[PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class

H.J. Lu hjl.tools@gmail.com
Mon Nov 23 20:54:00 GMT 2015

On Mon, Nov 23, 2015 at 1:57 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> 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.

There are no bits in tree_type_common I can move.  Instead,
this patch overloads side_effects_flag in tree_base.  Tested on
Linux/x86-64.  OK for trunk?


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