[cxx-abi-dev] Passing an empty class by value

[Jason Merrill]

On 12/11/2015 01:37 PM, Jason Merrill wrote:
> On 12/10/2015 07:32 PM, Richard Smith wrote:
>> On 10 December 2015 at 16:15, John McCall <rjmccall at apple.com> wrote:
>>
>>>> On Dec 10, 2015, at 4:11 PM, Nelson, Clark <clark.nelson at intel.com>
>>> wrote:
>>>>
>>>> It has come to my attention that GCC and clang generate incompatible
>>>> code
>>>> for passing an argument of an empty class type.
>>>>
>>>> clang seems to completely ignore arguments and parameters of empty
>>>> class
>>>> type -- which seems to make a certain amount of sense.
>>>>
>>>> OTOH, as far as I understand it, GCC effectively treats an empty class
>>>> equivalently to a class containing a single member with some character
>>>> type -- which also seems pretty reasonable.
>>>>
>>>> Should the C++ ABI come down on one side or the other of this question?
>>>>
>>>> This is really the sort of question a psABI should settle. But of
>>>> course
>>>> the C language doesn't actually support a structure with no members, so
>>>> it's not too surprising if a psABI doesn't nail down what should happen
>>>> for this.
>>>
>>> It’s valid as a C extension in GCC.  If there are platforms where we
>>> use a
>>> different rule from GCC, we should come to some understanding with them.
>>>
>>> Because of the GCC extension, C++ can’t really use different rules
>>> from C.
>>
>>
>> (For x86_64:) GCC uses different rules for C and C++. In C, they do not
>> pass anything. In C++, they pass a 1-byte object on the stack. Clang uses
>> the same rules for C and C++, passing nothing in both cases.
>>
>> A careful reading of the x86_64 psABI suggests that clang is right in
>> both
>> cases; the eightbyte corresponding to the 1-byte empty struct
>> parameter is
>> never classified (at all), so should occupy neither a register nor
>> memory,
>> but it's not really especially clear.
>>
>> In any case, I think GCC should be the one to change here, because its C
>> and C++ ABIs don't match.
>
> Agreed.

On further discussion, it came up that the C and C++ ABIs really can't 
match.  In GNU C, an empty struct has size 0; in C++ it has size 1.  We 
could finesse this difference for argument passing, as suggested here, 
but we really can't paper over the difference in general.  What if you 
have a class containing an empty class?  GNU C still says that has size 
0, but it's less clear that this makes sense for C++.  Significantly, 
this does not fit the ABI definition of an empty class.

I also notice that the ABI says "If the base ABI does not specify rules 
for empty classes, then an empty class has size and alignment 1."

Jason