From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 46488 invoked by alias); 6 Apr 2017 19:16:05 -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 46457 invoked by uid 89); 6 Apr 2017 19:16:04 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-6.9 required=5.0 tests=BAYES_00,GIT_PATCH_1,RP_MATCHES_RCVD,SPF_PASS autolearn=ham version=3.3.2 spammy= X-HELO: mx2.suse.de Received: from mx2.suse.de (HELO mx2.suse.de) (195.135.220.15) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 06 Apr 2017 19:16:03 +0000 Received: from relay1.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 1807DAC30; Thu, 6 Apr 2017 19:16:03 +0000 (UTC) Date: Thu, 06 Apr 2017 19:16:00 -0000 User-Agent: K-9 Mail for Android In-Reply-To: References: <21E940B5-C8C4-4A86-8C15-49A86547DD87@suse.de> <20170405160333.GR4425@redhat.com> <20170405160849.GV17461@tucnak> <20170406075104.GA17461@tucnak> <7d17b3b7-2d38-6184-8bd6-eb9f96f87912@redhat.com> <50936a77-870a-5156-1f5e-b1e0327498b6@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [PATCH] Add a new type attribute always_alias (PR79671) To: Bernd Edlinger ,Florian Weimer ,Jakub Jelinek CC: Jonathan Wakely ,GCC Patches ,Jason Merrill ,Jeff Law From: Richard Biener Message-ID: <7FF41E4E-8C71-431D-A955-BED818241913@suse.de> X-SW-Source: 2017-04/txt/msg00328.txt.bz2 On April 6, 2017 8:12:29 PM GMT+02:00, Bernd Edlinger wrote: >On 04/06/17 19:47, Florian Weimer wrote: >> On 04/06/2017 07:39 PM, Bernd Edlinger wrote: >>> On 04/06/17 16:17, Florian Weimer wrote: >>>>> Here is what I want to write in the doc: >>>>> >>>>> @item typeless_storage >>>>> @cindex @code{typeless_storage} type attribute >>>>> A type declared with this attribute behaves like a character type >>>>> with respect to aliasing semantics. >>>>> This is attribute is similar to the @code{may_alias} attribute, >>>>> except that it is not restricted to pointers. >>>> >>>> As Jakub pointed out, this is not what we need here. An object of >type >>>> char does *not* have untyped storage. Accessing it as a different >type >>>> is still undefined. >>>> >>> >>> but, do you agree that this is valid in C11? >>> >>> typedef char char_a[4]; >>> >>> int >>> main (void) >>> { >>> char_a a =3D {1,2,3,4}; >>> short *b =3D (short *) &a; >>> >>> b[1] =3D 0; >>> >>> if (a[0] =3D=3D 1 && a[1] =3D=3D 2 && a[2] =3D=3D 3 && a[3] =3D=3D 4) >>> abort(); >>> >>> exit(0); >>> } >>> >>> >>> all I want to do is replace "char" with a different type. >> >> Thanks a lot for posting a concrete example. >> >> The effective type of a[2] and [3] is char. The character type >wildcard >> in 6.5(7) only applies to the type of the lvalue expression ysed for >the >> access, not the effective type of the object being accessed. The >type >> of the LHS of the assignment expression is short. So the access is >> undefined. >> > >exactly *that* is what I want to make valid with that attribute, which >would be also useful in C and kernel code, IMHO. > >But isn't the effective type changed by the assignment b[1] =3D 0; >as described in 6.5(6): >"If a value is stored into an object having no declared type through an >lvalue having a type that is not a character type, then the type of the >lvalue becomes the effective type of the object for that access and for >subsequent accesses that do not modify the stored value." Yes. I think the example is valid. At least GCCs memory model makes it so. Richard. > > >Bernd.