220 35603 <1938158.E5InEKLV7f@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Rotational shift operator Date: Wed, 29 Nov 2017 10:50:39 -0800 Lines: 55 Approved: news@gmane.org Message-ID: <1938158.E5InEKLV7f@tjmaciei-mobl1> References: <69593098-e2cc-487e-bd14-9001b27aa7c1@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 1511981455 10925 195.159.176.226 (29 Nov 2017 18:50:55 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 29 Nov 2017 18:50:55 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBB4D7TIAKGQEUTXBVAQ@isocpp.org Wed Nov 29 19:50:46 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f197.google.com ([209.85.220.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eK7RR-0001y4-2X for gclcip-std-proposals@m.gmane.org; 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[2001:19f0:5:2221::1:3]) by mx.google.com with ESMTPS id x3si2379686qtf.197.2017.11.29.10.50.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 29 Nov 2017 10:50:47 -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 (jfdmzpr04-ext.jf.intel.com [134.134.139.73]) by smtp.macieira.info (Postfix) with ESMTPSA id 4A4F12DA6 for ; Wed, 29 Nov 2017 10:50:46 -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:35603 Archived-At: On Wednesday, 29 November 2017 09:43:20 PST Hyman Rosen wrote: > A. If the platform always causes a trap when indirecting through a null > pointer, > the check can be eliminated. That's the case. Therefore, the check can be eliminated. The special case of the Linux kernel was that on some MCU, under some special circumstance, the zero page *was* mapped and therefore didn't trap. So the GCC developers added a special flag to allow that particular case to work. In other words, this is *exactly* what you're asking for. > Notice your own contradictory claims - you first say that the compiler > should > assume that programmers know what they are doing, then you say that > compilers > should eliminate code that those programmers have written. The compiler should assume programmers know what they are doing, which means programmers don't write UB code. Since they don't, the compiler is allowed to discard any UB from the list of possibilities. The example from Qt was more or less like this: if (d->size + newitems > d->alloc) realloc(d->size + newitems); However, due to constant propagation through the inlining, it knows that d->alloc = d->size and it knows newitems is a constant value (1). So it reduced the expression to: if (d->size + 1 > d->size) Since signed integer overflow is UB, the compiler is allowed to assume I've written proper code to ensure that d->size + 1 will not overflow (I have), in which case the comparison is a success. I agree that if the compiler cannot tell things apart, it should leave the comparison alone. But it *could* tell things apart. So it should optimise the comparison and not leave a useless compare-and-branch which will never be true. -- 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/1938158.E5InEKLV7f%40tjmaciei-mobl1. .