This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Richard Smith <richard at metafoo dot co dot uk>
- Cc: Matthijs van Duin <matthijsvanduin at gmail dot com>, Michael Matz <matz at suse dot de>, Jonathan Wakely <jwakely dot gcc at gmail dot com>, GCC Development <gcc at gcc dot gnu dot org>, cfe-commits <cfe-commits at lists dot llvm dot org>
- Date: Tue, 16 Feb 2016 12:25:49 -0800
- Subject: Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Authentication-results: sourceware.org; auth=none
- References: <CAMe9rOrsV-zohnj=31_DDYSxUDyRDYL0anTi_NJm5vqogF9URQ at mail dot gmail dot com> <CAMe9rOq5QL9J0F-isO1njJyDJcZ08gF4zUk4vXNONWSfUtSzwA at mail dot gmail dot com> <CAH6eHdR0vJkYuYeKU=zyhthOY0KT-m8C=m2AtZ5GfVeyPOA=jw at mail dot gmail dot com> <CAOfiQqmKnONCdEi=coKaAKO_0Ux5X1BdYZ5t98gCU+LN21xKvw at mail dot gmail dot com> <CAH6eHdTxMXA7BEEHB7y6c5dRG_Hur5=CmdrQWMRn3zRGHnGZuA at mail dot gmail dot com> <CAMe9rOr9CCDjVVdYq7SoCDewg6uLnegEvT3Rrf2jg05SSyM_GQ at mail dot gmail dot com> <CAOfiQqnK-MFNhjcnHPPBpFfcJxR5rkdj26x8HcfMOiibCtevXA at mail dot gmail dot com> <CAMe9rOpM5Cs4zthCTAyaBEx3TWAQ7QNKv=k9mrqNzvj-upGcUw at mail dot gmail dot com> <20160211104729 dot GA3522 at squirrel dot local> <CAMe9rOrVQ2RJJVwKio_yxjGaj8RXcu5oS6K55EWQ0K5xHPtn_Q at mail dot gmail dot com> <CAALWOA_9JdscgL+_1R4RToSOKdwEmS2+P5fctF4RDayjZUS4GA at mail dot gmail dot com> <CAH6eHdQteUNnyPXmuGmwNDgoheotDvJcEM1=qni6-VdEOySUQQ at mail dot gmail dot com> <alpine dot LSU dot 2 dot 20 dot 1602111527090 dot 20277 at wotan dot suse dot de> <CAMe9rOpkV9jdNGfifLOg-uUqGbXwynBiYJTTh2hPcpq+TO3mNg at mail dot gmail dot com> <alpine dot LSU dot 2 dot 20 dot 1602111547280 dot 20277 at wotan dot suse dot de> <CAMe9rOq0SoxOh=WtXJ_VE7CBioRARqub9G59SM+WhbL_VWcaRA at mail dot gmail dot com> <CAALWOA8-NCP+dTf35+PqcemzpPPR+7-Odvt+hUVTTsCmCbhWaQ at mail dot gmail dot com> <CAMe9rOpsgqN5XOSdCbTtwoRz+KbaZxr0s0OMBhRG4Yg=ayT=-Q at mail dot gmail dot com> <CAMe9rOqQLZgEfMBDudAekW_DUDJbofx+bj-O753Brk4YrREa+w at mail dot gmail dot com> <CAOfiQqnyde8eJPsNi=WhmGexZawD9efzeRKX7xgCN39D39BZtg at mail dot gmail dot com>
On Tue, Feb 16, 2016 at 12:22 PM, Richard Smith <richard@metafoo.co.uk> wrote:
> On Tue, Feb 16, 2016 at 10:24 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Fri, Feb 12, 2016 at 11:39 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> > On Fri, Feb 12, 2016 at 6:58 AM, Matthijs van Duin
>> > <matthijsvanduin@gmail.com> wrote:
>> >> On 11 February 2016 at 16:31, H.J. Lu <hjl.tools@gmail.com> wrote:
>> >>> struct A {
>> >>> static void foo (void) ();
>> >>> static int xxx;
>> >>> };
>> >>
>> >> What about it? It's an empty struct. (And it declares a function and
>> >> a variable in the namespace of A, which however do not have any
>> >> relevant impact here.)
>> >>
>> >
>> > Thanks for all the feedbacks. Here is the new proposal:
>> >
>> > 1. "empty type". An empty type is a trivially-copyable aggregate
>> > occupying zero bytes (excluding any padding).
>> > 2. No memory slot nor register should be used to pass or return an object
>> > of empty type.
>> >
>> > Footnote: Array of empty type can only passed by reference in C/C++.
>> >
>>
>> I updated intel386, x86-64 and IA MCU psABIs:
>>
>> https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI
>>
>> to specify:
>>
>> Empty type is defined as a trivially-copyable aggregate occupying zero bytes
>> (excluding any padding).
>
> I think this is now extremely unclear. Does an empty struct in C++
> occupy zero bytes? sizeof applied to it will produce at least 1.
Can it be considered as padding?
>> No memory slot nor register should be used to pass or
>> return an object object of empty type.
>>
>> with footnote: Array of empty type can only passed by reference in C and C++.
>>
>> Any comments?
>>
>> Thanks.
>>
>>
>> --
>> H.J.
--
H.J.
- References:
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct