Re: [Xen-devel] [PATCH v2 01/16] xen/mm: Don't use _{g, m}fn for defining INVALID_{G, M}FN

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 20/06/17 11:32, Jan Beulich wrote:
> > > > On 20.06.17 at 12:06, <tim@xxxxxxx> wrote:
> > At 03:36 -0600 on 20 Jun (1497929778), Jan Beulich wrote:
> > > > > > On 20.06.17 at 11:14, <tim@xxxxxxx> wrote:
> > > > At 01:32 -0600 on 20 Jun (1497922345), Jan Beulich wrote:
> > > > > > > > On 19.06.17 at 18:57, <julien.grall@xxxxxxx> wrote:
> > > > > > --- a/xen/include/xen/mm.h
> > > > > > +++ b/xen/include/xen/mm.h
> > > > > > @@ -56,7 +56,7 @@
> > > > > >
> > > > > >  TYPE_SAFE(unsigned long, mfn);
> > > > > >  #define PRI_mfn          "05lx"
> > > > > > -#define INVALID_MFN      _mfn(~0UL)
> > > > > > +#define INVALID_MFN      (mfn_t){ ~0UL }
> > > > > While I don't expect anyone to wish to use a suffix expression on
> > > > > this constant, for maximum compatibility this should still be fully
> > > > > parenthesized, I think. Of course this should be easy enough to
> > > > > do while committing.
> > > > >
> > > > > Are you able to assure us that clang supports this gcc extension
> > > > > (compound literal for non-compound types)
> > > > AIUI this is a C99 feature, not a GCCism.
> > > Most parts of it yes (it is a gcc extension in C89 mode only), but the
> > > specific use here isn't afaict: Compound literals outside of functions
> > > are static objects, and hence couldn't be used as initializers of other
> > > objects.
> > Ah, I see.  So would it be better to use
> >
> >   #define INVALID_MFN ((const mfn_t) { ~0UL })
> >
> > ?
> While I think we should indeed consider adding the const, the above
> still is a static object, and hence still not suitable as an initializer as
> per C99 or C11. But as long as gcc and clang permit it, we're fine.
Actually this solutions breaks on GCC 4.9 provided by Linaro ([1] 
4.9-2016-02 and 4.9-2017.01).
This small reproducer does not compile with -std=gnu99 (used by Xen) but 
compile with this option. Jan, have you tried 4.9 with this patch?
typedef struct
{
    unsigned long i;
} mfn_t;

mfn_t v = (const mfn_t){~0UL};

Cheers,

[1] https://releases.linaro.org/components/toolchain/binaries/

> Jan
--
Julien Grall

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