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

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Jan 27, 2016 at 1:03 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> 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 ;-)
>

I opened  a clang bug:

https://llvm.org/bugs/show_bug.cgi?id=26337

I propose the following definitions:

i. An empty record is:
   1. A class without member.  Or
   2. An array of empty records.  Or
   3. A class with empty records.
ii. An empty record type for parameter passing is POD for the purpose of layout
and
   1. A class without member.  Or
   2. A class with empty records.

/* An empty record is:
   1. A class without member.  Or
   2. An array of empty records.  Or
   3. A class with empty records.  */

/* Returns true if TYPE is an empty record or an array of empty records.  */

static bool
is_empty_record_or_array_of_empty_record (tree type)
{
  if (CLASS_TYPE_P (type))
    {
      if (CLASSTYPE_EMPTY_P (type))
        return true;

      tree field;

      for (field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
        if (TREE_CODE (field) == FIELD_DECL
            && !DECL_ARTIFICIAL (field)
            && !is_empty_record_or_array_of_empty_record (TREE_TYPE (field)))
          return false;
      return true;
    }
  else if (TREE_CODE (type) == ARRAY_TYPE)
    return is_empty_record_or_array_of_empty_record (TREE_TYPE (type));
  return false;
}

/* Returns true if TYPE is POD for the purpose of layout and
   1. A class without member.  Or
   2. A class with empty records.  */

static bool
is_empty_record_for_parm (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;

  tree field;

  for (field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
    if (TREE_CODE (field) == FIELD_DECL
        && !DECL_ARTIFICIAL (field)
        && !is_empty_record_or_array_of_empty_record (TREE_TYPE (field)))
      return false;

  return true;
}


-- 
H.J.