220 24186 article Path: news.gmane.org!not-for-mail From: Edward Catmur Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal: Change the specified behavior when control flow reaches the end of non-void functions Date: Thu, 4 Feb 2016 02:57:30 -0800 (PST) Lines: 152 Approved: news@gmane.org Message-ID: References: <5115b78c-8fb4-4c72-a278-f74c636f5217@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1253_803925918.1454583450835" X-Trace: ger.gmane.org 1454583464 6618 80.91.229.3 (4 Feb 2016 10:57:44 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 4 Feb 2016 10:57:44 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDZLZTXF7UJBBG65ZS2QKGQE4EFIAPA@isocpp.org Thu Feb 04 11:57:44 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f198.google.com ([209.85.220.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aRHbW-0002l4-HO for gclcip-std-proposals@m.gmane.org; Thu, 04 Feb 2016 11:57:38 +0100 Original-Received: by mail-qk0-f198.google.com with SMTP id e124sf76207057qkc.3 for ; Thu, 04 Feb 2016 02:57:38 -0800 (PST) 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 :content-type: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=4EfIbirp/JQ2AoOL6/gltH5W6drBaLY4Cu7EclnSvoM=; b=ZpihU5pTp6TU4cR9ilz9KLb+jLGjC4w8IhxGrTP8tZjrSYwi2H1/4K3Rrw5D1or6nB Ks478AlrGkW28SgM1IyY4PPCxOM6Q5IoPeLFatFA/arREHllX1AWxbIKcjosA0CeRF50 DMLOJddwOqY1lQeBqd/36y8tb9u4wT2BTRJWK/N1/csqq3bJCcNfZa1aTklwbUW4cfZi vXOboF7nX4pMmxEkTsx/oxw+uvy4Beb2MPutBQ5Dp8O9KRKB7zcCdFad7NNfPhaWaS01 8muap3l9MUviDFyMKjUyS57i/5YKW/jFJqbKb984nXGaZAO40NXj2QMRM7m08ibTKEZ8 Wf2A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:content-type: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=4EfIbirp/JQ2AoOL6/gltH5W6drBaLY4Cu7EclnSvoM=; b=cfM+3M2zm8AL4nus8OoANijnWjdMQal/6t/tUxOfmsZ2PFY9Q1ndC1+PJOcUMBjP9c G3TbLjsrZ/6FbVHKlwFyxIrxRQHl7B9pM1WFT1UU0vv3XYb2z2rp0WM4YNqAu1/XJSB5 MWJsaPKALRSWRxdVp8O2c+mqA0KHcFtZAtiXj1JsbsWtjyP2A3W2p2E2zZYvNwcJ2dAH fT4dNAuls/1hYi/EyVAMXjaHwLUZDMhxGJJsvRBRL6usrslHbMx0GJPmrTNAUSQFZzu1 CeNeHHCaypWwHEE2/5nYnQKGXmvJLR+ElET5X3Q7igSUIMyeqIp5420EqnoFviYLYT9o A/Bg== X-Gm-Message-State: AG10YOR1vHjt0SNXkZZgMQ0hDO0EcqacT0dIp7Re4lrL1c0u3IeyS4wYI0wnkDW7FN561w== X-Received: by 10.13.202.7 with SMTP id m7mr6117906ywd.54.1454583452775; Thu, 04 Feb 2016 02:57:32 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.67.46 with SMTP id k14ls2268591igt.44.canary; Thu, 04 Feb 2016 02:57:31 -0800 (PST) X-Received: by 10.50.136.193 with SMTP id qc1mr108946igb.6.1454583451719; Thu, 04 Feb 2016 02:57:31 -0800 (PST) In-Reply-To: X-Original-Sender: ed@catmur.co.uk 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:24186 Archived-At: ------=_Part_1253_803925918.1454583450835 Content-Type: multipart/alternative; boundary="----=_Part_1254_936790487.1454583450835" ------=_Part_1254_936790487.1454583450835 Content-Type: text/plain; charset=UTF-8 On Wednesday, 3 February 2016 12:02:55 UTC, Tony V E wrote: > > Since it is currently undefined behaviour, you could ask your compiler > vendor to offer it as a compiler option, and then we could get some > experience as to how valuable it is. > Both clang and gcc have this in ubsan (-fsanitize=undefined), specifically as -fsanitize=return (clang's implementation seems better than gcc's). They don't turn it on by default as it pessimizes valid code. Thiago has already given examples; here's another that comes up fairly often: enum e { a, b, c }; enum E { A, B, C }; E f(e v) { switch (v) { case a: return A; case b: return B; case c: return C; } } Currently clang compiles this to mov edi, eax; ret. With -fsanitize=return the instruction count quadruples, including a branch. The proposal would be worse than the current ubsan offering, not just in making it compulsory but also because calling std::terminate discards information on the nature and location of the error; ubsan outputs the source location and describes the error: "main.c:3:3: missing return statement", or some such. -- --- 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 email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at https://groups.google.com/a/isocpp.org/group/std-proposals/. ------=_Part_1254_936790487.1454583450835 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Wednesday, 3 February 2016 12:02:55 UTC, Tony V E = wrote:
Since it is currently undefined behaviour, yo= u could ask your compiler vendor to offer it as a compiler option, and then= we could get some experience as to how valuable it is.

Both clang and gcc have this in ubsan (-fsanitize= =3Dundefined), specifically as -fsanitize=3Dreturn (clang's implementat= ion seems better than gcc's).

They don't t= urn it on by default as it pessimizes valid code. Thiago has already given = examples; here's another that comes up fairly often:
enum e { a, b= , c };
enum E { A, B= , C };
E f(e v) {=C2=A0 sw= itch (v) {
=C2=A0 =C2=A0 case a: return A;
=C2=A0 =C2=A0 case b: return B;
=C2=A0 =C2=A0 case c: return C;
=C2=A0 }
}

Currently clang compiles t= his to mov edi, eax; ret. With=C2=A0-fsanitize=3Dreturn=C2=A0the instructio= n count quadruples, including a branch.

The propos= al would be worse than the current ubsan offering, not just in making it co= mpulsory but also because calling std::terminate discards information on th= e nature and location of the error; ubsan outputs the source location and d= escribes the error: "main.c:3:3: missing return statement", or so= me such.

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