[PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class

[H.J. Lu]

On Fri, Nov 20, 2015 at 2:17 PM, Jason Merrill <jason@redhat.com> wrote:
> On 11/20/2015 01:52 PM, H.J. Lu wrote:
>>
>> On Tue, Nov 17, 2015 at 4:22 AM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>>
>>> On Tue, Nov 17, 2015 at 12:01 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>
>>>> Empty record should be returned and passed the same way in C and C++.
>>>> This patch adds LANG_HOOKS_EMPTY_RECORD_P for C++ empty class, which
>>>> defaults to return false.  For C++, LANG_HOOKS_EMPTY_RECORD_P is defined
>>>> to is_really_empty_class, which returns true for C++ empty classes.  For
>>>> LTO, we stream out a bit to indicate if a record is empty and we store
>>>> it in TYPE_LANG_FLAG_0 when streaming in.  get_ref_base_and_extent is
>>>> changed to set bitsize to 0 for empty records.  Middle-end and x86
>>>> backend are updated to ignore empty records for parameter passing and
>>>> function value return.  Other targets may need similar changes.
>>>
>>>
>>> Please avoid a new langhook for this and instead claim a bit in
>>> tree_type_common
>>> like for example restrict_flag (double-check it is unused for
>>> non-pointers).
>>
>>
>> There is no bit in tree_type_common I can overload.  restrict_flag is
>> checked for non-pointers to issue an error when it is used on
>> non-pointers:
>>
>>
>> /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/template/qualttp20.C:19:38:
>> error: ‘__restrict__’ qualifiers cannot be applied to ‘AS::L’
>>     typedef typename T::L __restrict__ r;// { dg-error "'__restrict__'
>> qualifiers cannot" "" }
>
>
> The C++ front end only needs to check TYPE_RESTRICT for this purpose on
> front-end-specific type codes like TEMPLATE_TYPE_PARM; cp_type_quals could
> handle that specifically if you change TYPE_RESTRICT to only apply to
> pointers.
>

restrict_flag is also checked in this case:

[hjl@gnu-6 gcc]$ cat x.i
struct dummy { };

struct dummy
foo (struct dummy __restrict__ i)
{
  return i;
}
[hjl@gnu-6 gcc]$ gcc -S x.i -Wall
x.i:4:13: error: invalid use of ‘restrict’
 foo (struct dummy __restrict__ i)
             ^
x.i:4:13: error: invalid use of ‘restrict’
[hjl@gnu-6 gcc]$

restrict_flag can't also be used to indicate `i' is an empty record.


H.J.