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Re: [PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class
- From: Andrew Pinski <pinskia at gmail dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Patrick Palka <patrick at parcs dot ath dot cx>, Richard Biener <richard dot guenther at gmail dot com>, Jason Merrill <jason at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Mon, 23 Nov 2015 22:00:24 -0800
- Subject: Re: [PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class
- Authentication-results: sourceware.org; auth=none
- References: <1447758067-4440-1-git-send-email-hjl dot tools at gmail dot com> <CAFiYyc0M7O9-6QQT-JLZ4M8PuPii6cw497cgoiR35Ez8tOBCRg at mail dot gmail dot com> <CAMe9rOqu_ERwmLdeYZ2dM=Z9phz2_mm=b7F1jU0n+P0hyLogEQ at mail dot gmail dot com> <564F9BE9 dot 5040003 at redhat dot com> <CAMe9rOqZ_8dBuy8wOyki=Z-A2OB5zJdHE6Yxa0Zz5a6Z3N7i6Q at mail dot gmail dot com> <CAFiYyc3V_GLj6GhXA7K_0+VVP5GmtnYtvVRf9PTF_kGMFaUB3Q at mail dot gmail dot com> <CAMe9rOqTgpaOSs=7or3HvL9KT5Ui+f6xw4zgTJkFY5sOx13b5w at mail dot gmail dot com> <CA+C-WL-edsY9rWzE09XGs6Vhgp9Xn2KR50wJ3vDpwKkVh9fbZw at mail dot gmail dot com> <CAMe9rOpc0mPZx95Ns554aJpWAodNXguiAH7kVC+56VQf4MbyxQ at mail dot gmail dot com>
On Mon, Nov 23, 2015 at 9:53 PM, 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: â__restrict__â qualifiers cannot be applied to âAS::Lâ
>>>>>>> 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 ârestrictâ
>>>>> foo (struct dummy __restrict__ i)
>>>>> ^
>>>>> x.i:4:13: error: invalid use of ârestrictâ
>>>>> [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;
This above should not be needed as TYPE_FIELDS should include one
already. Or do you have prove it does not?
Thanks,
Andrew
>
> Does it work with variable argument list? Did you run GCC
> testsuite for both i686 and x86-64?
>
>
> --
> H.J.