From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 13147 invoked by alias); 7 Apr 2015 13:12:58 -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 13130 invoked by uid 89); 7 Apr 2015 13:12:58 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.0 required=5.0 tests=AWL,BAYES_00,SPF_HELO_PASS,SPF_PASS,T_RP_MATCHES_RCVD autolearn=ham version=3.3.2 X-Spam-User: qpsmtpd, 2 recipients X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES256-GCM-SHA384 encrypted) ESMTPS; Tue, 07 Apr 2015 13:12:57 +0000 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id t37DCsdu000710 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 7 Apr 2015 09:12:54 -0400 Received: from localhost (ovpn-116-96.ams2.redhat.com [10.36.116.96]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id t37DCrrr008008; Tue, 7 Apr 2015 09:12:54 -0400 Date: Tue, 07 Apr 2015 13:12:00 -0000 From: Jonathan Wakely To: Hans-Peter Nilsson Cc: Richard Henderson , libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Subject: Re: [libstdc++/65033] Give alignment info to libatomic Message-ID: <20150407131252.GB9755@redhat.com> References: <54DD19B7.6060401@redhat.com> <20150403141333.GY9755@redhat.com> <20150407094458.GA9755@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) X-SW-Source: 2015-04/txt/msg00237.txt.bz2 On 07/04/15 06:51 -0400, Hans-Peter Nilsson wrote: >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. That shouldn't be possible because the C++ standard says it's an error to use alignas with a less strict alignment than would be used if it was omitted, i.e. an error to use alignas with a value less than the result alignof would give. However, G++ doesn't reject it (PR65685). It still won't be possible here, because the alignas value we use is not less than alignof(_Tp). >Your question quoted just below seems to indicate a similar >worry. I was thinking about cases like this: struct __attribute__((packed)) Bad { char c; std::atomic a; }; But G++ ignores the packed attribute here, which is good (Clang doesn't seem to ignore it, and mis-aligns the atomic). >> > > 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? Yes, alignas sets the alignment requirement, alignof gets it.