220 39332 article Path: news.gmane.org!.POSTED!not-for-mail From: David Brown Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Alternative proposal for mapping P0709 Deterministic Exceptions into C Date: Tue, 24 Jul 2018 11:16:14 +0200 Lines: 38 Approved: news@gmane.org Message-ID: References: <6a65c934-5d2a-4e75-b88d-9eaaee338bd3@isocpp.org> <87240a3d-623f-4f7f-8e7c-fa8c9482fa72@isocpp.org> <5a449c86-b0f4-4b07-b5d6-21b7adbfd63b@isocpp.org> 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 1532423659 16191 195.159.176.226 (24 Jul 2018 09:14:19 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 24 Jul 2018 09:14:19 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDP6R2XHXIARB2G43PNAKGQEET5FTYQ@isocpp.org Tue Jul 24 11:14:14 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lj1-f198.google.com ([209.85.208.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fhtOX-00045x-LQ for gclcip-std-proposals@m.gmane.org; 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Original-Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1fhtOU-00042q-Ok for std-proposals@isocpp.org; Tue, 24 Jul 2018 11:14:10 +0200 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 31 Original-X-Complaints-To: usenet@blaine.gmane.org In-Reply-To: <5a449c86-b0f4-4b07-b5d6-21b7adbfd63b@isocpp.org> X-Original-Sender: david@westcontrol.com X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 195.159.176.226 is neither permitted nor denied by best guess record for domain of gclcip-std-proposals@m.gmane.org) smtp.mailfrom=gclcip-std-proposals@m.gmane.org 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:39332 Archived-At: On 23/07/18 10:11, Niall Douglas wrote: > Overall, do we really need this 'expected' kind of type to be quite > so constrained and magic, to require an even address etc. etc. > In other words, if the type was actually something that was > specified as a 'normal' C/C++ struct without any of those gymnastics > and data packing or require some carry register etc., would the code > be that dramatically worse that we should require such magic or > constraints than a regular type without all the magic and alignment > requirements? > > Not speaking for the above proposal, but in general, it is currently > believed that using something like the CPU carry bit ought to be zero > overhead on most CPUs, whereas a discriminant stored in the layout would > not be zero overhead on more CPUs. For some cpus, there are cheap flags like this. (Some cpus even have "return with carry flag set" and "return with carry flag clear" instructions that are tailor-made for such usage.) On many bigger cpus, however, working with flags can cause stalls in the processor pipelines - the flag becomes a bottleneck dependency. It is precisely for that reason that RISC-V and some other newer designs have no flags. > > The situation of RISC-V, which has no status register, is a particular > unknown. Our current best guess is that either we keep the discriminant > in the structure, or we fiddle with the return address e.g. offset it by > four. There are no good options on that architecture. > > Niall > -- 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/pj6qkr%24elo%241%40blaine.gmane.org. .