220 35726 <2686086.MHsyEc3O6d@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: Sun, 03 Dec 2017 22:28:15 -0800 Lines: 69 Approved: news@gmane.org Message-ID: <2686086.MHsyEc3O6d@tjmaciei-mobl1> References: <69593098-e2cc-487e-bd14-9001b27aa7c1@isocpp.org> <2168917.U4FlfNKMk3@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" X-Trace: blaine.gmane.org 1512368900 19037 195.159.176.226 (4 Dec 2017 06:28:20 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 4 Dec 2017 06:28:20 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBAWWSPIQKGQEWZ23APY@isocpp.org Mon Dec 04 07:28:16 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f197.google.com ([209.85.216.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eLkEe-0004WS-LM for gclcip-std-proposals@m.gmane.org; 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[209.222.10.136]) by mx.google.com with ESMTPS id q190si4480869qkf.453.2017.12.03.22.28.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 03 Dec 2017 22:28:18 -0800 (PST) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 209.222.10.136 as permitted sender) client-ip=209.222.10.136; Original-Received: from tjmaciei-mobl1.localnet (fmdmzpr04-ext.fm.intel.com [192.55.54.39]) by smtp.macieira.info (Postfix) with ESMTPSA id 4969B3416 for ; Sun, 3 Dec 2017 22:28:17 -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 209.222.10.136 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:35726 Archived-At: On Sunday, 3 December 2017 22:19:08 PST Hyman Rosen wrote: > On Mon, Dec 4, 2017 at 12:31 AM, Thiago Macieira > > wrote: > > Let's try this one: > > extern const int v; > > const int v = 0; > > void f() { if (v) foo(); else bar(); } > > > > Should the compiler remove the test for v and make an unconditional call > > to > > foo()? > > Yes. Why? See that "extern" on the line above? It means the variable v exists in memory somewhere. Since that's the case, I can mremap() the page where it exists and change its value from 0 to non-zero. According to you, the compiler should obey the programmer blindly so long as something is possible in hardware. This is. > This is a trick question. > > No, actually it's not. > > If you answer "yes", then you're being specious. You're fine with some > > optimisations but not others. if you answer "no", you're being crazy. > > As I have said over and over, optimization based on propagating values along > code paths and determining that certain relations must hold is perfectly > fine. The > thing that is wrong is assuming that undefined behavior does not happen and > using > that assumption to eliminate code where the compiler sees undefined behavior > must occur. Do you want to revise that statement after my hack above? > > Optimisation must not change the meaning of the code, and the meaning of > > that > > comparison is an always false. > > Wrong. a + 1 < a can be true only is signed overflow occurs. It's always false because we said it is. Therefore it is. If you change the rules, then the results change. But the rules have not changed. > > The compiler should believe the programmer and > > understand that the comparison was intended to always be false. > > There was a real programmer who wrote that code. I think, therefore I am real. I wrote some code that the compiler concluded was a signed integer comparison to itself. I know it couldn't overflow. The compiler believed me. -- 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/2686086.MHsyEc3O6d%40tjmaciei-mobl1. .