.47.05 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 12 Jan 2015 19:47:05 -0800 (PST) Received-SPF: pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:4001:c05::22b as permitted sender) client-ip=2607:f8b0:4001:c05::22b; Original-Received: by mail-ig0-f171.google.com with SMTP id z20so15005325igj.4 for ; Mon, 12 Jan 2015 19:47:05 -0800 (PST) X-Received: by 10.50.73.136 with SMTP id l8mr18739271igv.5.1421120825125; Mon, 12 Jan 2015 19:47:05 -0800 (PST) Original-Sender: metafoo@gmail.com Original-Received: by 10.107.136.223 with HTTP; Mon, 12 Jan 2015 19:47:05 -0800 (PST) In-Reply-To: <2233213.JephozMv50@tjmaciei-mobl4> X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:4001:c05::22b as permitted sender) smtp.mail=metafoo@gmail.com; dkim=pass header.i=@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:15615 Archived-At: --089e01294836f6f1e3050c807998 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Mon, Jan 12, 2015 at 7:01 PM, Thiago Macieira wrote: > On Tuesday 13 January 2015 09:18:13 David Krauss wrote: > > > On 2015=E2=80=9301=E2=80=9313, at 6:43 AM, Thiago Macieira > wrote: > > > > > > Note that the difference is whether the intrinsic functions are > considered > > > > > constexpr or not. For GCC, __builtin_ldexp is constexpr but isn't for > Clang: > > For GCC, most of is constexpr because it has supported > compile-time > > evaluation of constant floating-point expressions since time immemorial= .. > > Only in C++11 (once constexpr extensions were forbidden) did this > extension > > become nonconforming. > > > > I almost finished a paper for Urbana, =E2=80=9CRefining floating-point > constexpr,=E2=80=9D > > but ultimately I had enough other material, and the implementations kep= t > > evolving under my feet. I should finish it and find a presenter for > Lenexa. > > Co-authors are welcome! > > I believe you should leave those functions in cmath be optionally > constexpr. > Compilers do not have to make them constexpr if they are not equipped for > evaluating them at compile time, like GCC was. It doesn't seem unreasonable to require more of C++17 implementations. I think it's much better to make the functions unconditionally constexpr and require the implementations to do the right thing. One consideration, though: C supports controlling the floating point environment in various ways, and C++ pays lipservice to this by providing the 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 with compile-time evaluation of floating-point expressions. --=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/. --089e01294836f6f1e3050c807998 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On M= on, Jan 12, 2015 at 7:01 PM, Thiago Macieira <thiago@macieira.org&g= t; wrote:
On Tues= day 13 January 2015 09:18:13 David Krauss wrote:
> > On 2015=E2=80=9301=E2=80=9313, at 6:43 AM, Thiago Macieira <thiago@macieira.org> wrote:
> >
> > Note that the difference is whether the intrinsic functions are c= onsidered
>
> > constexpr or not. For GCC, __builtin_ldexp is constexpr but isn&#= 39;t for
Clang:
> For GCC, most of <cmath> is constexpr because it has supported c= ompile-time
> evaluation of constant floating-point expressions since time immemoria= l.
> Only in C++11 (once constexpr extensions were forbidden) did this exte= nsion
> become nonconforming.
>
> I almost finished a paper for Urbana, =E2=80=9CRefining floating-point= constexpr,=E2=80=9D
> but ultimately I had enough other material, and the implementations ke= pt
> evolving under my feet. I should finish it and find a presenter for Le= nexa.
> Co-authors are welcome!

I believe you should leave those functions in cmath be optionally co= nstexpr.
Compilers do not have to make them constexpr if they are not equipped for evaluating them at compile time, like GCC was.

<= div>It doesn't seem unreasonable to require more of C++17 implementatio= ns. I think it's much better to make the functions unconditionally cons= texpr and require the implementations to do the right thing.

=
One consideration, though: C supports controlling the floating p= oint environment in various ways, and C++ pays lipservice to this by provid= ing the <cfenv> header. If we ever start also supporting "#pragm= a 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 &= quot;interesting" interactions with compile-time evaluation of floatin= g-point expressions.

--

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