From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:48830) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ghDjw-0007Ms-OR for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:17:49 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ghDjv-0006Pd-Rz for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:17:48 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43304) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ghDjv-0006P3-Jm for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:17:47 -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> From: Thomas Huth Message-ID: Date: Wed, 9 Jan 2019 14:17:42 +0100 MIME-Version: 1.0 In-Reply-To: <87y37udljk.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: "=?UTF-8?Q?Daniel_P._Berrang=c3=a9?=" , peter.maydell@linaro.org, pbonzini@redhat.com, qemu-devel@nongnu.org, Richard Henderson On 2019-01-09 14:10, Markus Armbruster wrote: > Thomas Huth writes: >=20 >> 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? >>> >>> I was looking at the "info gcc" docs on RHEL7, gcc-4.8.5-16.el7_4.1.x= 86_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 h= as >> 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 GC= C >> v4.8, too. If we later find problems, we could still switch back to >> gnu99 instead. Other opinions? >=20 > Switchinh back could be somewhat painful if we already started using C1= 1 > features. And if we don't plan to, then what exactly will -std=3Dgnu11 > buy us? With C11, we get safety for the "duplicated typedef" problem that we run into regularly again and again, see e.g.: https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html Thomas