From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58849) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aOqIB-0003H3-BB for qemu-devel@nongnu.org; Thu, 28 Jan 2016 12:23:40 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aOqI6-0000KC-8V for qemu-devel@nongnu.org; Thu, 28 Jan 2016 12:23:35 -0500 References: <1453219845-30939-1-git-send-email-eblake@redhat.com> <1453219845-30939-37-git-send-email-eblake@redhat.com> <878u39iuqf.fsf@blackfin.pond.sub.org> From: Eric Blake Message-ID: <56AA4E8F.2030808@redhat.com> Date: Thu, 28 Jan 2016 10:23:27 -0700 MIME-Version: 1.0 In-Reply-To: <878u39iuqf.fsf@blackfin.pond.sub.org> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Grld4J902L5MK46rh9Gid4hmd6uwKvgPn" Subject: Re: [Qemu-devel] [PATCH v9 36/37] RFC: qapi: Adjust layout of FooList types List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Michael Roth , Alexander Graf , qemu-devel@nongnu.org, "open list:sPAPR" , marcandre.lureau@redhat.com, David Gibson This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --Grld4J902L5MK46rh9Gid4hmd6uwKvgPn Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 01/28/2016 08:34 AM, Markus Armbruster wrote: > Eric Blake writes: >=20 >> By sticking the next pointer first, we don't need a union with >> 64-bit padding for smaller types. On 32-bit platforms, this >> can reduce the size of uint8List from 16 bytes (or 12, depending >> on whether 64-bit ints can tolerate 4-byte alignment) down to 8. >> It has no effect on 64-bit platforms (where alignment still >> dictates a 16-byte struct); but fewer anonymous unions is still >> a win in my book. >> >> However, this requires visit_start_list() and visit_next_list() >> to gain a size parameter, to know what size element to allocate. >> >> I debated about going one step further, to allow for fewer casts, >> by doing: >> typedef GenericList GenericList; >> struct GenericList { >> GenericList *next; >> }; >> struct FooList { >> GenericList base; >> Foo value; >> }; >> so that you convert to 'GenericList *' by '&foolist->base', and >> back by 'container_of(generic, GenericList, base)' (as opposed to >> the existing '(GenericList *)foolist' and '(FooList *)generic'). >> But doing that would require hoisting the declaration of >> GenericList prior to inclusion of qapi-types.h, rather than its >> current spot in visitor.h; it also makes iteration a bit more >> verbose through 'foolist->base.next' instead of 'foolist->next'. Should I attempt this? >> typedef struct GenericList >> { >> - union { >> - void *value; >> - uint64_t padding; >> - }; >> struct GenericList *next; >> + char padding[]; >> } GenericList; >=20 > Less trickery, I like it. >=20 > Member padding appears to be unused. or just leave it at this? >> bool visit_start_list(Visitor *v, const char *name, GenericList **lis= t, >> - Error **errp) >> + size_t size, Error **errp) >> { >> - bool result =3D v->start_list(v, name, list, errp); >> + bool result; >> + >> + assert(list ? size : !size); >=20 > Tighter than size !=3D 0 would be size >=3D GenericList. Same elsewher= e. Makes sense. >=20 > Rest looks good. Didn't look as closely as for the previous patches > (getting tired), but so far I like the idea. Okay, I'll keep it and drop the RFC. --=20 Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org --Grld4J902L5MK46rh9Gid4hmd6uwKvgPn Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 Comment: Public key at http://people.redhat.com/eblake/eblake.gpg Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iQEcBAEBCAAGBQJWqk6PAAoJEKeha0olJ0NqkP4H/2+eo2newo6WtPC72oo0fXjY mtcq6158fsgLcgCmJ9Va5NmhHSOvJ6HQsgetmuav6KqQklpTje2+uXz37bJFMOgA k0RZU03lQijpAToI41m9IqBS7ccmxlC85FqoCzIq8kUhdf2o3gUUFOD2wsgiFAi9 v7nu6hsqHMa2v8wdCxM/RDuJ4yZQku4i8JC9terXHMkwN4JekJKuNdu4y8xY4KOF 6OvF/WAPOaJYlBcWD1wTFo3JCrl2aaO9UUxPPDhKxyo0rAeILVtQqbqFrDIuMGjw DNZszeyuFNXjCcvPenCIUCMy1eM2FMuv3MoOmUvBg6+Bbr7bdyX6f2pkZ2WaEcY= =HV/p -----END PGP SIGNATURE----- --Grld4J902L5MK46rh9Gid4hmd6uwKvgPn--