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

[Thomas Huth <thuth@redhat.com>]

On 2019-01-09 15:20, Markus Armbruster wrote:
> Thomas Huth <thuth@redhat.com> writes:
> 
>> On 2019-01-09 14:10, Markus Armbruster wrote:
>>> Thomas Huth <thuth@redhat.com> writes:
>>>
>>>> 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?
>>>
>>> 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=gnu11
>>> 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
> 
> That's a compilation failure.  "Support is experimental" makes me afraid
> of run time failures.
> 
> 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