220 35508 <3031585.uCQqKE2Qzv@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: Fri, 24 Nov 2017 21:37:32 -0800 Lines: 40 Approved: news@gmane.org Message-ID: <3031585.uCQqKE2Qzv@tjmaciei-mobl1> References: <69593098-e2cc-487e-bd14-9001b27aa7c1@isocpp.org> <1527647.07Qi8WvUZa@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 1511588255 21238 195.159.176.226 (25 Nov 2017 05:37:35 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 25 Nov 2017 05:37:35 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBH4D4TIAKGQE3R374EA@isocpp.org Sat Nov 25 06:37:31 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f200.google.com ([209.85.128.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eIT9d-00053E-0F for gclcip-std-proposals@m.gmane.org; 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[78.47.120.188]) by mx.google.com with ESMTP id 140si8031386wmv.53.2017.11.24.21.37.35 for ; Fri, 24 Nov 2017 21:37:35 -0800 (PST) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 78.47.120.188 as permitted sender) client-ip=78.47.120.188; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:10a8:5dac:57fd:2747]) by gondolin.macieira.info (Postfix) with ESMTPSA id 4FC3111A585 for ; Fri, 24 Nov 2017 21:37:34 -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 78.47.120.188 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:35508 Archived-At: 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. [*] unless the processor is smart enough to do macro-op fusing of three instructions. The Intel x86 processors' microcode can fuse CMP+Jcc, but I don't know of three-instruction fusing. -- 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/3031585.uCQqKE2Qzv%40tjmaciei-mobl1. .