Re: [Xen-devel] [PATCH for-4.6] xen/public: arm: Use __typeof__ rather than typeof

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.10.15 at 19:08, <julien.grall@xxxxxxxxxx> wrote:
> On 23/10/15 15:55, Ian Campbell wrote:
>> On Fri, 2015-10-23 at 15:44 +0100, Julien Grall wrote:
>>> Thank you for the explanation. I think we can do the same as x86 does
>>> i.e:
>>>
>>> #define set_xen_guest_handle_raw(hnd, val)                  \
>>>     do { if ( sizeof(hnd) == 8 ) *(uint64_t *)&(hnd) = 0;   \
>>>          (hnd).p = val;                                     \
>>>     } while ( 0 )
>> 
>> This evaluates hnd twice, which I assumed we wanted to avoid.
>> 
>> But if that is OK for x86 in this situation then there is no harm doing it
>> on ARM too[0].
>> 
>> But in that case I think we would just do
>>      (hnd).q = val ; (hnd).p = val
>> rather than messing with &, casts and *.
> 
> Which is, based on the ISO C spec [1], unspecified. See 6.2.6.1#7:
> 
> "When a value is stored in a member of an object of union type, the
> bytes of the object representation that do not correspond to that member
> but do correspond to other members take unspecified values, but the
> value of the union object shall not thereby become a trap
> representation."

Question is - why would you need both assignments? Afaics the
.q one would suffice.

Jan


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