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

[H.J. Lu]

On Tue, Jan 26, 2016 at 12:44 PM, Marc Glisse <marc.gli...@inria.fr> wrote:
> On Tue, 26 Jan 2016, H.J. Lu wrote:
>
>> On Tue, Jan 26, 2016 at 12:23 PM, Marc Glisse <marc.gli...@inria.fr>
>> wrote:
>>>
>>> On Tue, 26 Jan 2016, H.J. Lu wrote:
>>>
>>>> On Tue, Jan 26, 2016 at 11:27 AM, Jason Merrill <ja...@redhat.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> On 12/14/2015 05:08 PM, H.J. Lu wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> +  if (abi_version_at_least (10))
>>>>>> +    TYPE_EMPTY_RECORD (t) = is_really_empty_class (t);
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> This should use is_empty_class or CLASSTYPE_EMPTY_P.  We don't want to
>>>>> change how classes with just a vptr are passed.
>>>>>
>>>>> Otherwise, it looks OK to me.
>>>>
>>>>
>>>>
>>>> Is true_type an empty class here?  is_empty_class returns false
>>>> on this:
>>>
>>>
>>>
>>> It isn't empty in the usual C++ sense (we can't apply the empty base
>>> optimization to something that derives from it, for instance), or the one
>>> described in the itanium ABI (the relevant one here I guess). On the
>>> other
>>> hand, it is rather useless to pass it by value, so a different notion of
>>
>>
>> llvm/clang treats it as empty class and I think it should be treated
>> as "empty" class.
>
>
> Is it still empty if there are several empty members? Is there a clear
> definition somewhere of what empty means? I guess it makes sense to
> recursively allow "empty" members for this purpose.

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;
}

-- 
H.J.