Re: PING^1: [PATCH] Add TYPE_EMPTY_RECORD for C++ empty class

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Mar 16, 2016 at 7:33 AM, Jason Merrill <jason@redhat.com> wrote:
> On 03/16/2016 07:55 AM, H.J. Lu wrote:
>>
>> On Tue, Mar 15, 2016 at 7:51 PM, Jason Merrill <jason@redhat.com> wrote:
>>>
>>> On 03/15/2016 08:25 PM, Joseph Myers wrote:
>>>>
>>>>
>>>> On Tue, 15 Mar 2016, H.J. Lu wrote:
>>>>
>>>>> On Tue, Mar 15, 2016 at 3:34 PM, Joseph Myers <joseph@codesourcery.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> On Tue, 15 Mar 2016, H.J. Lu wrote:
>>>>>>
>>>>>>> On Tue, Mar 15, 2016 at 2:39 PM, Joseph Myers
>>>>>>> <joseph@codesourcery.com>
>>>>>>> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> I'm not sure if the zero-size arrays (a GNU extension) are
>>>>>>>> considered
>>>>>>>> to
>>>>>>>> make a struct non-empty, but in any case I think the tests should
>>>>>>>> cover
>>>>>>>> such arrays as elements of structs.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> There are couple tests for structs with members of array
>>>>>>> of empty types.  testsuite/g++.dg/abi/empty14.h has
>>>>>>
>>>>>>
>>>>>>
>>>>>> My concern is the other way round - structs with elements such as
>>>>>> "int a[0];", an array [0] of a nonempty type.  My reading of the
>>>>>> subobject
>>>>>> definition is that such an array should not cause the struct to be
>>>>>> considered nonempty (it doesn't result in any int subobjects).
>>>>>
>>>>>
>>>>>
>>>>> This is a test for struct with zero-size array, which isn't treated
>>>>> as empty type.  C++ and C are compatible in its passing.
>>>>
>>>>
>>>>
>>>> Where is the current definition of empty types you're proposing for use
>>>> in
>>>> GCC?  Is the behavior of this case clear from that definition?
>>>
>>>
>>>
>>> "An empty type is a type where it and all of its subobjects (recursively)
>>> are of structure, union, or array type.  No memory slot nor register
>>> should
>>> be used to pass or return an object of empty type."
>>>
>>> It seems to me that such a struct should be considered an empty type
>>> under
>>> this definition, since a zero-length array has no subobjects.
>>
>>
>> Since zero-size array is GCC extension, we can change it.   Do we
>> want to change its passing for C?
>
>
> I would think so; it seems to follow clearly from this definition.  I have
> trouble imagining that anyone would ever pass an object containing a
> zero-length array by value, so it shouldn't matter much either way, but I
> consistency is good.
>

This requires change in both C and C++ frontends.

-- 
H.J.