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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.06.17 at 10:55, <julien.grall@xxxxxxx> wrote:

> 
> 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 ...
>>
>>> 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).
> 
> Actually did you build with -std=gnu99? I just tried 4.9.3 for x86 and
> also 4.8 for ARM64 on Ubuntu Trusty. Both are broken.

Ah, indeed - that fails with 4.9.3 but succeeds with 5.2.0. And
it's not the const getting in the way here. I notice this difference
in their documentation (4.9.3 first, then 7.1.0):

Compound literals for scalar types and union types are also allowed,
but then the compound literal is equivalent to a cast.

Compound literals for scalar types and union types are also allowed.
In the following example the variable i is initialized to the value 2,
the result of incrementing the unnamed object created by the
compound literal.

        int i = ++(int) { 1 };

It is especially this example clarifying that newer compilers don't
treat this like a cast anymore (albeit a casted expression alone is
fine as initializer in 4.9.3, so there must be more to the failure).

While I still view this as a compiler bug (as it accepts the code in
default mode), as a workaround  I guess we'll need to accept a
gcc < 5 conditional in the header, which we would really have
wanted to avoid.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel