Re: PING^1: [PATCH] Add TYPE_EMPTY_RECORD for C++ empty class

[Jakub Jelinek <jakub@redhat.com>]

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.
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...

	Jakub