Re: Moving to C11? (was Re: Redefinition of typedefs (C11 feature))

[Thomas Huth <thuth@redhat.com>]

On 14/09/2020 15.46, Eduardo Habkost wrote:
> On Mon, Sep 14, 2020 at 07:39:09AM +0200, Thomas Huth wrote:
>> On 13/09/2020 04.51, Eduardo Habkost wrote:
>>> On Sat, Sep 12, 2020 at 08:45:19AM +0200, Thomas Huth wrote:
>>>> On 11/09/2020 22.06, Eduardo Habkost wrote:
>>>>> On Fri, Sep 11, 2020 at 08:06:10PM +0100, Peter Maydell wrote:
>>>>>> On Fri, 11 Sep 2020 at 19:49, Eduardo Habkost <ehabkost@redhat.com> wrote:
>>>>>>>
>>>>>>> I'm wondering: do our supported build host platforms all include
>>>>>>> compilers that are new enough to let us redefine typedefs?
>>>>>>>
>>>>>>> The ability to redefine typedefs is a C11 feature which would be
>>>>>>> very useful for simplifying our QOM boilerplate code.  The
>>>>>>> feature is supported by GCC since 2011 (v4.6.0)[1], and by clang
>>>>>>> since 2012 (v3.1)[2].
>>>>>>
>>>>>> In configure we mandate either GCC v4.8 or better, or
>>>>>> clang v3.4 or better, or XCode Clang v5.1 or better
>>>>>> (Apple uses a different version numbering setup to upstream).
>>>>>> So you should probably double-check that that xcode clang has
>>>>>> what you want, but it looks like we're good to go otherwise.
>>>>>
>>>>> Can anybody confirm if the following is accurate?
>>>>>
>>>>> https://gist.github.com/yamaya/2924292#file-xcode-clang-vers-L67
>>>>> # Xcode 5.1 (5B130a)
>>>>> Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
>>>>> Target: x86_64-apple-darwin13.1.0
>>>>> Thread model: posix
>>>>>
>>>>> If we know we have GCC 4.8+ or clang 3.4+, can we move to C11 and
>>>>> start using -std=gnu11?
>>>>
>>>> You don't have to switch to gnu11, redefintions of typedefs are already
>>>> fine in gnu99, they are a gnu extension there to the c99 standard.
>>>>
>>>> See also:
>>>> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=7be41675f7cb16b
>>>>
>>>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg585581.html
>>>
>>> They still trigger a warning with gnu99 on clang:
>>>
>>> $ clang --version
>>> clang version 10.0.0 (Fedora 10.0.0-2.fc32)
>>> Target: x86_64-unknown-linux-gnu
>>> Thread model: posix
>>> InstalledDir: /usr/bin
>>> $ cat test.c
>>> typedef struct A A;
>>> typedef struct A A;
>>> $ clang -std=gnu11 -c test.c
>>> $ clang -std=gnu99 -c test.c
>>> test.c:2:18: warning: redefinition of typedef 'A' is a C11 feature [-Wtypedef-redefinition]
>>> typedef struct A A;
>>
>> Ah, right, I forgot about that ... so for clang, we silence that warning
>> via CFLAGS in the configure script. See commit e6e90feedb706b1.
> 
> Nice, I hadn't seen that.  This means we don't need C11 for
> supporting redefinition of typedefs.
> 
> Now, do we have other reasons for not moving to C11?  It would be
> nice to make QEMU_GENERIC unnecessary and just use _Generic, for
> example.

See https://www.mail-archive.com/qemu-devel@nongnu.org/msg585581.html
... c11 is still "experimental" in GCC 4.8, so I think we likely have to
wait 'till summer next year - then we do not have to support
RHEL7/CentOS7 anymore according our support policy, and thus we can bump
the minimum required compiler versions.

 Thomas