Re: [PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class

[Patrick Palka <patrick@parcs.ath.cx>]

On Tue, Nov 24, 2015 at 12:53 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Nov 23, 2015 at 7:22 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
>> On Mon, Nov 23, 2015 at 3:53 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> 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?
>>>
>>
>> Hi,
>>
>> Coincidentally a few months ago I was experimenting with making
>> empty-struct function arguments zero-cost (and thus making them behave
>> the same way as in GNU C).  My approach (patch attached) was to assign
>> empty-struct arguments to a virtual register (instead of on the stack
>> or to a hard register) during RTL call expansion.  These
>> virtual-register assignments would then be trivially DCE'd later.
>> This approach seemed to work surprisingly well with minimal code
>> changes.  I wonder what
>> your thoughts are on this approach..
>
> I don't think it works for C++ class.  empty_record_or_union_type_p
> missed:
>
>     for (binfo = TYPE_BINFO (type), i = 0;
>            BINFO_BASE_ITERATE (binfo, i, base_binfo); ++i)
>         if (!is_really_empty_class (BINFO_TYPE (base_binfo)))
>           return false;

Yeah, your TYPE_EMPTY_RECORD flag covers more instances of empty
structs than this predicate does.

>
> Does it work with variable argument list?   Did you run GCC
> testsuite for both i686 and x86-64?

Hmm, I don't think it works with variable argument lists, at least not
perfectly. And I just finished running the testsuite on x86-64 and
observed a failure in struct-layout-1.exp which makes no sense to me.
Now I remember why I didn't pursue this change any further.