Re: [Xen-devel] Compile errors with gcc 4.8

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-02-07 at 08:59 +0000, M A Young wrote:
> On Thu, 7 Feb 2013, Andrew Cooper wrote:
> 
> > On 06/02/2013 20:37, M A Young wrote:
> >
> >> There are two types of problem, the first is from
> >> xen/common/compat/memory.c where the error is
> >>
> >> memory.c: In function 'compat_memory_op':
> >> /builddir/build/BUILD/xen-4.2.1/xen/include/public/arch-x86/xen.h:35:33:
> >> error: typedef '__guest_handle_const_compat_memory_exchange_t' locally
> >> defined but not used [-Werror=unused-local-typedefs]
> >>       typedef struct { type *p; } __guest_handle_ ## name
> >>                                   ^
> >> /builddir/build/BUILD/xen-4.2.1/xen/include/public/arch-x86/xen.h:43:5:
> >> note: in expansion of macro '___DEFINE_XEN_GUEST_HANDLE'
> >>       ___DEFINE_XEN_GUEST_HANDLE(const_##name, const type)
> >>       ^
> >> /builddir/build/BUILD/xen-4.2.1/xen/include/public/arch-x86/xen.h:44:41:
> >> note: in expansion of macro '__DEFINE_XEN_GUEST_HANDLE'
> >>   #define DEFINE_XEN_GUEST_HANDLE(name)   __DEFINE_XEN_GUEST_HANDLE(name,
> >> name)
> >>                                           ^
> >> memory.c:261:13: note: in expansion of macro 'DEFINE_XEN_GUEST_HANDLE'
> >>               DEFINE_XEN_GUEST_HANDLE(compat_memory_exchange_t);
> >>               ^
> >>
> >> so we are defining something that isn't used.
> >
> > But it is used, by guest_handle_cast, albeit through several layers of
> > macros.
> >
> > I would tentativly object to the use of "__attribute__((unused));" to
> > work around what appears to be a gcc bug.  If 4.8 support is desperately
> > wanted, then a better solution would be to specify
> > -Wno-unused-local-typedefs to disable the buggy feature.
> 
> It looked to me as if the DEFINE macro does two typedefs and the code only 
> uses the first, so gcc 4.8 was correct in its objections. The suggested 
> workaround of using -Wno-unused-local-typedefs isn't very portible as 
> earlier versions of gcc don't recognize it.

We have a cc-option macro in the makefiles to enable these sorts of
options to be enabled conditionally.

However it seems a bit odd to be using DEFINE_XEN_GUEST_HANDLE in this
manner, should it not be declared in some header? I can see a
DEFINE_COMPAT_HANDLE(compat_memory_exchange_t) in
xen/include/compat/memory.h. Perhaps the one in memory.c can just be
dropped or otherwise modified to use this?

Ian.


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