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

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Sep 14, 2020 at 05:36:30PM +0200, Thomas Huth wrote:
> 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.

Thanks Thomas and Daniel for the pointers.  Staying with gnu99
for a little longer sounds reasonable, now that we typedef
redefinitions are allowed.

-- 
Eduardo