[PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class

Richard Biener richard.guenther@gmail.com
Mon Nov 23 09:59:00 GMT 2015

On Sat, Nov 21, 2015 at 12:46 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 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.

I'm sure this error can be done during parsing w/o relying on TYPE_RESTRICT.

But well, use any other free bit (but do not enlarge
tree_type_common).  Eventually
you can free up a bit by putting sth into type_lang_specific currently
using bits
in tree_type_common.


> H.J.

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