220 38152 <5B0520CD.30204@gmx.net> article Path: news.gmane.org!.POSTED!not-for-mail From: Jens Maurer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: memcpy/memset/etc overloads for volatile memory Date: Wed, 23 May 2018 10:05:33 +0200 Lines: 62 Approved: news@gmane.org Message-ID: <5B0520CD.30204@gmx.net> References: <2300193.1RphyUdPvq@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" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1527062609 2395 195.159.176.226 (23 May 2018 08:03:29 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 23 May 2018 08:03:29 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux i686; rv:6.0.1) Gecko/20110830 Thunderbird/6.0.1 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDPMTYGK64JRBTWBSTMAKGQEQVL7SMY@isocpp.org Wed May 23 10:03:25 2018 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 1fLOk0-0000WH-Bi for gclcip-std-proposals@m.gmane.org; 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[212.227.15.18]) by mx.google.com with ESMTPS id e185-v6si1384848wmg.9.2018.05.23.01.05.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 23 May 2018 01:05:34 -0700 (PDT) Received-SPF: pass (google.com: domain of jens.maurer@gmx.net designates 212.227.15.18 as permitted sender) client-ip=212.227.15.18; Original-Received: from [192.168.2.101] ([188.101.152.160]) by mail.gmx.com (mrgmx001 [212.227.17.190]) with ESMTPSA (Nemesis) id 0MF5FT-1fb4Ew3JjX-00GHP7 for ; Wed, 23 May 2018 10:05:33 +0200 In-Reply-To: X-Provags-ID: V03:K1:wChMfsA4F8fQmZ0rQ3dRjvTc3sni4Pf0zfl6Gh8/Ugt9ahP/FYJ 0OSbr0bVMJ9OXNuDdUySasXI5OV4vgnnOQ5Dec9wahvLTuNR/wIiWdaoAk0NGxLeSBFI0OD pI+bbEO3H8tF9pOzDJALBq4/O55Hj8RUSt0cfLWPXDJKir4uIJ5vTFYw4+M07gogCtsS2wL WBOWuyJywBQe5mUt04mvA== X-UI-Out-Filterresults: notjunk:1;V01:K0:7vYJghFVFs8=:QeuLIVssy8r8wFEYEOppRl 00muSM1LtKneyWcdh1r5g1M6Pyjx/mZxlMHLqbqi5kOCjwL+gXGMTYKDkKzmAmmzPuWR34CZL UwPk4DsrAv2CjRkRw1PFkBg/It7d9J/KXl1dnIkYjwonULyiKGAF1ETLt8StrD2scYZ313T89 fUytFel1kTEPcdWxvLSr/yTDCDTrRtDD9+UCwra8QHPDC8yBgEbuuaopTQaVXAk7vjQuyg0y2 v2S9+6T8o6tRcr5NDyFD2n9Emwkhov2JNIJp5H4svfOQBABYhN0jPNr4PkJV82mMXUj5lwAq7 /NOlk/3pz8/zTnsvDv92dz0lIs5bihvx5Q2v0Jr1WXZL3FO1LbLDyH9yWbtoLHJuadgaODT/M 4eHPqoDto8gq07qemqZNX34VZc0DNMwAoCWIlIvJ57OIRNse357x2ZZhtZntudE5r3MEam4EE a1NI2KRNmSVGyCx8N2khT4yaRZwEAuS742/ydMJXizrA+SyvVcrHBRUKIEVepaXn2Agu9JfEg kVOWNuNs8CeG2UnEfjQOEdEjeQDQhc2QDb2hWkqfEmsQwnnM06DzFbhgDM4zh1uCijQp5VXIr z8cNTEewEZDy8ZhXmgOV6VmfZOyHWzix5veUgRMclbl8qWqtQVmDgC7DC71Nss0aly2w1Kz/L 8Y5CSVg8xG+H0Bs78sDfYTKA4Q1knRE41FeUoc2x5Buzb8KxVKXEusfPeapD6L9WhbucUUMoX jnfLqOQP+nO4KjobWVHmorNbyOsDv0oV+e6PboZ5UWy7qIWlch3LKs7a9I7gGbs2sFadZNGF X-Original-Sender: Jens.Maurer@gmx.net X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of jens.maurer@gmx.net designates 212.227.15.18 as permitted sender) smtp.mailfrom=Jens.Maurer@gmx.net Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:38152 Archived-At: On 05/19/2018 03:24 AM, dgutson . wrote: >=20 >=20 > El vie., 18 de mayo de 2018 21:16, Thiago Macieira > escribi=C3=B3: >=20 > On Friday, 18 May 2018 14:13:15 PDT Andrey Semashev wrote: > > What is "the right thing"? Does it have to be a byte-granular opera= tion? > > Does it have to be a forward iteration? Is this behavior the same o= n all > > implementations? You have to answer on all these questions in the > > standard wording so that everyone can rely on the exact memory acce= ss > > pattern, which is important in case of volatile memory. > > > > If it is not important in your case then you don't need volatile an= d > > hence the overload. >=20 > On the other hand, if your memory storage requires a specific behavio= ur which > cannot be standardised, maybe you should implement the copying direct= ly in > your code. It's not like bytewise memcpy and memset are particularly = difficult > to implement... >=20 >=20 > Oh yes you don't know how hard it can get... :) avoid prefetching, avoid = SSE/vectorization, ensure right order of operations (WC regions). > So there are lots of details that a too smart toolchain may mess. > However I will think if an execution policy can help here in copy. I'm no= t talking about copying but also memsetting. > The linux kernel has a nice memcpy_toio/fromio worth to look at. "volatile" only conveys a single bit of information, but modern architectures need a lot more data to actually do what you want if the "memory" isn't your usual kind of RAM with a cache hierarchy on top. For example, it might make a difference whether you copy bytewise or wordwise (and both options might be useful in different circumstances). For example, you might need to expressly synchronize writes with later reads. Since you mention the Linux kernel, note that it has a lot more than just a volatile memcpy to deal with all these issues. Maybe there's a general enough abstraction hidden in there somewhere that we could eventually standardize, but just having a volatile memcpy is neither here nor there. Jens --=20 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 e= mail 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/5B0520CD.30204%40gmx.net. .