220 38129 article Path: news.gmane.org!.POSTED!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: memcpy/memset/etc overloads for volatile memory Date: Sat, 19 May 2018 00:13:15 +0300 Lines: 57 Approved: news@gmane.org Message-ID: References: <0e5ce74c-3e9e-4cf2-4afd-f7333ed82c5d@gmail.com> 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 1526677871 18067 195.159.176.226 (18 May 2018 21:11:11 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 18 May 2018 21:11:11 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCXY5TXHWYJRB3MD7XLQKGQEEHCBORA@isocpp.org Fri May 18 23:11:07 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f72.google.com ([209.85.215.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fJmeY-0004ca-Jo for gclcip-std-proposals@m.gmane.org; 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[109.173.28.102]) by smtp.googlemail.com with ESMTPSA id v64-v6sm1385403lje.56.2018.05.18.14.13.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 18 May 2018 14:13:15 -0700 (PDT) In-Reply-To: Content-Language: en-US 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=BTo7+eGh; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=andrey.semashev@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com 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:38129 Archived-At: On 05/18/18 23:53, dgutson . wrote: > > On Fri, May 18, 2018 at 5:46 PM, Andrey Semashev > > wrote: > > On 05/18/18 23:41, dgutson . wrote: > > This might be interesting. > > In some systems, the implementation of memcpy() & friends cannot > be used for memory-mapped devices. > Usually, mapped memory is marked as volatile. > > I'm proposing to add volatile pointers overloads for the > memcpy() function and friends, without specifying exactly what > should be different at the standard-level, but allowing > implementations to provide a particular behavior. I'm just > proposing to add the declaration to the standard. > > Particularly, in some x86-based systems, reading things like the > SPI should be done on byte-per-byte basis (rather than streamed > with any SIMD instruction). > > I'm not sure what the wording should exactly say, but I'm just > initiating the discussion. > > If it's not specified then the exact behavior wrt. memory accesses > is not going to be portable, which means it's not going to be useful. > > It is useful indeed: if I have a pointer to volatile memory, I cannot > use memcpy currently. > By adding the overload, I'm being guaranteed that I will be able to call > memcpy (and that the implementation shall DTRT, which is for example, > the byte per byte copying). > Besides the "DTRT" discussion, my point is: I want this overload to be > available. At least it should fall back to the non-volatile version. What is "the right thing"? Does it have to be a byte-granular operation? Does it have to be a forward iteration? Is this behavior the same on all implementations? You have to answer on all these questions in the standard wording so that everyone can rely on the exact memory access pattern, which is important in case of volatile memory. If it is not important in your case then you don't need volatile and hence the overload. Personally, I don't see much use in such an overload. Where memory access pattern is important I would rather spell it explicitly or at least use std::copy, which, unlike memcpy, does define the order of iteration and granularity. -- 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/dc37b3f7-74b8-b94f-0a1c-59bb99ecbccc%40gmail.com. .