220 33242 article Path: news.gmane.org!.POSTED!not-for-mail From: Pedro Alves Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal "is_constexpr" Date: Tue, 18 Jul 2017 14:20:34 +0100 Lines: 95 Approved: news@gmane.org Message-ID: 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: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1500384043 18281 195.159.176.226 (18 Jul 2017 13:20:43 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 18 Jul 2017 13:20:43 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDRZRD7VU4BBBJMWXDFQKGQER3BXAQA@isocpp.org Tue Jul 18 15:20:36 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f70.google.com ([209.85.215.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dXSQU-0004Dk-C0 for gclcip-std-proposals@m.gmane.org; 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[209.85.128.172]) by mx.google.com with ESMTPS id o202si1839428wmg.30.2017.07.18.06.20.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 18 Jul 2017 06:20:37 -0700 (PDT) Received-SPF: pass (google.com: domain of alves.ped@gmail.com designates 209.85.128.172 as permitted sender) client-ip=209.85.128.172; Original-Received: by mail-wr0-f172.google.com with SMTP id a10so29511615wrd.0 for ; Tue, 18 Jul 2017 06:20:37 -0700 (PDT) X-Received: by 10.223.163.197 with SMTP id m5mr1262508wrb.128.1500384036822; Tue, 18 Jul 2017 06:20:36 -0700 (PDT) Original-Received: from [192.168.0.101] (bl17-148-124.dsl.telepac.pt. [188.82.148.124]) by smtp.gmail.com with ESMTPSA id o68sm2955636wmi.21.2017.07.18.06.20.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 18 Jul 2017 06:20:35 -0700 (PDT) In-Reply-To: <803a7b14-34c2-4ee6-8c5a-154532143a6e@isocpp.org> X-Original-Sender: pedro@palves.net X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of alves.ped@gmail.com designates 209.85.128.172 as permitted sender) smtp.mailfrom=alves.ped@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:33242 Archived-At: On 07/17/2017 11:15 AM, dimeiyi24@gmail.com wrote: > =E2=96=A1 Motivation > In C++, the same semantic functions sometimes have to write two > different 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 a value is a compile time constant, but it can only be used for > macros, and v can only be one literal. > Can you provide a function of *is_constexpr*(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){ > if constexpr(*is_constexpr*(s)) { > auto tp=3Ds;while(*s) s++;return s-tp; > } > else return StrLenSSE4(s); > } Note that GCC's __builtin_constant_p _can_ be used for the above. It's how C++17 constexpr char_traits<>::length() etc. is currently implemented in libstdc++ trunk. (BTW, you can just use char_traits<>::length() in this case.) #include static constexpr bool constant_string_p(const char* s) { while (__builtin_constant_p(*s) && *s) s++; return __builtin_constant_p(*s); } static constexpr size_t constexpr_strlen(const char* s) { auto tp =3D s; while (*s) s++; return s - tp; } constexpr size_t my_strlen(const char* s) noexcept { if (constant_string_p(s)) return constexpr_strlen(s); return strlen(s); } static_assert (my_strlen ("") =3D=3D 0); static_assert (my_strlen ("hello") =3D=3D 5); static_assert (my_strlen ("he\0llo") =3D=3D 2); static const char array[] =3D "foo"; static_assert (my_strlen (array) =3D=3D 3); constexpr bool check() { char s[] =3D "str"; s[0] =3D 'l'; s[1] =3D '\0'; return my_strlen(s) =3D=3D 1; } static_assert(check()); int main (int argc, char** argv) { return my_strlen(argv[0]); } $ g++ constexpr_strlen.cc -o constexpr_strlen -g3 -O2 -std=3Dgnu++17=20 $ gdb --batch -q ./constexpr_strlen -ex "disassemble main" Dump of assembler code for function main(int, char**): 0x00000000004004c0 <+0>: sub $0x8,%rsp 0x00000000004004c4 <+4>: mov (%rsi),%rdi 0x00000000004004c7 <+7>: callq 0x400490 0x00000000004004cc <+12>: add $0x8,%rsp 0x00000000004004d0 <+16>: retq =20 End of assembler dump. $ --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. 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