From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:46864) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ghDeU-0002z4-R5 for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:12:11 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ghDcY-0000kW-De for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:10:19 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41920) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ghDcY-0000k2-3p for qemu-devel@nongnu.org; Wed, 09 Jan 2019 08:10:10 -0500 From: Markus Armbruster 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> Date: Wed, 09 Jan 2019 14:10:07 +0100 In-Reply-To: <89b89818-00b8-44b5-04db-4e2571533e84@redhat.com> (Thomas Huth's message of "Wed, 9 Jan 2019 13:52:02 +0100") Message-ID: <87y37udljk.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 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: "Daniel P. =?utf-8?Q?Berrang=C3=A9?=" , peter.maydell@linaro.org, pbonzini@redhat.com, qemu-devel@nongnu.org, Richard Henderson 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 the C s= tandard. >>>>> This repeatedly caused problems already, e.g. with duplicated typedef= s: >>>>> >>>>> 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 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" alrea= dy, >>>>> this seems to be a good choice. >>>> >>>> In 4.x gnu11 is marked as experimental. I'm not really comfortable >>>> 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 looked 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 in 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 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.x86_= 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." > > 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=3Dgnu11 buy us?