Re: [Qemu-devel] [PATCH] configure: Force the C standard to gnu11

[Daniel P . Berrangé]

On Wed, Jan 09, 2019 at 02:20:02PM +0100, Thomas Huth wrote:
> On 2019-01-09 13:58, Daniel P. Berrangé wrote:
> > 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.
> What about the duplicated typedef problem? See:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
> 
> That one occured with Clang, but I think we've had plenty of these in
> the past with GCC, too...

IIUC, That's only a problem because we don't pass any  -std  flag, and
so get the compilers default, which may be gnu11.  If we explicitly
set -std=gnu99, that problem will be reported by patchew, travis and
maintainers own build tests, and thus won't get anywhere near git master.

Regards,
Daniel
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