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>]

On 23/06/17 09:58, Tim Deegan wrote:
> At 09:41 +0100 on 23 Jun (1498210893), Julien Grall wrote:
> > Hi Jan,
> >
> > On 23/06/17 09:30, Jan Beulich wrote:
> > > > > > On 22.06.17 at 20:31, <julien.grall@xxxxxxx> wrote:
> > > > 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?
> > > That's sort of an odd question - you've sent the patch, so I would
> > > have expected you to have made sure it doesn't break (and
> > > while it was me to add the const, this was discussed, and you don't
> > > make clear whether that's the issue). In any event, I've tried ...
> > I don't personally try every single compiler every time I am writing a
> > patch... This is too complex given that different stakeholders (Linaro,
> > Debian, Ubuntu,...) provide various binaries with their own patches on top.
> >
> > I asked you because I was wondering what is happening on x86 (I don't
> > have 4.9 x86 in hand) and to rule out a bug in the compiler provided by
> > Linaro.
> >
> > > > typedef struct
> > > > {
> > > >      unsigned long i;
> > > > } mfn_t;
> > > >
> > > > mfn_t v = (const mfn_t){~0UL};
> > > ... this now with 7.1.0, 6.3.0, 5.4.0, 5.2.0, and 4.9.3, and all
> > > of them compile this without errors or warnings (at -Wall -W).
> > > For 4.9.3 I've also specifically taken care to try not only the
> > > x86 compiler, but also the arm32 and arm64 ones. So I'm afraid
> > > I lack enough detail to understand what the issue is and what a
> > > solution may look like.
> > I don't have much except the following error:
> >
> > /tmp/test.c:6:1: error: initializer element is not constant
> >   mfn_t v = (const mfn_t){~0UL};
> >   ^
> >
> > If it works for you on 4.9, then it might be a bug in the GCC provided
> > by Linaro and will report it.
> This fails for me on x86 gcc 4.9.4, using -xc -std=gnu99.  The
> complaint is valid, as Jan pointed out: the literal is a static object
> and so not a valid initializer.  GCC also complains about the
> 'debug' version for the same reason. :(
>
> Using a plain initializer works OK for both debug and non-debug:
>
>   mfn_t v = {~0UL};
>
> but I haven't checked whether other compilers like that as well.
This seems to be a bug in GCC up to 5.0:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64856

Cheers,

--
Julien Grall

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