From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 63010 invoked by alias); 6 Apr 2015 01:07:17 -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 62983 invoked by uid 89); 6 Apr 2015 01:07:16 -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; Mon, 06 Apr 2015 01:07:14 +0000 Received: by arjuna.pair.com (Postfix, from userid 3006) id 60CFD8A362; Sun, 5 Apr 2015 21:07:10 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by arjuna.pair.com (Postfix) with ESMTP id 2A6888A21F; Sun, 5 Apr 2015 21:07:10 -0400 (EDT) Date: Mon, 06 Apr 2015 01:07:00 -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: <20150403141333.GY9755@redhat.com> Message-ID: References: <54DD19B7.6060401@redhat.com> <20150403141333.GY9755@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/msg00176.txt.bz2 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? Those questions wouldn't even be asked if we used _S_alignment for the fake-pointer too, just as a matter of defensive programming. > Instead of changing every case in the condition to include sizeof why > not just do it afterwards using sizeof(_Tp), in the _S_alignment > calculation? Doh. > We know sizeof(_Tp) == sizeof(corresponding integer type) because > that's the whole point of the conditionals! See attachment. > > > @@ -216,7 +216,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > > is_lock_free() const noexcept > > { > > // Produce a fake, minimally aligned pointer. > > - void *__a = reinterpret_cast(-__alignof(_M_i)); > > + void *__a = reinterpret_cast(-_S_alignment); > > return __atomic_is_lock_free(sizeof(_M_i), __a); > > } > > If _M_i is aligned to _S_alignment then what difference does the > change above make? > > It doesn't matter if the value is per-object if we've forced all such > objects to have the same alignment, does it? > > 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? > The attached patch against trunk should have the same result with much > less effort. > > It doesn't include the changes to the reinterpret_cast > expressions to produce a minimally aligned pointer, but I think this > is progress, thanks :-) Progress is good. :) brgds, H-P