From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 85144 invoked by alias); 7 Apr 2015 10:52:01 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 85117 invoked by uid 89); 7 Apr 2015 10:51:59 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.7 required=5.0 tests=AWL,BAYES_00,RCVD_IN_DNSWL_NONE,SPF_PASS autolearn=ham version=3.3.2 X-Spam-User: qpsmtpd, 2 recipients X-HELO: arjuna.pair.com Received: from arjuna.pair.com (HELO arjuna.pair.com) (209.68.5.131) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Tue, 07 Apr 2015 10:51:58 +0000 Received: by arjuna.pair.com (Postfix, from userid 3006) id BFDFF8A242; Tue, 7 Apr 2015 06:51:54 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by arjuna.pair.com (Postfix) with ESMTP id BF2298A0C3; Tue, 7 Apr 2015 06:51:54 -0400 (EDT) Date: Tue, 07 Apr 2015 10:52:00 -0000 From: Hans-Peter Nilsson To: Jonathan Wakely cc: Richard Henderson , libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Subject: Re: [libstdc++/65033] Give alignment info to libatomic In-Reply-To: <20150407094458.GA9755@redhat.com> Message-ID: References: <54DD19B7.6060401@redhat.com> <20150403141333.GY9755@redhat.com> <20150407094458.GA9755@redhat.com> User-Agent: Alpine 2.02 (BSF 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-IsSubscribed: yes X-SW-Source: 2015-04/txt/msg00228.txt.bz2 On Tue, 7 Apr 2015, Jonathan Wakely wrote: > On 05/04/15 21:07 -0400, Hans-Peter Nilsson wrote: > > On Fri, 3 Apr 2015, Jonathan Wakely wrote: > > > > > On 03/04/15 05:24 -0400, Hans-Peter Nilsson wrote: > > > > On Thu, 2 Apr 2015, Hans-Peter Nilsson wrote: > > > > > Why then use __alignof(_M_i) (the object-alignment) > > > > > instead of _S_alignment (the deduced alas insufficiently > > > > > increased type-alignment)? > > > > > > Isn't the object aligned to _S_alignment? > > > > We did specify that with the alignas. Is the alignof always > > exactly the same as an alignas, if one is specified? (And will > > that not change in a future amendment, standard and/or > > implementation?) Either way, is there a test-case to guard all > > this? > > The language guarantees that's what alignas() does, if the argument is > a valid alignment (which it must be if we derive it from some other > type's alignment). I'm more worried about alignof reporting a higher value for a specific object than alignas to be wrong. Your question quoted just below seems to indicate a similar worry. > > > Or is it different if a std::atomic is included in some other > > > struct and the user forces a different alignment on it? I don't think > > > we really need to support that, users shouldn't be doing that. > > > > Why do we even need to ask those questions, when the patch takes > > care of the per-type business without doubt? > > Well if we know the object is guaranteed to be correctly aligned we > might not even need a fake, minimally aligned pointer. We could go > back to passing &_M_i or just a null pointer to __atomic_is_lock_free. The target has a say in __atomic_is_lock_free and could impose some crazy pointer-value-specific condition like "only in the first half of a page" (anything can happen to work around chip errata) so I suggest staying with an alignment-generated pointer. No, I only dreamt that up, shoot it down if we only care for current targets. > The whole point of alignas() is to fix the alignment to a known value. And alignof and __alignof to report *exactly* that, I hope? brgds, H-P