220 38131 article Path: news.gmane.org!.POSTED!not-for-mail From: "dgutson ." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: memcpy/memset/etc overloads for volatile memory Date: Fri, 18 May 2018 22:24:37 -0300 Lines: 153 Approved: news@gmane.org Message-ID: References: <2300193.1RphyUdPvq@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="0000000000005fecf4056c84ef4a" X-Trace: blaine.gmane.org 1526692971 26178 195.159.176.226 (19 May 2018 01:22:51 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 19 May 2018 01:22:51 +0000 (UTC) To: std-proposals Original-X-From: std-proposals+bncBDE3NBMV6UFBBY7Z7XLQKGQETY6SXCY@isocpp.org Sat May 19 03:22:47 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f200.google.com ([209.85.220.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fJqa2-0006bM-0x for gclcip-std-proposals@m.gmane.org; Sat, 19 May 2018 03:22:42 +0200 Original-Received: by mail-qk0-f200.google.com with SMTP id f19-v6sf514404qkm.23 for ; 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[209.85.220.41]) by mx.google.com with SMTPS id x25-v6sor6925201qto.101.2018.05.18.18.24.51 for (Google Transport Security); Fri, 18 May 2018 18:24:51 -0700 (PDT) Received-SPF: pass (google.com: domain of danielgutson@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:ac8:2fe1:: with SMTP id m30-v6mr11973912qta.43.1526693091075; Fri, 18 May 2018 18:24:51 -0700 (PDT) In-Reply-To: <2300193.1RphyUdPvq@tjmaciei-mobl1> X-Original-Sender: danielgutson@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=RbM641x1; spf=pass (google.com: domain of danielgutson@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=danielgutson@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:38131 Archived-At: --0000000000005fecf4056c84ef4a Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable El vie., 18 de mayo de 2018 21:16, Thiago Macieira escribi=C3=B3: > 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 operation= ? > > Does it have to be a forward iteration? Is this behavior the same on al= l > > 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. > > On the other hand, if your memory storage requires a specific behaviour > which > cannot be standardised, maybe you should implement the copying directly i= n > your code. It's not like bytewise memcpy and memset are particularly > difficult > to implement... > 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 not talking about copying but also memsetting. The linux kernel has a nice memcpy_toio/fromio worth to look at. > It's the SIMD-optimised with cacheline-alignment and such that are. > > -- > 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/2300193.1Rph= yUdPvq%40tjmaciei-mobl1 > . > --=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/CAFdMc-0wber%2BdL_ugAcg7OBxLoajOOU8mMqLmS-aKTWHR= cCghQ%40mail.gmail.com. --0000000000005fecf4056c84ef4a Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


= El vie., 18 de mayo de 2018 21:16, Thiago Macieira <thiago@macieira.org> escribi=C3=B3:
On Friday, 18 May 2018 14:13:15 PDT Andrey Semas= hev wrote:
> What is "the right thing"? Does it have to be a byte-granula= r operation?
> Does it have to be a forward iteration? Is this behavior the same on a= ll
> implementations? You have to answer on all these questions in the
> standard wording so that everyone can rely on the exact memory access<= br> > pattern, which is important in case of volatile memory.
>
> If it is not important in your case then you don't need volatile a= nd
> hence the overload.

On the other hand, if your memory storage requires a specific behaviour whi= ch
cannot be standardised, maybe you should implement the copying directly in =
your code. It's not like bytewise memcpy and memset are particularly di= fficult
to implement...

Oh yes you don't know how hard it can get... :) avoid pr= efetching, avoid SSE/vectorization, ensure right order of operations (WC re= gions).
So there are lots of details that a too smar= t toolchain may mess.
However I will think if an exe= cution policy can help here in copy. I'm not talking about copying but = also memsetting.
The linux kernel has a nice memcpy_= toio/fromio worth to look at.



It's the SIMD-optimised with cacheline-alignment and such that are.

--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org=
=C2=A0 =C2=A0Software Architect - Intel Open Source Technology Center



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