From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:47001) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ghDf5-00032A-IQ for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:12:49 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ghDRm-0001qD-BQ for qemu-devel@nongnu.org; Wed, 09 Jan 2019 07:59:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37164) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ghDRl-0001pd-Iu for qemu-devel@nongnu.org; Wed, 09 Jan 2019 07:59:01 -0500 Date: Wed, 9 Jan 2019 12:58:54 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20190109125854.GM3998@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <1546857926-5958-1-git-send-email-thuth@redhat.com> <20190109105818.GG3998@redhat.com> <20190109114459.GK3998@redhat.com> <89b89818-00b8-44b5-04db-4e2571533e84@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <89b89818-00b8-44b5-04db-4e2571533e84@redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Thomas Huth Cc: qemu-devel@nongnu.org, Richard Henderson , pbonzini@redhat.com, peter.maydell@linaro.org On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote: > On 2019-01-09 12:44, Daniel P. Berrang=C3=A9 wrote: > > On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote: > >> On 2019-01-09 11:58, Daniel P. Berrang=C3=A9 wrote: > >>> On Mon, Jan 07, 2019 at 11:45:26AM +0100, Thomas Huth wrote: > >>>> Different versions of GCC and Clang use different versions of the = C standard. > >>>> This repeatedly caused problems already, e.g. with duplicated type= defs: > >>>> > >>>> https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.ht= ml > >>>> > >>>> or with for-loop variable initializers: > >>>> > >>>> https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.ht= ml > >>>> > >>>> To avoid these problems, we should enforce the C language version = to the > >>>> same level for all compilers. Since our minimum compiler versions = are > >>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" al= ready, > >>>> this seems to be a good choice. > >>> > >>> In 4.x gnu11 is marked as experimental. I'm not really comfortabl= e > >>> using experimental features - even if its warning free there's a ri= sk > >>> it would silently mis-compile something. > >>> > >>> gnu99 is ok with 4.x - it is merely "incomplete". > >> > >> gnu11 has the big advantage that it also fixes the problem with > >> duplicated typedefs that are reported by older versions of Clang. > >> > >> Are you sure about the experimental character in 4.x? I just looked = at > >> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it s= ays: > >> > >> "A fourth version of the C standard, known as C11, was published in = 2011 > >> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts o= f > >> this standard, enabled with -std=3Dc11 or -std=3Diso9899:2011." > >> > >> It does not say anything about "experimental" there. The word > >> "experimental" is only used for the C++ support, but we hardly have = C++ > >> code in QEMU -- if you worry about that, I could simply drop the > >> "-std=3Dgnu++11" part from my patch? > >=20 > > I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x= 86_64: > >=20 > > "3.4 Options Controlling C Dialect > >=20 > > ....snip... > >=20 > > 'gnu11' > > 'gnu1x' > > GNU dialect of ISO C11. Support is incomplete and > > experimental. The name 'gnu1x' is deprecated." >=20 > Ok. Looks like the "Support is incomplete and experimental" sentence ha= s > been removed with GCC 4.9.0 here. So GCC 4.8 is likely pretty close > already. IMHO we could give it a try and enable gnu11 for QEMU with GCC > v4.8, too. If we later find problems, we could still switch back to > gnu99 instead. Other opinions? Our code is already cleanly compiling with gnu99 standard - the problem is merely that we sometimes introduce regressions due to not enforcing that standard level. I don't think the features in gnu11 are compelling enough to justify using something that's declared experimental. As long as we always have a -std=3Dgnu99 flag set, it will avoid the regressions we've seen. Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|