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

[Jason Merrill <jason@redhat.com>]

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.

Jason