On 02/27/2016 02:57 AM, Gabriel Dos Reis wrote:
On Fri, Feb 26, 2016 at 8:01 PM, Jason Merrill <[email protected]> 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 <[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.
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.
I also notice that the ABI says "If the base ABI does not specify rules
for empty classes, then an empty class has size and alignment 1."
Jason
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