Re: [Xen-devel] [PATCH 4/4] xen/arm: correctly handle an empty array of platform descs.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.05.13 at 16:34, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> On Fri, 2013-05-17 at 11:39 +0100, Jan Beulich wrote:
>> >> And btw., for both 32- and 64-bit ARM, other than for x86, I see
>> >> empty structure instances occupy zero bytes (and hence distinct
>> >> symbols end up at the same address), so the compiler is conflicting
>> >> with itself here.
>> > 
>> > I imagine this is as much to do with the architecture ABI as the
>> > compiler.
>> 
>> So do I, but this doesn't make this any less of a compiler bug.
> 
> Is there a requirement (e.g. from the C standard) that an empty struct
> take at least one byte, or more likely I suppose something about
> different instances having different addresses which sort of implies it?

Yes: "Two pointers compare equal if and only if both are null pointers,
both are pointers to the same object (including a pointer to an object
and a subobject at its beginning) or function, both are pointers to one
past the last element of the same array object, or one is a pointer to
one past the end of one array object and the other is a pointer to the
start of a different array object that happens to immediately follow
the first array object in the address space."

Gcc of course can, as an extension, support code that doesn't match
this, but then they need to not do the visibility based optimization
(and I would assume that, unless the architecture ABI specifically
provides for it, this should be disabled by default).

Jan


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