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

[Richard Smith]

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.
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