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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: Jonathan Wakely <jwakely dot gcc at gmail dot com>
- Cc: Florian Weimer <fw at deneb dot enyo dot de>, GCC Development <gcc at gcc dot gnu dot org>, cfe-commits <cfe-commits at lists dot llvm dot org>
- Date: Mon, 8 Feb 2016 07:42:53 -0800
- Subject: Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
- Authentication-results: sourceware.org; auth=none
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On Mon, Feb 8, 2016 at 7:02 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 8 February 2016 at 13:54, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Sun, Feb 7, 2016 at 12:52 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> The standard-layout POD is well defined:
>>
>> https://en.wikipedia.org/wiki/C%2B%2B11#Modification_to_the_definition_of_plain_old_data
>>
>> Here is the updated proposal for Intel386, x86-64 and IA MCU psABIs:
>>
>> 1. "collection". A collection is a structure, union or C++ class.
>
> These are all "class types". Why invent a new term?
Because it applies to both C and C++. There is no class in C.
>> 2. "empty collection". An empty collection is:
>> a. A collection without member. Or
>
> What about base classes?
>
> What about bit-fields of length 0?
Is a collection with them standard-layout POD type?
>> b. A collection with only empty collections. Or
>
> What does "with" mean? Only members, or bases too?
Is "A collection with only members of empty collections" better?
>> c. An array of empty collections.
>> 3. "empty record". An empty record is Plain Old Data (POD) for the purpose
>> of standard-layout and
>
> "For the purposes of standard-layout" doesn't mean anything.
>
> A type is a standard-layout type, or it isn't.
How about "An empty record is standard-layout Plain Old Data (POD)
type and ..."?
> Do you mean "An empty record is a standard-layout type and..."
>
>> a. A collection without member. Or
>> b. A collection with only empty collections.
>
> ?
>
Is "A collection with only members of empty collections" better?
>
>> 4. No memory slot nor register should be used to pass or return an object of
>> empty record.
--
H.J.