220 39336 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 15:16:42 +0200 Lines: 35 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> <4e6835ac-6975-4c89-81c4-fc038d004cb7@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 1532438089 24015 195.159.176.226 (24 Jul 2018 13:14:49 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 24 Jul 2018 13:14:49 +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+bncBDP6R2XHXIARBR6N3TNAKGQEPMY4LYY@isocpp.org Tue Jul 24 15:14:45 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lj1-f200.google.com ([209.85.208.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fhx9J-000693-24 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 1fhx9D-000627-U9 for std-proposals@isocpp.org; Tue, 24 Jul 2018 15:14:39 +0200 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 28 Original-X-Complaints-To: usenet@blaine.gmane.org In-Reply-To: <4e6835ac-6975-4c89-81c4-fc038d004cb7@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:39336 Archived-At: On 24/07/18 12:38, Niall Douglas wrote: > The pipeline bubbles you refer to only occur when testing more than > one CPU flag at a time. One of the main reasons for choosing the > carry bit as return discriminant is because there are instructions > for testing just the carry bit alone on all such architectures. Thus > such partial flags stalls will never occur in this specific use > case. > I am reluctant to be so confident about the effects here - I really don't think you can generalise and say these stalls will never occur. I think it will depend a lot on the cpu ISA, the details of its implementation (scheduling, out-of-order execution, register renaming, pipelining, speculative execution, etc.), and the type of code you are dealing with. On many cpus, there are quite a lot of instructions that use or affect the carry flag (or any other flags) - these pose a challenge for scheduling. They also pose a challenge for the compiler, if it would want to use such instructions between the call to "foo" and the check for "foo.error". It may be fair to say that using the carry flag here would not cause much in the way of stalling or delays - but I would not say "never" without a lot more thorough analysis with a range of different processors. On a related point, many compilers do not track the state of flags across instruction pattern boundaries - /any/ use of a carry flag like this would involve significant engineering effort. -- 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/pj78nm%24loi%241%40blaine.gmane.org. .