From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:38738) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ghEpR-000779-Pw for qemu-devel@nongnu.org; Wed, 09 Jan 2019 09:27:34 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ghEpQ-0007vS-LI for qemu-devel@nongnu.org; Wed, 09 Jan 2019 09:27:33 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37366) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ghEpQ-0007um-DF for qemu-devel@nongnu.org; Wed, 09 Jan 2019 09:27:32 -0500 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> <87y37udljk.fsf@dusky.pond.sub.org> <87k1jdewvc.fsf@dusky.pond.sub.org> From: Thomas Huth Message-ID: Date: Wed, 9 Jan 2019 15:27:26 +0100 MIME-Version: 1.0 In-Reply-To: <87k1jdewvc.fsf@dusky.pond.sub.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US 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: Markus Armbruster Cc: peter.maydell@linaro.org, Richard Henderson , qemu-devel@nongnu.org, pbonzini@redhat.com On 2019-01-09 15:20, Markus Armbruster wrote: > Thomas Huth writes: >=20 >> On 2019-01-09 14:10, Markus Armbruster wrote: >>> Thomas Huth writes: >>> >>>> 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 th= e C standard. >>>>>>>> This repeatedly caused problems already, e.g. with duplicated ty= pedefs: >>>>>>>> >>>>>>>> https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.= html >>>>>>>> >>>>>>>> or with for-loop variable initializers: >>>>>>>> >>>>>>>> https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.= html >>>>>>>> >>>>>>>> To avoid these problems, we should enforce the C language versio= n to the >>>>>>>> same level for all compilers. Since our minimum compiler version= s are >>>>>>>> GCC v4.8 and Clang v3.4 now, and both basically support "gnu11" = already, >>>>>>>> this seems to be a good choice. >>>>>>> >>>>>>> In 4.x gnu11 is marked as experimental. I'm not really comforta= ble >>>>>>> using experimental features - even if its warning free there's a = risk >>>>>>> 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 looke= d at >>>>>> https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Standards.html and it= says: >>>>>> >>>>>> "A fourth version of the C standard, known as C11, was published i= n 2011 >>>>>> as ISO/IEC 9899:2011. GCC has limited incomplete support for parts= of >>>>>> 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 hav= e C++ >>>>>> code in QEMU -- if you worry about that, I could simply drop the >>>>>> "-std=3Dgnu++11" part from my patch? >>>>> >>>>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1= .x86_64: >>>>> >>>>> "3.4 Options Controlling C Dialect >>>>> >>>>> ....snip... >>>>> >>>>> 'gnu11' >>>>> 'gnu1x' >>>>> GNU dialect of ISO C11. Support is incomplete and >>>>> experimental. The name 'gnu1x' is deprecated." >>>> >>>> Ok. Looks like the "Support is incomplete and experimental" sentence= has >>>> 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? >>> >>> Switchinh back could be somewhat painful if we already started using = C11 >>> features. And if we don't plan to, then what exactly will -std=3Dgnu= 11 >>> buy us? >> >> With C11, we get safety for the "duplicated typedef" problem that we r= un >> into regularly again and again, see e.g.: >> >> https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html >=20 > That's a compilation failure. "Support is experimental" makes me afrai= d > of run time failures. >=20 > If we truly want C11, shouldn't we bump minimum required GCC to 4.9? That's not possible, since we claim to support RHEL7 / CentOS7 that is still using GCC v4.8. Thomas