220 28969 <6a135136-1acf-465d-994e-fc3c60fb2764@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: rianquinn@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: uint16_t data =x= static_cast(1) + 1; Date: Wed, 19 Oct 2016 05:59:32 -0700 (PDT) Lines: 146 Approved: news@gmane.org Message-ID: <6a135136-1acf-465d-994e-fc3c60fb2764@isocpp.org> References: <47e7f79f-8676-4a63-b7d3-09d64219d575@isocpp.org> <15C675BC-1FAC-4196-9978-FAB54B0DE897@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_35_87710164.1476881972850" X-Trace: blaine.gmane.org 1476881990 16878 195.159.176.226 (19 Oct 2016 12:59:50 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 19 Oct 2016 12:59:50 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDN6PG7E2ALRBOO4TXAAKGQEP64VSIY@isocpp.org Wed Oct 19 14:59:45 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f69.google.com ([209.85.214.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bwqT5-0003AL-HS for gclcip-std-proposals@m.gmane.org; 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Wed, 19 Oct 2016 05:59:41 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.227.79 with SMTP id d76ls3921145ith.4.canary; Wed, 19 Oct 2016 05:59:34 -0700 (PDT) X-Received: by 10.36.77.75 with SMTP id l72mr552888itb.1.1476881974113; Wed, 19 Oct 2016 05:59:34 -0700 (PDT) In-Reply-To: <15C675BC-1FAC-4196-9978-FAB54B0DE897@gmail.com> X-Original-Sender: rianquinn@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:28969 Archived-At: ------=_Part_35_87710164.1476881972850 Content-Type: multipart/alternative; boundary="----=_Part_36_1039713435.1476881972850" ------=_Part_36_1039713435.1476881972850 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable These are the flags that we are using: -Wall -Wextra -Wpedantic -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor=20 -Wold-style-cast -Wcast-align -Woverloaded-virtual -Wconversion=20 -Wsign-conversion -DGSL_THROW_ON_CONTRACT_VIOLATION -fpic -fexceptions=20 -fstack-protector-strong -mstackrealign -mmmx -msse -msse2 -msse3 -mssse3= =20 -msse4.1 -msse4.2 -msse4 -mavx -maes -ffreestanding -mno-red-zone I think -Wconversion is the offending flag.=20 - Rian On Wednesday, October 19, 2016 at 6:55:09 AM UTC-6, David Krauss wrote: > > > On 2016=E2=80=9310=E2=80=9319, at 8:31 PM, dgutson . >=20 > wrote: > > > > On Tue, Oct 18, 2016 at 8:11 PM, > wrote: > >> Writing a hypervisor in C++, and at least on Intel that are a lot of=20 >> things that require 8 and 16 bit integers. To my surprise, this generate= s a=20 >> warning about conversions: >> >> uint16_t data1 =3D 1; >> uint16_t data2 =3D data1 + 1; // warning on possible overflow >> > > Interestingly, g++ (5.4) does not complain with that snippet, until you d= o=20 > { } initialization: > > > I tried GCC and Clang with -Wall -Wextra -Woverflow -Wnarrowing and=20 > couldn=E2=80=99t get any diagnoses. > > With braces, the narrowing conversion makes the program ill-formed. It is= =20 > conforming to issue a warning for an ill-formed program, but it is=20 > nonconforming to be quiet. The standard doesn=E2=80=99t usually different= iate=20 > between error messages and warning messages. > > To silence the warning requires a static_cast on the sum: data2{=20 > static_cast< uint16_t >( data1 + 1 ) }. > > It would be nice to redo C++ without integer promotion, but perhaps=20 > allowing the same optimization as a matter of undefined overflow, much as= =20 > floating-point computations don=E2=80=99t have guaranteed imprecision.=20 > Unfortunately, we=E2=80=99re pretty much stuck with it. > > --=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/6a135136-1acf-465d-994e-fc3c60fb2764%40isocpp.or= g. ------=_Part_36_1039713435.1476881972850 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
These are the flags that we are using:

= -Wall -Wextra -Wpedantic -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -W= old-style-cast -Wcast-align -Woverloaded-virtual -Wconversion -Wsign-conver= sion -DGSL_THROW_ON_CONTRACT_VIOLATION -fpic -fexceptions -fstack-protector= -strong -mstackrealign -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 = -msse4 -mavx -maes -ffreestanding -mno-red-zone

I = think -Wconversion is the offending flag.=C2=A0

- = Rian



On Wednesday, October 19, 2016 at 6:5= 5:09 AM UTC-6, David Krauss wrote:

On 2016=E2=80=9310=E2=80=9319, at 8:31 PM, dgutson . <daniel...@gmai= l.com> wrote:



On Tue, Oct 18, 2016 at 8:11 PM, <rian...= @gmail.com> wrote:
Writing a hypervisor in C++, and at least = on Intel that are a lot of things that require 8 and 16 bit integers. To my= surprise, this generates a warning about conversions:

=
uint16_t data1 =3D 1;
uint16_t data2 =3D data1 + 1; =C2=A0//= warning on possible overflow

Interestingly, g++ (5.4) does not complain with that snippet, until you = do { } initialization:
<= br>
I tried GCC and Clang with -Wall -Wextra -Woverflow -Wnarrowi= ng and couldn=E2=80=99t get any diagnoses.

With br= aces, the narrowing conversion makes the program ill-formed. It is conformi= ng to issue a warning for an ill-formed program, but it is nonconforming to= be quiet. The standard doesn=E2=80=99t usually differentiate between error= messages and warning messages.

To silence the war= ning requires a static_cast on the sum: data2{ stati= c_cast< uint16_t >( data1 + 1 ) }.

It= would be nice to redo C++ without integer promotion, but perhaps allowing = the same optimization as a matter of undefined overflow, much as floating-p= oint computations don=E2=80=99t have guaranteed imprecision. Unfortunately,= we=E2=80=99re pretty much stuck with it.

<= /blockquote>

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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/6a135136-1acf-465d-994e-fc3c60fb2764= %40isocpp.org.
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