On Wed, 27 Jan 2016, Jakub Jelinek wrote:
On Wed, Jan 27, 2016 at 09:10:14AM +0100, Marc Glisse wrote:
Revised:
/* Returns true if TYPE is POD of one-byte or less in size for the purpose
of layout and an empty class or an class with empty classes. */
static bool
is_empty_record (tree type)
{
if (type == error_mark_node)
return false;
if (!CLASS_TYPE_P (type))
return false;
if (CLASSTYPE_NON_LAYOUT_POD_P (type))
return false;
gcc_assert (COMPLETE_TYPE_P (type));
if (CLASSTYPE_EMPTY_P (type))
return true;
if (int_size_in_bytes (type) > 1)
return false;
That's completely arbitrary :-(
Yeah. Because (adapted to be compilable with C):
struct A1 {}; struct A2 {};
struct B1 { struct A1 a; struct A2 b; }; struct B2 { struct A1 a; struct A2 b;
};
struct C1 { struct B1 a; struct B2 b; }; struct C2 { struct B1 a; struct B2 b;
};
struct D1 { struct C1 a; struct C2 b; }; struct D2 { struct C1 a; struct C2 b;
};
struct E1 { struct D1 a; struct D2 b; }; struct E2 { struct D1 a; struct D2 b;
};
struct F1 { struct E1 a; struct E2 b; }; struct F2 { struct E1 a; struct E2 b;
};
struct G1 { struct F1 a; struct F2 b; }; struct G2 { struct F1 a; struct F2 b;
};
struct H1 { struct G1 a; struct G2 b; }; struct H2 { struct G1 a; struct G2 b;
};
struct I1 { struct H1 a; struct H2 b; }; struct I2 { struct H1 a; struct H2 b;
};
struct J1 { struct I1 a; struct I2 b; }; struct J2 { struct I1 a; struct I2 b;
};
struct K1 { struct J1 a; struct J2 b; };
int v;
__attribute__((noinline, noclone))
struct K1 foo (int a, struct K1 x, int b)
{
v = a + b;
return x;
}
struct K1 k, m;
void
bar (void)
{
m = foo (1, k, 2);
}
then would have a different calling convention between C and C++,
so where is the argument that we change anything just to make the two
compatible? Though, of course, those two will never be compatible,
it is enough to add struct L1 { int a; struct K1 b; int c; }; and
that structure has 1024+8 bytes in C++ and 8 bytes in C.
I don't know how empty classes are used in C in practice, but it could
make sense to have ABI compatibility as long as no empty struct is used as
a member of another struct (I also suggested an attribute to let
C++ use the same layout as C here: PR63579). But then the usual definition
of empty would be sufficient.
As clang generates different code for the above between C and C++, it
clearly special cases for some reason just the most common case.
IMHO it is not worth to change GCC ABI...
I was interested in this change because it improves C++, C compatibility
was a convenient excuse ;-)
--
Marc Glisse
