220 39378 <1957969.uOyN2Jk7ag@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Alternative proposal for mapping P0709 Deterministic Exceptions into C Date: Tue, 24 Jul 2018 12:27:57 -0700 Lines: 46 Approved: news@gmane.org Message-ID: <1957969.uOyN2Jk7ag@tjmaciei-mobl1> References: <6a65c934-5d2a-4e75-b88d-9eaaee338bd3@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 1532460358 12120 195.159.176.226 (24 Jul 2018 19:25:58 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 24 Jul 2018 19:25:58 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBQX33XNAKGQE4FG5S6Q@isocpp.org Tue Jul 24 21:25:54 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f197.google.com ([209.85.216.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fi2wS-000319-L3 for gclcip-std-proposals@m.gmane.org; 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[2001:19f0:5:2221::1:3]) by mx.google.com with ESMTPS id w43-v6si2957427qvj.262.2018.07.24.12.28.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 24 Jul 2018 12:28:02 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 2001:19f0:5:2221::1:3 as permitted sender) client-ip=2001:19f0:5:2221::1:3; Original-Received: from tjmaciei-mobl1.localnet (jfdmzpr04-ext.jf.intel.com [134.134.139.73]) by smtp.macieira.info (Postfix) with ESMTPSA id AFB32143E7 for ; Tue, 24 Jul 2018 12:28:00 -0700 (PDT) In-Reply-To: X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2001:19f0:5:2221::1:3 as permitted sender) smtp.mailfrom=thiago@macieira.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:39378 Archived-At: On Tuesday, 24 July 2018 06:16:42 PDT David Brown wrote: > 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. That's entirely QoI. If the compiler that targets this particular system where setting or testing a particular flag is slow, the compiler can simply choose another way of returning the information. Let's take another example of a CPU without flags: IA-64. Instead of having flags, the CPU has 1-bit predicate registers, which are set or unset depending on the comparison operation that was performed. So IA-64 could then use one of the scratch predicate registers as an extra return value. There are 10 of them. Another example is MIPS: it has no carry flag and no predicate register. In that case, it would need to use an extra register (currently used for scratch) with either a zero or non-zero value, which would allow the caller to use the BNEZ instruction to jump to the failure path. Fortunately, MIPS has plenty of registers, so it can do that. This is up to the ABI documents for each platform to determine. -- 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/1957969.uOyN2Jk7ag%40tjmaciei-mobl1. .