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Date: Tue, 26 Jan 2016 22:21:00 -0000
Message-ID: <CAMe9rOrFyPmyvnxZtohyC05mXUPPQ6DWk3aCXU8RWts+w9GH+w@mail.gmail.com>
Subject: Re: PING^1: [PATCH] Add TYPE_EMPTY_RECORD for C++ empty class
From: "H.J. Lu" <hjl.tools@gmail.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>, Jason Merrill <jason@redhat.com>, 	Richard Biener <richard.guenther@gmail.com>, Markus Trippelsdorf <markus@trippelsdorf.de>
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On Tue, Jan 26, 2016 at 1:40 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Tue, Jan 26, 2016 at 01:21:52PM -0800, H.J. Lu wrote:
>> Like this:
>>
>> /* Returns true if TYPE is POD 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;
>>
>>   tree field;
>>
>>   for (field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
>>     if (TREE_CODE (field) == FIELD_DECL
>>         && !DECL_ARTIFICIAL (field)
>>         && !is_empty_record (TREE_TYPE (field)))
>>       return false;
>>
>>   return true;
>> }
>
> So you say that K1 in e.g.:
>
> struct A1 {}; struct A2 {};
> struct B1 { A1 a; A2 b; }; struct B2 { A1 a; A2 b; };
> struct C1 { B1 a; B2 b; }; struct C2 { B1 a; B2 b; };
> struct D1 { C1 a; C2 b; }; struct D2 { C1 a; C2 b; };
> struct E1 { D1 a; D2 b; }; struct E2 { D1 a; D2 b; };
> struct F1 { E1 a; E2 b; }; struct F2 { E1 a; E2 b; };
> struct G1 { F1 a; F2 b; }; struct G2 { F1 a; F2 b; };
> struct H1 { G1 a; G2 b; }; struct H2 { G1 a; G2 b; };
> struct I1 { H1 a; H2 b; }; struct I2 { H1 a; H2 b; };
> struct J1 { I1 a; I2 b; }; struct J2 { I1 a; I2 b; };
> struct K1 { J1 a; J2 b; };
> int v;
> __attribute__((noinline, noclone))
> K1 foo (int a, K1 x, int b)
> {
>   v = a + b;
>   return x;
> }
> K1 k, m;
> void
> bar (void)
> {
>   m = foo (1, k, 2);
> }
>
> is empty class?  What does clang do with this?
>
>         Jakub

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;

  tree field;

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

  return true;
}


-- 
H.J.