On Tue, Nov 24, 2015 at 8:28 AM, Patrick Palka <patr...@parcs.ath.cx> wrote:
> On Tue, Nov 24, 2015 at 12:53 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>> On Mon, Nov 23, 2015 at 7:22 PM, Patrick Palka <patr...@parcs.ath.cx> wrote:
>>> On Mon, Nov 23, 2015 at 3:53 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>> On Mon, Nov 23, 2015 at 1:57 AM, Richard Biener
>>>> <richard.guent...@gmail.com> wrote:
>>>>> On Sat, Nov 21, 2015 at 12:46 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>>>> On Fri, Nov 20, 2015 at 2:17 PM, Jason Merrill <ja...@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.guent...@gmail.com> wrote:
>>>>>>>>>
>>>>>>>>> On Tue, Nov 17, 2015 at 12:01 PM, H.J. Lu <hjl.to...@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.
I tried a similar approach and got quite a few C++ failures in
gcc testsuite. There were more failures on i686 than x86-64.
See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60336
for more details. My current patch passes all tests on i686 and
x86-64.
--
H.J.
