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

[Richard Smith]

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.
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