220 35627 article Path: news.gmane.org!.POSTED!not-for-mail From: Hyman Rosen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Rotational shift operator Date: Wed, 29 Nov 2017 16:47:03 -0500 Lines: 92 Approved: news@gmane.org Message-ID: References: <69593098-e2cc-487e-bd14-9001b27aa7c1@isocpp.org> <8176d75b-8df7-40ba-af35-d8213937d5bc@isocpp.org> <1873023.0OMUIGm3aL@tjmaciei-mobl1> <4a4ac39b-d14f-8aea-1739-6c819cd1f8c6@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="089e082fa0ecbb74e8055f26144b" X-Trace: blaine.gmane.org 1511992049 23284 195.159.176.226 (29 Nov 2017 21:47:29 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 29 Nov 2017 21:47:29 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDKJJPVBQMKRB3OV7TIAKGQE5RCHR7Q@isocpp.org Wed Nov 29 22:47:25 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f199.google.com ([209.85.128.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eKACN-0005NL-ED for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id z66sor770538wmb.10.2017.11.29.13.47.25 for (Google Transport Security); Wed, 29 Nov 2017 13:47:25 -0800 (PST) Received-SPF: pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.28.190.12 with SMTP id o12mr158218wmf.148.1511992044777; Wed, 29 Nov 2017 13:47:24 -0800 (PST) Original-Received: by 10.28.57.214 with HTTP; Wed, 29 Nov 2017 13:47:03 -0800 (PST) In-Reply-To: <4a4ac39b-d14f-8aea-1739-6c819cd1f8c6@gmail.com> X-Original-Sender: hyman.rosen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=VQESUlIz; spf=pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hyman.rosen@gmail.com; dmarc=pass (p=NONE sp=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:35627 Archived-At: --089e082fa0ecbb74e8055f26144b Content-Type: text/plain; charset="UTF-8" On Wed, Nov 29, 2017 at 2:00 PM, Matthew Woehlke wrote: > On 2017-11-29 12:43, Hyman Rosen wrote: > > If I write *p = 1; if (!p) foo(); then the compiler should > > not eliminate the test. > > Why not? In no case will `foo()` execute. If `p`, the condition is > false. If `!p`, the condition is irrelevant because the preceding line > will cause a SEGV. You are making assumptions that don't always hold. On SunOS, given the following program pz.cpp: int main() { char *volatile p = 0; return *p; } built like this: gcc -o pz pz.cpp /usr/lib/0@0.so.1 running it does not cause a SEGV: % ./pz % echo $? 0 -- 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/CAHSYqdbtHNzYeJnvPwnLTkFt4auuRQt2BLTkCR-hn1bW0MO9OA%40mail.gmail.com. --089e082fa0ecbb74e8055f26144b Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On W= ed, Nov 29, 2017 at 2:00 PM, Matthew Woehlke <mwoehlke.floss@gmail= ..com> wrote:
On 2017-11-29 12:43, Hyman Rosen wrote:
> If I write *p =3D 1; if (!p) foo(); then the compiler should
> not eliminate the test.

Why not? In no case will `foo()` execute. If `p`, the condition is false. If `!p`, the condition is irrelevant because the preceding line
will cause a SEGV.

You are making assumptions that don= 't always hold.=C2=A0

On SunOS, given the following progr= am pz.cpp:
=
int main() { char *volatile p =3D 0; return *p; }
built like this:
gcc -o= pz pz.cpp /usr/lib/0@0.so.1
running it does not cause= a SEGV:
% ./pz
% echo $?
0

=C2=A0

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