220 35575 <1887594.mdY3tAJtOz@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Rotational shift operator Date: Mon, 27 Nov 2017 15:49:15 -0800 Lines: 56 Approved: news@gmane.org Message-ID: <1887594.mdY3tAJtOz@tjmaciei-mobl1> References: <69593098-e2cc-487e-bd14-9001b27aa7c1@isocpp.org> <38eba22a-1305-4b45-86a8-2d2e3c9f26ae@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1511826560 16743 195.159.176.226 (27 Nov 2017 23:49:20 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 27 Nov 2017 23:49:20 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRB7OI6LIAKGQEIPL52CQ@isocpp.org Tue Nov 28 00:49:15 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f198.google.com ([209.85.216.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eJT9D-0003es-6M for gclcip-std-proposals@m.gmane.org; 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[2001:19f0:5:2221::1:3]) by mx.google.com with ESMTPS id z81si11269923qkz.106.2017.11.27.15.49.17 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 27 Nov 2017 15:49:17 -0800 (PST) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 2001:19f0:5:2221::1:3 as permitted sender) client-ip=2001:19f0:5:2221::1:3; Original-Received: from tjmaciei-mobl1.localnet (jfdmzpr03-ext.jf.intel.com [134.134.139.72]) by smtp.macieira.info (Postfix) with ESMTPSA id 93C242AC9 for ; Mon, 27 Nov 2017 15:49:16 -0800 (PST) In-Reply-To: X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2001:19f0:5:2221::1:3 as permitted sender) smtp.mailfrom=thiago@macieira.org 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:35575 Archived-At: On Monday, 27 November 2017 14:15:48 PST Hyman Rosen wrote: > C compilers were intended to be "close to the machine," so their arithmetic > was supposed to be "whatever the machine did." No, they were not. C was designed to be "portable assembly", which meant it's not "whatever the machine did" but instead a known set of rules known to work the same on all machines. Sometimes that meant reducing to a subset of what the machine could not, sometimes it meant some machines needed to do more work compared to others. > Signed overflow was > formally undefined because compilers could run on hardware with > 1's-complement arithmetic, or with hardware traps on overflow, or whatever, > and defining the effects in the language could badly penalize programs > running on hardware that really wanted to do the arithmetic differently. Right. > That's qualitatively different from compilers deliberately adopting a model > not represented by the hardware in order to *cause* programs to have > undefined behavior. On a processor where signed overflow naturally wraps > around (which is virtually all of them), the compiler should not make > signed overflow be undefined behavior and then eliminate code which checks > for overflow after the fact. I understand you believe this, but that's not a shared belief. Others believe that, since the language says you cannot do something, compilers are free to assume you did not do it. > And programmers have from time immemorial wanted to overlay different types > on the same area of memory and interpret the object representation of one > type as the object representation of another. In Fortran, they used > EQUIVALENCE to do that. In C and C++, they used unions or just plain old > casting from one pointer type to another. (That's how the X Window event > system works.) Taking that away in the formal language definition doesn't > change the fact that people want to do this, have written programs that do > it, and that those programs break at the whim of compiler writers, who are > there in the meetings to make sure that nothing gets into the standard that > would "negatively impact optimization." I agree we need a way to fix this, since we need to do that. That means specifying the behaviour properly, not getting rid of all UB. -- Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org Software Architect - Intel Open Source Technology Center -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/1887594.mdY3tAJtOz%40tjmaciei-mobl1. .