220 33238 <93d6544c-8e99-4f68-ab84-b355a0ce5ca9@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Erich Keane Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal "is_constexpr" Date: Mon, 17 Jul 2017 15:27:49 -0700 (PDT) Lines: 107 Approved: news@gmane.org Message-ID: <93d6544c-8e99-4f68-ab84-b355a0ce5ca9@isocpp.org> References: <803a7b14-34c2-4ee6-8c5a-154532143a6e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1454_174121304.1500330469796" X-Trace: blaine.gmane.org 1500330474 25277 195.159.176.226 (17 Jul 2017 22:27:54 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 17 Jul 2017 22:27:54 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCWZXNPJO4ION45UZMCRUBFA7HVMQ@isocpp.org Tue Jul 18 00:27:48 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f200.google.com ([209.85.216.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dXEUU-000696-CL for gclcip-std-proposals@m.gmane.org; Tue, 18 Jul 2017 00:27:46 +0200 Original-Received: by mail-qt0-f200.google.com with SMTP id p25sf1572199qtp.4 for ; Mon, 17 Jul 2017 15:27:52 -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=23M2CwtyTl36fLPsM3LsJO6mPQMx2b6N/bAO4fsYxos=; b=U/1L+sO93YVGo1oQC+jiaQ1l7d9psaJ5+2a7/moMH5wj+8lEx/SmsIjNyUFJTVAkyj QFGMA2D75SYpYGCDK/mkrIp2oPIuJMgniY6TrRhDDdXrqRV5ThzyX41GGxL4G1xK6b29 RXElxZGZDfMHHKXsOnMZm2iuKCQUhJCvZ5onQBpWa6wi4DfgRo4UTv4lPGd+aOMzK0uS rRBMccy+yJDVB51PaBUyh8YK9V4vPZ1BQYqfScYrnobmIQ3Gj67MTyiWnavcaiEeg72A sTUWXahpNhjxbtb5DhaI80FdfqXYNlKsxMYZTufDUNYzpARHiaXsc62m/QS/9+StBKHf 6IVQ== 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=23M2CwtyTl36fLPsM3LsJO6mPQMx2b6N/bAO4fsYxos=; b=CFl8AwjFZ4tRF9JTmJFXXgHUPFhKDk1hhTF9iMXL7zjA5daBGDb+3c501eQx8N2Ksu /FnaSPCh+bjeyZHrdZsSDxMbgmDykbmkKHpA1Ow/Lnket/xDduOWDdDbHbA+gOh6S7rG wWYZbQaOnHAaCVPwGPBeOTyOAgO7+EKzXGJiwFw4i0IX3e33yo7n6ptprTWfSroHG5ln qqEFGgnlC5ySUJj0DVVs+PHBnSqE9IThGMgrpgWHT2J7zi+R461OKWAY2IQNncBTE+Sj amqNeDbooRJlkl0us5WktnqJqjEDmd9W+NmuiXPjJ51ePx0ic7CK+AdIOtzEETSuVxxB Y8ow== X-Gm-Message-State: AIVw1111sgCJNRtRpWRSuj/xGZp1sjrp/p0DcCtZX5pZArW2Bllr51qa jDxZw0IoJcaWb04Q X-Received: by 10.237.33.213 with SMTP id m21mr15821521qtc.88.1500330471590; Mon, 17 Jul 2017 15:27:51 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.71.72 with SMTP id t69ls6286083itb.0.canary-gmail; Mon, 17 Jul 2017 15:27:50 -0700 (PDT) X-Received: by 10.31.153.84 with SMTP id b81mr87711vke.14.1500330470263; Mon, 17 Jul 2017 15:27:50 -0700 (PDT) In-Reply-To: <803a7b14-34c2-4ee6-8c5a-154532143a6e@isocpp.org> X-Original-Sender: erich.keane@verizon.net 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:33238 Archived-At: ------=_Part_1454_174121304.1500330469796 Content-Type: multipart/alternative; boundary="----=_Part_1455_1090816499.1500330469796" ------=_Part_1455_1090816499.1500330469796 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable In Kona, Daveed proposed this:=20 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0595r0.html it made it through EWG, and seems to accomplish your goal. Additionally,= =20 since "is_constexpr" is a compile-time constant, the constant folder level= =20 of optimization will just 'fix' it for the 'false' case. BTW: No progress was made by Daveed in Toronto, but no idea if that was on= =20 purpose, or just a ball dropped. On Monday, July 17, 2017 at 3:15:53 AM UTC-7, dime...@gmail.com wrote: > > =E2=96=A1 Motivation > In C++, the same semantic functions sometimes have to write two different= =20 > versions, a constexpr version, an efficient general version. > For example: > constexpr auto StrLen(const char *s) {auto tp=3Ds;while(*s) s++;return s-= tp;} > auto StrLenSSE4(const char *s){...} > =E2=96=A1 Proposal > Gcc has a built-in function __builtin_constant_p (v) to determine whether= =20 > a value is a compile time constant, but it can only be used for macros, a= nd=20 > v can only be one literal. > Can you provide a function of *is_constexpr*(v) to determine whether v is= =20 > a compile time constant? > In this way, the function above can use a unified interface. > constexpr auto StrLen(const char *s){ > if constexpr(*is_constexpr*(s)) { > auto tp=3Ds;while(*s) s++;return s-tp; > } > else return StrLenSSE4(s); > } > --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/93d6544c-8e99-4f68-ab84-b355a0ce5ca9%40isocpp.or= g. ------=_Part_1455_1090816499.1500330469796 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
In Kona, Daveed proposed this: http://www.open-std.org/jtc= 1/sc22/wg21/docs/papers/2017/p0595r0.html

it made it through EWG, an= d seems to accomplish your goal.=C2=A0 Additionally, since "is_constex= pr" is a compile-time constant, the constant folder level of optimizat= ion will just 'fix' it for the 'false' case.

BTW: No= progress was made by Daveed in Toronto, but no idea if that was on purpose= , or just a ball dropped.

On Monday, July 17, 2017 at 3:15:53 AM UTC= -7, dime...@gmail.com wrote:
=E2=96=A1 Mo= tivation
In C++, the same semantic functions sometimes have to wr= ite two different versions, a constexpr version, an efficient general versi= on.
For example:
constexpr auto StrLen(const char *s) {auto tp=3Ds;wh= ile(*s) s++;return s-tp;}
auto StrLenSSE4(const char *s){...}
=E2=96=A1 Proposal
Gcc has a built-in function _= _builtin_constant_p (v) to determine whether a value is a compile time cons= tant, but it can only be used for macros, and v can only be one literal.Can you provide a function of is_constexpr<= /font>(v) to determine whether v is a compile time constant?
In this way= , the function above can use a unified interface.
constexpr auto StrLen(= const char *s){
=C2=A0 if constexpr(is_constexpr(s)) {
=C2=A0= =C2=A0=C2=A0 auto tp=3Ds;while(*s) s++;return s-tp;
=C2=A0 }
=C2=A0 e= lse return StrLenSSE4(s);
}

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