220 37378 <3371255.CqAHtthudB@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: `offsetof` in a macro-less, modular world Date: Sat, 17 Mar 2018 10:34:02 -0700 Lines: 40 Approved: news@gmane.org Message-ID: <3371255.CqAHtthudB@tjmaciei-mobl1> References: <3998341.rfsqbO5A2U@tjmaciei-mobl1> <8fdfc016-2eb8-4888-9531-66310c80b457@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 1521307926 15926 195.159.176.226 (17 Mar 2018 17:32:06 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 17 Mar 2018 17:32:06 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBDNDWXKQKGQEXYI4DVQ@isocpp.org Sat Mar 17 18:32:02 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f199.google.com ([209.85.220.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1exFgV-00040a-PM for gclcip-std-proposals@m.gmane.org; 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[209.222.10.136]) by mx.google.com with ESMTPS id c8si616227qth.15.2018.03.17.10.34.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 17 Mar 2018 10:34:04 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 209.222.10.136 as permitted sender) client-ip=209.222.10.136; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:4183:2825:a294:7ca8]) by smtp.macieira.info (Postfix) with ESMTPSA id C1876A7DC for ; Sat, 17 Mar 2018 10:34:03 -0700 (PDT) In-Reply-To: <8fdfc016-2eb8-4888-9531-66310c80b457@isocpp.org> X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 209.222.10.136 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:37378 Archived-At: On Saturday, 17 March 2018 03:11:11 PDT Jakob Riedle wrote: > as a first contribution to this survey :-), I recently used it to > determine, whether two attributes (used in two different memory layout > modes of a class) shaddow each other. > Here is the link to the spot (Line 667): > https://github.com/DuffsDevice/tinyutf8/blob/master/include/tinyutf8.h I think I understand what your optimisation is trying to do here: support the case where struct NON_SSO has tail padding, in which case the last byte in struct SSO aliases no members in NON_SSO. Is that it? If so, the code is dead in all modern platforms, since sizeof(size_t) == sizeof(void *) and ditto for alignof. So NON_SSO will never have tail padding and it will be very hard for you to test this anywhere. Is this an optimisation you really want to keep in your code? Though I don't see any other way to test for padding at a specific position aside from offsetof. That seems to be a legitimate use, though fragile and dangerous. I also think that your code in that condition is UB. You can't use t_sso and t_non_sso at the same time -- only one member of a union can be active at a time. Since this function is supposed to disable SSO, you must leave the t_non_sso member of the union active at the end of your function, but the last one it would have used is t_sso. Your last_byte type may be suffering from the same problem. -- 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/3371255.CqAHtthudB%40tjmaciei-mobl1. .