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

[Jason Merrill]

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.

Jason