On Wed, Jan 09, 2019 at 01:52:02PM +0100, Thomas Huth wrote: > On 2019-01-09 12:44, Daniel P. Berrangé wrote: > > On Wed, Jan 09, 2019 at 12:25:43PM +0100, Thomas Huth wrote: > >> On 2019-01-09 11:58, Daniel P. Berrangé 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 typedefs: > >>>> > >>>> 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" already, > >>>> 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=c11 or -std=iso9899:2011." > >> > >> 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=gnu++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?
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=gnu99 flag set, it will avoid the regressions we've seen. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|