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

[Jason Merrill]

On 02/29/2016 02:10 AM, Richard Smith wrote:
> On 28 February 2016 at 19:26, Jason Merrill <jason at redhat.com> wrote:
>
>> On 02/27/2016 02:57 AM, Gabriel Dos Reis wrote:
>>
>>> On Fri, Feb 26, 2016 at 8:01 PM, Jason Merrill <jason at redhat.com> wrote:
>>>
>>> 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.
>>>>
>>>
>>> Five years ago, we determined that it would make sense (read: provably
>>> safe) to optimize that case too -- even for arrays.  See section 6.2 of
>>>         http://www.axiomatics.org/~gdr/formal-cxx/layout-popl11.pdf
>>> for the "layman" summary.
>>>
>>
>> Sure.  But my point is that this represents a change to the ABI, not a
>> simple implementation bug as I initially thought.  If we want to omit
>> passing some parameters, we need some specification of which parameters
>> qualify, and I'm rather uncomfortable with that level of invention at this
>> point.
>
> The x86_64 psABI already classifies an empty struct of size <= 16 as
> NO_CLASS, so it is not passed in registers nor memory. The recent
> discussion on the GCC and LLVM dev lists (subject line "Update Intel386,
> x86-64 and IA MCU psABIs for passing/returning empty struct") concluded by
> adding this rule to the i386, x86-64, and Itanium psABI documents:
>
> "An empty type is a type where it and all of its subobjects (recursively)
> are of class, structure, union, or array type.  No memory slot nor register
> should be used to pass or return an object of empty type."
>
> ... which effectively extends that rule to empty types of all sizes.

Thanks for pointing me at that discussion.  I don't think that thread 
has quite concluded, but that rule seems promising.

Jason