From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/4977 Path: news.gmane.org!not-for-mail From: Jens Gustedt Newsgroups: gmane.linux.lib.musl.general Subject: Re: [PATCH 3/3] stddef: Define max_align_t Date: Mon, 28 Apr 2014 14:22:44 +0200 Message-ID: <1398687764.7944.50.camel@eris.loria.fr> References: <1398649434-23560-1-git-send-email-raj.khem@gmail.com> <1398649434-23560-3-git-send-email-raj.khem@gmail.com> <20140428020328.GV26358@brightrain.aerifal.cx> <20140428101156.GF12324@port70.net> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg="pgp-sha1"; protocol="application/pgp-signature"; boundary="=-EeicpOIQqYHC4p2Nu4n8" X-Trace: ger.gmane.org 1398687784 6047 80.91.229.3 (28 Apr 2014 12:23:04 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 28 Apr 2014 12:23:04 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-4981-gllmg-musl=m.gmane.org@lists.openwall.com Mon Apr 28 14:22:58 2014 Return-path: Envelope-to: gllmg-musl@plane.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1WekaH-00020Z-Jk for gllmg-musl@plane.gmane.org; Mon, 28 Apr 2014 14:22:57 +0200 Original-Received: (qmail 16122 invoked by uid 550); 28 Apr 2014 12:22:56 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 16114 invoked from network); 28 Apr 2014 12:22:56 -0000 X-IronPort-AV: E=Sophos;i="4.97,944,1389740400"; d="scan'";a="70770545" In-Reply-To: <20140428101156.GF12324@port70.net> Face: 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Cj15cu23XZI4/826Px/xqB37z547sPH374oOvnKTsOb+1zuCoImZARM8ZiLlKkXcWOcoFRJ+Do/CwFO4e2DEe+9Ti+ZXA8ny/cU+X4bMKRZxZIVEiqXI6/+NKuUzPHb7z40ovnX3zx+Ctuq38HyuqWG7Tu+A0AAAAASUVORK5CYII= X-Mailer: Evolution 3.2.3-0ubuntu6 Xref: news.gmane.org gmane.linux.lib.musl.general:4977 Archived-At: --=-EeicpOIQqYHC4p2Nu4n8 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Am Montag, den 28.04.2014, 12:11 +0200 schrieb Szabolcs Nagy: > i think i386 abi is non-conforming to the c11 alignment requirements now: > long long has 8 byte alignment, but in a struct/union it has only 4 > (this is why the attrs are needed above) >=20 > long long x; // _Alignof(x) =3D=3D 8 > struct {long long x;} y; // _Alignof(y.x) =3D=3D 4 I don't think that it is non-conforming _Alignof of a type only tells you on what alignments the programmer may place objects of the corresponding type (if he deals with this manually) and gives no guarantee what the implementation itself choses under all circumstances this holds especially if a type has "extended alignment", I think > i think the standard requires that all (addressable) long long objects > should have the same alignment (or stricter) than _Alignof(x) >=20 > max_align_t is defined to be the "greatest alignment supported in all > contexts", i don't know why it is not just This only concerns "fundamental alignments". Also, this sentence has an implicit "minimum" operator that comes from the "in all contexts" if the alignment restriction is 8 in some context and 4 in others, the result is 4 and not 8. Perhaps all of this can be made consistent on i386 by having _Alignof(max_align_t) to be 4 and declare 8 byte and 16 byte aligned types as having "extended alignment" I am not sure that I remember correctly, but it seems to me that i386 allows for 4 byte alignment of all types, only that this results in suboptimal code > typedef char max_align_t __attribute__((aligned(__BIGGEST_ALIGNMENT__)))= ; >=20 > which gives 16 byte alignment on i386 gcc, i thought it was supported > in all contexts I think this just not necessary and even counter productive. > if gcc and clang went with the same definition we should follow, but > this makes the type less meaningful By looking at the page that you linked to, my impression is that they got it wrong. I think basically typedef union max_align_t max_align_t; union max_align_t { long double a; uintmax_t b; void* c; max_align_t* d; }; should do the trick. All other alignments should be considered as extended alignments. Jens --=20 :: INRIA Nancy Grand Est :: http://www.loria.fr/~gustedt/ :: :: AlGorille ::::::::::::::: office Nancy : +33 383593090 :: :: ICube :::::::::::::: office Strasbourg : +33 368854536 :: :: ::::::::::::::::::::::::::: gsm France : +33 651400183 :: :: :::::::::::::::::::: gsm international : +49 15737185122 :: --=-EeicpOIQqYHC4p2Nu4n8 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) iEYEABECAAYFAlNeSBQACgkQD9PoadrVN+IZnQCfSl5HpTPP00Yz/ggcKjjEaS6/ e/sAoJmGdm6x3Ww/kOAz6DuX+Ur1dD94 =wPQY -----END PGP SIGNATURE----- --=-EeicpOIQqYHC4p2Nu4n8--