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Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- From: Richard Smith <richard at metafoo dot co dot uk>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: 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: Mon, 8 Feb 2016 14:35:58 -0800
- Subject: Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Authentication-results: sourceware.org; auth=none
- References: <CAMe9rOofYT89yzF6jxFZFG2=QVQuQpEC7DP0HV_epjeawCf_sA at mail dot gmail dot com> <87d1s8wc4w dot fsf at mid dot deneb dot enyo dot de> <CAMe9rOp_6ZpR1YSJZ7AiLpa+FPRGJoLm5fTPkiH5BgF4Tkg3RA at mail dot gmail dot com> <87r3gouvv2 dot fsf at mid dot deneb dot enyo dot de> <CAMe9rOo2tPZKZEgpgFbbSnkyBiONzS9vfntn_QsxRPbVHZGmUw at mail dot gmail dot com> <87k2mgutzr dot fsf at mid dot deneb dot enyo dot de> <CAMe9rOoMujhjExxKWrz-Dv8uE33X=Upvi_X_HZsfeN0GS+=56A at mail dot gmail dot com> <CAMe9rOruCOvtM2D2AmG94A1_8Og8Kfu7tYF4De=XNfzUG+eh5g at mail dot gmail dot com> <CAH6eHdQci21ZhZ4odOfJRjC8s8HjuJ5X73LfK6oqvxCtOnzK6w at mail dot gmail dot com> <CAMe9rOpJXNUNvusTmwFb3riOxXyXLTGTquSMKqZkBe=nbmt_QQ at mail dot gmail dot com> <CAH6eHdQcvj7EY3jXgtUBQePfikiTbcuqsNFpDH7SWyxD628peQ at mail dot gmail dot com> <CAMe9rOrsV-zohnj=31_DDYSxUDyRDYL0anTi_NJm5vqogF9URQ at mail dot gmail dot com> <CAH6eHdQtnzMO6gLLRsczDdvmx0exkqXxyY-WvZ6RjuM7a3=85A at mail dot gmail dot com> <CAH6eHdQQjpOwD2kNCZVjH+w6N0Wh=gbdaaCcjg21iEbaHaAE5A 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>
On Mon, Feb 8, 2016 at 1:40 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, Feb 8, 2016 at 12:38 PM, Richard Smith <richard@metafoo.co.uk> wrote:
> > On Mon, Feb 8, 2016 at 12:05 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >>
> >> On Mon, Feb 8, 2016 at 11:33 AM, Jonathan Wakely <jwakely.gcc@gmail.com>
> >> wrote:
> >> > On 8 February 2016 at 19:23, Richard Smith wrote:
> >> >> "POD for the purpose of layout" is defined in the Itanium C++ ABI here:
> >> >>
> >> >> http://mentorembedded.github.io/cxx-abi/abi.html#definitions
> >> >
> >> > Thanks. So there's no problem using "POD for the purposes of layout",
> >> > and the change to "POD for the purpose of standard-layout" was
> >> > unnecessary and just confused things.
> >>
> >> Here is the revised proposal:
> >>
> >> 1. "class type". A class type is a structure, union or C++ class.
> >> 2. "empty class type". An empty class type is:
> >> a. A class type without member. Or
> >> b. A class type with only members of empty class types. Or
> >
> >
> > (a) is a special case of (b).
> >
> >> c. An array of empty class types.
> >
> >
> > It seems confusing to call an array a class type. Instead, how about:
> >
> > 2. An empty type is either an array of empty types or a class type where
> > every member is of empty type.
> >
> >> 3. "empty record". An empty record is Plain Old Data (POD) for the
> >> purposes of layout and
> >> a. A class type without member. Or
> >> b. A class type with only members of empty class types.
> >
> >
> > (a) is a special case of (b).
> >
> >> 4. No memory slot nor register should be used to pass or return an object
> >> of empty record.
> >
> >
> > Objects of array type are never passed or returned (but if through some
> > language extension they were, we'd want this rule to apply). So you don't
> > need rule 3 and this can be just:
> >
> > 3. No memory slot nor register should be used to pass or return an object
> > of empty type.
>
> Thanks very much for your inputs. Here is the proposal:
>
> 1. "class type". A class type is a structure, union or C++ class.
> 2. "empty type". An empty type is either an array of empty types or a
> class type where every member is of empty type.
> 3. No memory slot nor register should be used to pass or return an object
> of empty type.
David Majnemer points out that we also need to say something about
base classes. We could handle that case like this:
2. "empty type". An empty type is a type where it and all of its
subobjects are of class or array type.
Following the C++ rules, this also means that a class that contains
only unnamed bitfields is empty, because unnamed bitfields are not
subobjects, but might be worth explicitly stating for the C case. That
also matches Clang's behavior.
- References:
- 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
- Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct