220 21914 article Path: news.gmane.org!not-for-mail From: Oliver Kowalke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: is await an extension of the do-notation? (was Re: Re: Resumable expressions p0114r0 vs async/await P0057R0) Date: Wed, 21 Oct 2015 12:44:43 +0200 Lines: 181 Approved: news@gmane.org Message-ID: References: <639f0012-8cb4-4db3-82a6-8d042a3497f9@isocpp.org> <401aa118-ed0b-4c7b-92cf-3fbd706eff9d@isocpp.org> <1d15dbc4-e0c1-4df1-86db-014e11f14d26@isocpp.org> <56114953.4030701@wanadoo.fr> <561B403D.4030403@gmail.com> <561C07D1.3080304@gmail.com> <561C213F.1060009@gmail.com> <561C3875.2050701@gmail.com> <86924069-44d0-447f-bf8a-a42e46579832@isocpp.org> <561D79A7.8080007@gmail.com> <894616d6-9609-4038-b2bc-d2c62df21e0f@isocpp.org> <99bdb3ae-9214-4440-8572-4c47a61eda46@isocpp.org> <58628096-14ae-42b2-89d3-c0bd2183bbc0@isocpp.org> <561DC957.8080603@gmail.com> <2e09660f-d688-474a-8811-85fb041f68b8@isocpp.org> <8b78abb4-16b6-4b65-8513-6e2c17317647@isocpp.org> <779be861-3b2a-427a-b36b-ad2874c81129@isocpp.org> <767364f8-785d-4f8f-9bc0-852df56ee4eb@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=089e015383e818e0ed05229b123b X-Trace: ger.gmane.org 1445424308 1524 80.91.229.3 (21 Oct 2015 10:45:08 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 21 Oct 2015 10:45:08 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCLZHP5J7MHRBLWZTWYQKGQECPDKVSY@isocpp.org Wed Oct 21 12:45:07 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f69.google.com ([209.85.220.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZoqtE-0001sq-S3 for gclcip-std-proposals@m.gmane.org; Wed, 21 Oct 2015 12:45:05 +0200 Original-Received: by padda3 with SMTP id da3sf15351738pad.0 for ; Wed, 21 Oct 2015 03:45:03 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:content-type:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=dBuLhrke8WpuaQ5MAQ2uDMaMgYuzWEr3NPPRE+Wb+RE=; b=NfJxImESAXMsb1Bk7ne+brjk/05ebf24+jDtIJs+5ZzQR5g5nsLjRCXQhIRCJuG6BF 13cFCVYuZCL4jzs7cPeYugAy7sP8xh/ETvx2DsRLL53ajeK+zkMgV6XPqsaN5xKVkzb0 b3GDeyNsMLbj1g/yu+X0QYV6x32U/CGizrdvLe9RIhCammFuLLrG59bRt8Prz87OKfpr WRPOEgFeT9k58PXpaj3Aw8Ke76vvmtB/wqz3vnPvOpUqOhW7CCpqdiJSZA35Bx7twncQ uR0asAHhhIrIMHOAqm/c2kuMFtGSqQP78/g5xFrlfg7aflayoVEb9tZLZ11161ejM+9S BYHA== X-Gm-Message-State: ALoCoQmkOXEOgCr8MUJijJpy/590CqiQnG7dIXqkVmdPiaweclyxYVX7LSKT4gnrPSLPZRkKd7Tq X-Received: by 10.66.137.43 with SMTP id qf11mr6960135pab.1.1445424303744; Wed, 21 Oct 2015 03:45:03 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.81.33 with SMTP id w1ls398690obx.68.gmail; Wed, 21 Oct 2015 03:45:02 -0700 (PDT) X-Received: by 10.202.63.212 with SMTP id m203mr5259608oia.15.1445424302717; Wed, 21 Oct 2015 03:45:02 -0700 (PDT) Original-Received: from mail-ob0-x229.google.com (mail-ob0-x229.google.com. [2607:f8b0:4003:c01::229]) by mx.google.com with ESMTPS id p67si4871829oib.97.2015.10.21.03.45.02 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 21 Oct 2015 03:45:02 -0700 (PDT) Received-SPF: pass (google.com: domain of oliver.kowalke@gmail.com designates 2607:f8b0:4003:c01::229 as permitted sender) client-ip=2607:f8b0:4003:c01::229; Original-Received: by obbwb3 with SMTP id wb3so37531743obb.0 for ; Wed, 21 Oct 2015 03:45:02 -0700 (PDT) X-Received: by 10.182.191.38 with SMTP id gv6mr5307286obc.7.1445424302447; Wed, 21 Oct 2015 03:45:02 -0700 (PDT) Original-Received: by 10.76.71.233 with HTTP; Wed, 21 Oct 2015 03:44:43 -0700 (PDT) In-Reply-To: X-Original-Sender: oliver.kowalke@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of oliver.kowalke@gmail.com designates 2607:f8b0:4003:c01::229 as permitted sender) smtp.mailfrom=oliver.kowalke@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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-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:21914 Archived-At: --089e015383e818e0ed05229b123b Content-Type: text/plain; charset=UTF-8 2015-10-21 11:09 GMT+02:00 Giovanni Piero Deretta : > if you refer to class execution_context from boost.context - it allows to >> pass data through a context switch. >> > > to be clear, I meant the execution_context as described in P0099R0. I just > reread it and as far as I can tell doesn't have a native facility for > parameter passing. I believe this is an important functionality, and the > switching primitive should at least allow passing a void* through. > yes, P0099R0 does not reflect this change - working on boost.fiber I encountered the need for passing parameters through the context switch. I added this functionality to boost.context - at the moment only of type void pointer. > > >> execution_context::current() is a thread-local, local static pointer to a >> execution_context. >> P0099R1 will probably reflect this. >> > > I still do not see what's the use case for current(). In a way it is a > wrong concept: an execution_context captures the state of an *halted* > thread of execution, so there is no valid execution_context state for the > currently running thread of execution. > for me, an execution context consits out of a stack and a set of CPU registers (stack pointer, instruction pointer, some other registers defined by the ABI). if you start/enter main() or start/enter a thread (thread-fn) you already have an execution context. You need a way to access this context. Especially in teh case of userland-threads/fibers the main execution-context can be handled as a fiber too. that's the case for boost.fiber were the main context (running main()) is handled as the other threads (no difference). That means I need a way to assign and access the main context. Without execution_context:current() I would need to hold and pass the current context to each place where it is need. For instance you can run a fiber (from boost.fiber) and executing coroutines from boost.coroutine2. both access the same current execution_context. Another example, boost.fiber allows directly call boost::this_fiber::yiled() in main() without to invoke any initialization functionality directly. > > >> sure that the simple context switch will take < 10 CPU cycles - but you >> need some maintenance code to >> >> > what maintenance code exactly? I've just looked at boost.context > implementation and the operator() seems pretty minimal. > the context switch using jump_fcontext() takes < 10 CPU cycles, while execution_context::operator() need more cycles because it has to update TLS. > My "fast" context switching implementation uses GCC inline assembly and a > large list of clobbered registers so that the switching code itself does > not need to conservatively save and restore all callee saved registers; The > code works and is fast, but it is tricky and flaky (I would not recommend > it for a pure library implementation, depending on the version, GCC might > not like the large clobber list and you tend to hit dark corners of the > optimiser) but it shows that with compiler help context switch does not > need to be slow. > yes, but boost.context was required to support a broad range of compilers and unfortunately MS decided to drop inline assembler for 64Bit MS VC. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. --089e015383e818e0ed05229b123b Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
2015= -10-21 11:09 GMT+02:00 Giovanni Piero Deretta <gpderetta@gmail.com&g= t;:
if you refer to class execution_context from boost.context - it allow= s to pass data through a context switch.

to be clear, I meant the execution_context as describ= ed in P0099R0. I just reread it and as far as I can tell doesn't have a= native facility for parameter passing. I believe this is an important func= tionality, and the switching primitive should at least allow passing a void= * through.

yes, P0099R0 does not = reflect this change - working on boost.fiber I encountered the need for pas= sing parameters through the context switch.
I added this func= tionality to boost.context - at the moment only of type void pointer.
=C2=A0
=C2=A0
execution_context::current() is a = thread-local, local static pointer to a execution_context.
P0= 099R1 will probably reflect this.
<= /span>

I still do not see what's the use case for current(). In= a way it is a wrong concept: an execution_context captures the state of an= *halted* thread of execution, so there is no valid execution_context state= for the currently running thread of execution.

<= /div>
for me, an execution context consits out of= a stack and a set of CPU registers (stack pointer, instruction pointer, so= me other registers defined by the ABI).
if you start/enter main() or start/enter a thread (thread-fn) you already = have an execution context. You need a way to access this context.
=
Especially in teh case of userland-threads/fiber= s the main execution-context can be handled as a fiber too. that's the = case for boost.fiber were the main context (running main())
is handled as the other threads (no difference). That = means I need a way to assign and access the main context. Without execution= _context:current()
I would need to hold= and pass the current context to each place where it is need.
For instance you can run a fiber (from boost.fiber) = and executing coroutines from boost.coroutine2. both access the same curren= t execution_context.
Another example, b= oost.fiber allows directly call boost::this_fiber::yiled() in main() withou= t to invoke any initialization functionality directly.
=C2=A0
=C2= =A0
=C2=A0sure that the simple co= ntext switch will take < 10 CPU cycles - but you need some maintenance c= ode to


what mainten= ance code exactly? I've just looked at boost.context implementation and= the operator() seems pretty minimal.

=
the context switch using jump_fcontext() takes < 10 CPU cycles, whi= le execution_context::operator() need more cycles because it has to update = TLS.
=C2=A0
My "= ;fast" context switching implementation uses GCC inline assembly and a= large list of clobbered registers so that the switching code itself does n= ot need to conservatively save and restore all callee saved registers; The = code works and is fast, but it is tricky and flaky (I would not recommend i= t for a pure library implementation, depending on the version, GCC might no= t like the large clobber list and you tend to hit dark corners of the optim= iser) but it shows that with compiler help context switch does not need to = be slow.

y= es, but boost.context was required to support a broad range of compilers an= d unfortunately MS decided to drop inline assembler for 64Bit MS VC.

--

---
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/<= /a>.
--089e015383e818e0ed05229b123b-- .