220 31554 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?Q?Andrzej_Krzemie=C5=84ski?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: First draft of [[unreachable]] attribute proposal Date: Sat, 11 Mar 2017 06:24:34 -0800 (PST) Lines: 151 Approved: news@gmane.org Message-ID: References: <74928e4c-8f1e-4036-93d8-2560c27b651d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2563_1377689398.1489242275045" X-Trace: blaine.gmane.org 1489242279 23123 195.159.176.226 (11 Mar 2017 14:24:39 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 11 Mar 2017 14:24:39 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDT2DGOJ34DBBJERSDDAKGQESOEOEYA@isocpp.org Sat Mar 11 15:24:34 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f199.google.com ([209.85.192.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cmhwd-0005Ev-8E for gclcip-std-proposals@m.gmane.org; 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Sat, 11 Mar 2017 06:24:36 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.68.202 with SMTP id p10ls4845470otg.18.gmail; Sat, 11 Mar 2017 06:24:35 -0800 (PST) X-Received: by 10.157.80.16 with SMTP id a16mr2742411oth.3.1489242275960; Sat, 11 Mar 2017 06:24:35 -0800 (PST) In-Reply-To: X-Original-Sender: akrzemi1@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:31554 Archived-At: ------=_Part_2563_1377689398.1489242275045 Content-Type: multipart/alternative; boundary="----=_Part_2564_494395188.1489242275045" ------=_Part_2564_494395188.1489242275045 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable W dniu pi=C4=85tek, 10 marca 2017 13:41:05 UTC+1 u=C5=BCytkownik Morwenn na= pisa=C5=82: > > Le vendredi 10 mars 2017 13:04:37 UTC+1, Andrzej Krzemie=C5=84ski a =C3= =A9crit : >> >> >> >> W dniu czwartek, 9 marca 2017 04:49:35 UTC+1 u=C5=BCytkownik Myriachan n= apisa=C5=82: >>> >>> I wrote up a proposal document for a new attribute, [[unreachable]],=20 >>> which lets you mark locations in code as being unreachable for optimiza= tion=20 >>> purposes, or to suppress certain warnings. >>> >>> This has been discussed on the mailing list before; there's a link to= =20 >>> that thread in the .pdf. >>> >>> https://1drv.ms/b/s!Agwy_QljRwlGgcoJeZMiimuq3H36Ww=20 >>> >>> >>> I have no idea what to do with it now. If you have any comments on the= =20 >>> content, or how to actually proceed, let me know! =3D^-^=3D >>> >>> >>> Under the contract proposal, anywhere where the statement is expected,= =20 >>> you can type: >>> >>> [[assert: false]]; >>> >>> While [[assert axiom: false]] is probably semantically equivalent to=20 >>> the proposed [[unreachable]], the former doesn't really convey the=20 >>> meaning. >>> >>> I would be more confortable adding [[unreachable]] to the default=20 >>> branch of a switch than the equivalent [[assert axiom: false]]. >>> >> Given that the expression in the assertion is a compile-time constant only= =20 [[assert: false]] is fine. But the interaction goes beyond only syntax. If= =20 we have attribute [[unreachable]] maybe we want it to behave like [[assert:= =20 false]] in every detail; that is: allow calling contract violation handlers= =20 in test builds, if this place in the code is reached. This way the hint can= =20 be used not only for optimization, but also for instrumentation. Regards, &rzej; --=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/f6b40768-7dbc-4019-b90b-f5c4398f0e25%40isocpp.or= g. ------=_Part_2564_494395188.1489242275045 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


W dniu pi=C4=85tek, 10 marca 2017 13:41:05 UTC+1 u= =C5=BCytkownik Morwenn napisa=C5=82:
Le vendredi 10 mars 2017 13:04:37 UTC+1, Andrzej Krz= emie=C5=84ski a =C3=A9crit=C2=A0:


W dniu czwartek, 9 marca 2017 04:49:35 UTC+1 u=C5=BC= ytkownik Myriachan napisa=C5=82:
I wrote up a proposal document for a new attribute, [[unreachable]], whic= h lets you mark locations in code as being unreachable for optimization pur= poses, or to suppress certain warnings.

This has been discussed on t= he mailing list before; there's a link to that thread in the .pdf.
<= br>https://1drv.ms/b/s!Agwy_QljRwlGgcoJe= ZMiimuq3H36Ww

I have no idea what to do with it now.=C2=A0 If yo= u have any comments on the content, or how to actually proceed, let me know= ! =3D^-^=3D


Under the contract proposal, anywhere where the stat= ement is expected, you can type:

[[assert: false]];

While [[assert axiom: false]] is probably semantically eq= uivalent to the proposed = [[unreachable]], the former doesn't really convey the meaning.
I would be more confortable adding [[unreachable]] to the default branch of a switch than the equivalent [[assert axiom: false]].

Given that the expression in the assertion is a compile-t= ime constant only [[assert: false]] is fine. But the interaction goes beyon= d only syntax. If we have attribute [[unreachable]] maybe we want it to beh= ave like [[assert: false]] in every detail; that is: allow calling contract= violation handlers in test builds, if this place in the code is reached. T= his way the hint can be used not only for optimization, but also for instru= mentation.

Regards,
&rzej;

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