220 38052 <0918afa6-6f04-fdc1-fd80-f2843a5e63f1@gmail.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: [[likely]] attribute for switch case Date: Tue, 8 May 2018 19:08:44 +0300 Lines: 53 Approved: news@gmane.org Message-ID: <0918afa6-6f04-fdc1-fd80-f2843a5e63f1@gmail.com> References: <9adde2c8-8306-4708-96f2-edb621d906c7@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"; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1525795603 13906 195.159.176.226 (8 May 2018 16:06:43 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 8 May 2018 16:06:43 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCXY5TXHWYJRBD4XY7LQKGQEKNBCESA@isocpp.org Tue May 08 18:06:38 2018 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 1fG58P-0003Ut-AF for gclcip-std-proposals@m.gmane.org; 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[109.173.28.102]) by smtp.googlemail.com with ESMTPSA id t19-v6sm3702412ljj.46.2018.05.08.09.08.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 08 May 2018 09:08:45 -0700 (PDT) In-Reply-To: <9adde2c8-8306-4708-96f2-edb621d906c7@isocpp.org> Content-Language: en-US X-Original-Sender: andrey.semashev@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=gKYMBzWQ; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=andrey.semashev@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=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:38052 Archived-At: On 05/08/18 18:48, david.bolvansky@gmail.com wrote: > Hello, >=20 > Since in C++20 we can write: >=20 > char * b(int e) { > if=C2=A0 ([[likely]] e =3D=3D 0) > return "0"; > else if (e =3D=3D 1) > return "1"; > =C2=A0 =C2=A0else return "undef"; > } >=20 > Can likely attribute be added even for switch case? Like for example: >=20 > switch(e) { > =C2=A0 =C2=A0 case 0 [[likely]]: > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return "0"; > =C2=A0 =C2=A0 case 1: > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "1"; > =C2=A0 =C2=A0 default: > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "undef"; >=20 > } >=20 >=20 > I read the "likely" attribute=20 > proposal=C2=A0http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p04= 79r0.html=20 > and they do not mention this use case for switches. N4741 includes and example with `switch`/`case` in [dcl.attr.likelihood]/3. I suspect your example with an `if` statement is not doing what you=20 think it does. The [[likely]] applies to the condition expression, which=20 basically says "this condition is likely to be executed" and not "this=20 condition is likely to be true". The correct example is also given in=20 the aforementioned section. This is the most unfortunate aspect of this proposal as it makes it=20 incompatible with __builtin_expect and tons of code that wraps it in macros= .. --=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/0918afa6-6f04-fdc1-fd80-f2843a5e63f1%40gmail.com= .. .