220 35513 <9a7a898c-9e14-532f-fc4a-11d0aa5a1bf7@gmail.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Rotational shift operator Date: Sat, 25 Nov 2017 15:08:07 +0300 Lines: 43 Approved: news@gmane.org Message-ID: <9a7a898c-9e14-532f-fc4a-11d0aa5a1bf7@gmail.com> References: <69593098-e2cc-487e-bd14-9001b27aa7c1@isocpp.org> <1527647.07Qi8WvUZa@tjmaciei-mobl1> <3031585.uCQqKE2Qzv@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 X-Trace: blaine.gmane.org 1511611690 22238 195.159.176.226 (25 Nov 2017 12:08:10 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 25 Nov 2017 12:08:10 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCXY5TXHWYJRBKN24XIAKGQEAIDT2LA@isocpp.org Sat Nov 25 13:08:06 2017 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 1eIZFc-0005Ov-2v for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id u3sor3312324lju.11.2017.11.25.04.08.09 for (Google Transport Security); Sat, 25 Nov 2017 04:08:09 -0800 (PST) Received-SPF: pass (google.com: domain of andrey.semashev@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.46.66.140 with SMTP id h12mr11139475ljf.140.1511611689018; Sat, 25 Nov 2017 04:08:09 -0800 (PST) Original-Received: from [192.168.1.2] (broadband-178-140-150-13.moscow.rt.ru. [178.140.150.13]) by smtp.googlemail.com with ESMTPSA id f126sm4080914lff.52.2017.11.25.04.08.07 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 25 Nov 2017 04:08:07 -0800 (PST) In-Reply-To: <3031585.uCQqKE2Qzv@tjmaciei-mobl1> Content-Language: ru X-Original-Sender: andrey.semashev@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=DnGc4qZK; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=andrey.semashev@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:35513 Archived-At: On 11/25/17 08:37, Thiago Macieira wrote: > On sexta-feira, 24 de novembro de 2017 21:14:19 PST Andrey Semashev wrote: >> On 11/25/17 03:09, Thiago Macieira wrote: >>> On sexta-feira, 24 de novembro de 2017 10:46:09 PST Nicol Bolas wrote: >>>> Bit-*rotation* is a less common operation than bit shifting. >>> >>> Proof: the fact that there's no built-in rotation operator in any major >>> language, not even in C that was supposed to be "portable assembly". >>> Especially in the olden days when you coudn't count on the compilers >>> understanding what you meant. >>> >>> And if you want an even less common operation: rotate with carry. >> >> That's not exactly a proof, more of a conservative approach of language >> designers. Every widespread CPU architecture includes rotation >> instructions because theys are important in some areas. > > No doubt they are, because unlike a high-level language, executing a rotation > in assembly couldn't be executed in one cycle due to the data dependency[*], > not to mention the need to use one extra register, possibly two. A dedicated > instruction for rotation (and one more for rotate-with-carry) can probably > execute in one cycle by just reusing the barrel-shifter. > > The high-level language doesn't need that because the compilers are smart > enough to notice the pattern of two-shifts-and-or as a rotation and write > assembly accordingly. Well, the compilers were not always that smart, and if we had the standard rotation functions or operators from the start, people wouldn't have to write assembler code and compilers wouldn't need to be taught to recognize certain code patterns. Reliability of this recognition has always been and will be a question of QoI, i.e. something that cannot be relied on. I'm not arguing for operators or functions approach here. I'm saying that dedicated rotation operations are long overdue in C and C++. -- 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/9a7a898c-9e14-532f-fc4a-11d0aa5a1bf7%40gmail.com. .