220 38053 <9fa8e6c3-f3c7-468e-bb0f-b9197a88eb80@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: david.bolvansky@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: [[likely]] attribute for switch case Date: Tue, 8 May 2018 09:29:26 -0700 (PDT) Lines: 158 Approved: news@gmane.org Message-ID: <9fa8e6c3-f3c7-468e-bb0f-b9197a88eb80@isocpp.org> References: <9adde2c8-8306-4708-96f2-edb621d906c7@isocpp.org> <0918afa6-6f04-fdc1-fd80-f2843a5e63f1@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3928_1786510966.1525796966258" X-Trace: blaine.gmane.org 1525796841 7436 195.159.176.226 (8 May 2018 16:27:21 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 8 May 2018 16:27:21 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDSMRTMFUIFRBZ5AY7LQKGQECJRQ6RA@isocpp.org Tue May 08 18:27:17 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f70.google.com ([209.85.213.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fG5SO-0001qK-Rr for gclcip-std-proposals@m.gmane.org; Tue, 08 May 2018 18:27:17 +0200 Original-Received: by mail-vk0-f70.google.com with SMTP id h62-v6sf12352789vke.1 for ; Tue, 08 May 2018 09:29:28 -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=sRkubCCK1MpViGEKJe+CnlWWu2HW3NJDTGnyn8hImj4=; b=K3f1sDFLru2fBxcCdNfL6Me4ypsX49j3USosapmQiwhsNRknfeMcfNEX6cn7mPMrz3 UeFkseXEUzgx9U2tfPoIkB3zFb7J8tfCwa7EuXFUDcK4i5j2zdEpxMJKrmV92IuiQkbm jQKJjeDWKsKuwOQuP0L0J81N05mcyVgKoqFEj7d2StE+slOzdNYg59hWckuBqBKcccZJ 9HvtDp2Xe324JTWmkNqZc2jum/7VKFGfpIEfUJk+7DSrdvgmTRlFZhaviH/nmCBojfKJ qwV1K/mpAYIj74iEYYJm3et3c1nySlhoebKuHEMbQmyguKQq4so249wPrwWlmo9Fq2O3 emaw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; 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=sRkubCCK1MpViGEKJe+CnlWWu2HW3NJDTGnyn8hImj4=; b=GNRWexvI3FRDAK033oM4RlrwJGVcjn0XVyhjVOZzI/cUO7f8yj/k51SYUr+Yq5oqwD 1azA9nEnvdeFodvJ+bQpoK418mEDxPu8oJm7Yqg68accKhuTFk9dAnlSx6u5Q9m4fCIb dJPIb8+EvD1KwP+RYRIUDDnUTJCxPto4VFYV7Ybdlto0RbKSP3l4yTHKphaNwttDk1ay NWFUB9Z595/wvYRlEfWfsZGt3cs9596e3FbYhL0qnN8PPXJbpzF1j+BTVTv7rBADyNiJ fNvyn1cVkfgu3Ohnp5OAALS48Qf9ZBkBI16cIdvMEgUtve/Rmc+459NJIctfTVQoKtXg 8Mkw== 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=sRkubCCK1MpViGEKJe+CnlWWu2HW3NJDTGnyn8hImj4=; b=BjvE1TEmPILziLsdrpb33iGqipT6VpgScSZvgzRZ/AmVYGb6mWfCoZyFRTKFtlmx1L fINNwPQtzwxaYE4ybVb14Sm6Rty5LWA+SVyIhyXCcTGNH6AWNnCGH4rDaiPDdtrWsQQy U4d+zA5klX3KvCXKfxDY9wVHrd0Z4XB6mDDfBLYifUl4+kMixRE4DCuChyf+w0BeZzfH tV2JjjSZJc3lMlaQCH5hQxmJgpNYuVQSk1WNJWjd6wCASkJK+VBOAqvujEuAJZRWuBJ1 h/u3GzBLHcUY2X8sGoVv8lSUbowNIy2r6sYraYWFeJUdBqn2rvDIhEom1FYpUVv8VbPq lOgw== X-Gm-Message-State: ALQs6tDmoGO+x7CD2lZcDdoQx6lkgfYHsdg1oGcb66L8xPBavFZBZPu2 STwnH0vJ6dV0ofWzNGJeNameyQ== X-Google-Smtp-Source: AB8JxZpo6HBPWXCwheEQz5jHBedGYfrAo2jnGs40EsPxG+i5mRBTCPS4PRHU7o1EooCeEek4lAs1jQ== X-Received: by 2002:a1f:848d:: with SMTP id g135-v6mr21534973vkd.11.1525796967749; Tue, 08 May 2018 09:29:27 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a1f:db45:: with SMTP id s66-v6ls965511vkg.12.gmail; Tue, 08 May 2018 09:29:26 -0700 (PDT) X-Received: by 2002:a1f:3095:: with SMTP id w143-v6mr6596136vkw.9.1525796966756; Tue, 08 May 2018 09:29:26 -0700 (PDT) In-Reply-To: <0918afa6-6f04-fdc1-fd80-f2843a5e63f1@gmail.com> X-Original-Sender: david.bolvansky@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:38053 Archived-At: ------=_Part_3928_1786510966.1525796966258 Content-Type: multipart/alternative; boundary="----=_Part_3929_329272358.1525796966258" ------=_Part_3929_329272358.1525796966258 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Thanks, N4741 really has it already. D=C5=88a utorok, 8. m=C3=A1ja 2018 18:08:48 UTC+2 Andrey Semashev nap=C3=AD= sal(-a): > > On 05/08/18 18:48, david.b...@gmail.com wrote:=20 > > Hello,=20 > >=20 > > Since in C++20 we can write:=20 > >=20 > > char * b(int e) {=20 > > if ([[likely]] e =3D=3D 0)=20 > > return "0";=20 > > else if (e =3D=3D 1)=20 > > return "1";=20 > > else return "undef";=20 > > }=20 > >=20 > > Can likely attribute be added even for switch case? Like for example:= =20 > >=20 > > switch(e) {=20 > > case 0 [[likely]]:=20 > > return "0";=20 > > case 1:=20 > > return "1";=20 > > default:=20 > > return "undef";=20 > >=20 > > }=20 > >=20 > >=20 > > I read the "likely" attribute=20 > > proposal=20 > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0479r0.html=20 > > and they do not mention this use case for switches.=20 > > N4741 includes and example with `switch`/`case` in=20 > [dcl.attr.likelihood]/3.=20 > > 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.=20 > > 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=20 > macros.=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/9fa8e6c3-f3c7-468e-bb0f-b9197a88eb80%40isocpp.or= g. ------=_Part_3929_329272358.1525796966258 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Thanks, N4741 really has it already.

D=C5=88a utoro= k, 8. m=C3=A1ja 2018 18:08:48 UTC+2 Andrey Semashev nap=C3=ADsal(-a):On 05/08/18 18:48, david.b...@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=A0 =C2=A0else return "undef";
> }
>=20
> Can likely attribute be added even for switch case? Like for examp= le:
>=20
> switch(e) {
> =C2=A0=C2=A0 =C2=A0 case 0 [[likely]]:
> =C2=A0=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return "0";
> =C2=A0=C2=A0 =C2=A0 case 1:
> =C2=A0=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "1";
> =C2=A0=C2=A0 =C2=A0 default:
> =C2=A0=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/p0479r0.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, whic= h=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 ma= cros.

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/9fa8e6c3-f3c7-468e-bb0f-b9197a88eb80= %40isocpp.org.
------=_Part_3929_329272358.1525796966258-- ------=_Part_3928_1786510966.1525796966258-- .