220 28997 <2f77b3f0-eb09-537d-6590-59f4d68875c3@gmail.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: uint16_t data =x= static_cast(1) + 1; Date: Thu, 20 Oct 2016 03:06:22 +0300 Lines: 39 Approved: news@gmane.org Message-ID: <2f77b3f0-eb09-537d-6590-59f4d68875c3@gmail.com> References: <47e7f79f-8676-4a63-b7d3-09d64219d575@isocpp.org> <1672903.oiJgXq1YU5@tjmaciei-mobl1> <3106991.qNzeP8ykmy@tjmaciei-mobl1> 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 1476921998 16912 195.159.176.226 (20 Oct 2016 00:06:38 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 20 Oct 2016 00:06:38 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCXY5TXHWYJRBAEVUDAAKGQEA2LTF5I@isocpp.org Thu Oct 20 02:06:33 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f69.google.com ([209.85.215.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bx0sJ-0002VF-57 for gclcip-std-proposals@m.gmane.org; Thu, 20 Oct 2016 02:06:23 +0200 Original-Received: by mail-lf0-f69.google.com with SMTP id n3sf18987210lfn.5 for ; Wed, 19 Oct 2016 17:06:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:content-transfer-encoding:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=pSFu85iHrpGn2eFs6Oi7eg8zQoYJXnnXp8DG61uQ+ZE=; b=dJA9jAb89nWZCCJ3nuKY+aZRR5UX+JTr7qRtTDvNBwcfgbMXMlKaNosmjzkomBEYpl s2FqiMxTltEVJOB+b0VXkUPx/DM2H1mWxsIo9slL/dTlmAyHFxiBrBe9JK5EUJSTGL0v rWG5ieklbPVdr0TMwa5zRQ2WT+bORAtweClJlO2OgnugFQgzbfoiNZvABML0B6imAPbJ /mcECZpUmjRtDFc76akQv+uCjw1rOI6yTr3nxkzfouGcriaxZubNwmnTJ+wEpaZP2Sev Q4C8yv7ZFbAt5k6dRNe2aV8Bf2jSK5hiOboC3XmEgd3COqLgsS8HAr9QPd5WyaFd3bID X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=pSFu85iHrpGn2eFs6Oi7eg8zQoYJXnnXp8DG61uQ+ZE=; b=cej0HXiZrzdr3ZDrBWrTnkmQfnnvAqV/q3AyR9yEwPXLO/zLEuA/p/qcO8LXoQpT4W N0re8t1wF2oEHtRZFYwaw75Lw3/dhIX8QBqugXNXivRxi717PSmR/+IUUQufTxh0tVF8 hlDs2yvjMalHBolJT7wDZ2LGKQaoI5mVriqULqlEha/gdpY/bdszDq2V59sZS35iB0AQ RlwrIeLFAmIsII/9MshabyNTU86p5ErCM+FlBRFtbTxKbMRonibTxOL9DPwXeIAu7NmH Uhodadzut+egmhcxPWBjW7AuAgyJX0Kw6778DGOrOPfFOpQexdA4RgP5N36I9jVgLIC3 X-Gm-Message-State: AA6/9RlOWG2AZsigBWIqH+cpxHWXZSoOeGOTNvhf+0RC7RvQwyNWu5ijjGqlZcJVJDPkfQ== X-Received: by 10.25.221.130 with SMTP id w2mr597826lfi.8.1476921985647; Wed, 19 Oct 2016 17:06:25 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.25.22.78 with SMTP id m75ls461807lfi.20.gmail; Wed, 19 Oct 2016 17:06:24 -0700 (PDT) X-Received: by 10.25.132.18 with SMTP id g18mr8552961lfd.7.1476921984505; Wed, 19 Oct 2016 17:06:24 -0700 (PDT) Original-Received: from mail-lf0-x241.google.com (mail-lf0-x241.google.com. [2a00:1450:4010:c07::241]) by mx.google.com with ESMTPS id o102si5676687lfi.9.2016.10.19.17.06.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 19 Oct 2016 17:06:24 -0700 (PDT) Received-SPF: pass (google.com: domain of andrey.semashev@gmail.com designates 2a00:1450:4010:c07::241 as permitted sender) client-ip=2a00:1450:4010:c07::241; Original-Received: by mail-lf0-x241.google.com with SMTP id l131so4609433lfl.0 for ; Wed, 19 Oct 2016 17:06:24 -0700 (PDT) X-Received: by 10.25.89.138 with SMTP id n132mr8620035lfb.68.1476921983843; Wed, 19 Oct 2016 17:06:23 -0700 (PDT) Original-Received: from [192.168.1.2] (broadband-90-154-68-136.nationalcablenetworks.ru. [90.154.68.136]) by smtp.googlemail.com with ESMTPSA id 84sm11378518ljf.33.2016.10.19.17.06.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 19 Oct 2016 17:06:23 -0700 (PDT) In-Reply-To: <3106991.qNzeP8ykmy@tjmaciei-mobl1> X-Original-Sender: andrey.semashev@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 2a00:1450:4010:c07::241 as permitted sender) smtp.mailfrom=andrey.semashev@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:28997 Archived-At: On 10/20/16 02:02, Thiago Macieira wrote: > Em quarta-feira, 19 de outubro de 2016, =C3=A0s 13:07:31 PDT, Myriachan e= screveu: >> On Wednesday, October 19, 2016 at 11:09:17 AM UTC-7, Thiago Macieira wro= te: >>> Actually, the compiler is required to perform the cast to int, due to >>> integer >>> promotion rules. But the operation doesn't change either way. >> >> The promotion to int is also very important to consider in avoiding a >> certain case of undefined behavior: >> >> std::uint16_t x =3D 0xFFFF; >> x *=3D x; > > Indeed, this causes a signed integer overflow, which is UB. > > If you'd written: > > x *=3D unsigned(x); > > Then this is no longer UB. I've always wondered why the language had allowed the implicit=20 conversion to int in such cases. I mean, if it didn't the implementation=20 could have compiled the code to use full-width 32-bit instructions and=20 then slice the result and have the same performance anyway. But that=20 wouldn't introduce problems like in the OP or the UB above. --=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/2f77b3f0-eb09-537d-6590-59f4d68875c3%40gmail.com= .. .