On 12/10/2015 07:32 PM, Richard Smith wrote:
On 10 December 2015 at 16:15, John McCall <[email protected]> wrote:
On Dec 10, 2015, at 4:11 PM, Nelson, Clark <[email protected]>
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
_______________________________________________
cxx-abi-dev mailing list
[email protected]
http://sourcerytools.com/cgi-bin/mailman/listinfo/cxx-abi-dev
