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List:       gcc
Subject:    Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
From:       Richard Smith <richard () metafoo ! co ! uk>
Date:       2016-02-19 21:07:30
Message-ID: CAOfiQqmYkZvq93OV14m7-f7pn5j+3o4RTOSuFuLM-s7n9MFwzg () mail ! gmail ! com
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On Fri, Feb 19, 2016 at 5:35 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Thu, 18 Feb 2016, Richard Smith wrote:
>
>> >> An empty type is a type where it and all of its subobjects
>> >> (recursively) are of class, structure, union, or array type.  No
>> >> memory slot nor register should be used to pass or return an object
>> >> of empty type.
>> >
>> > The trivially copyable is gone again.  Why is it not necessary?
>>
>> The C++ ABI doesn't defer to the C psABI for types that aren't
>> trivially-copyable. See
>> http://mentorembedded.github.io/cxx-abi/abi.html#normal-call
>
> Hmm, yes, but we don't want to define something for only C and C++, but
> language independend (so far as possible).  And given only the above
> language I think this type:
>
> struct S {
>   S() {something();}
> };
>
> would be an empty type, and that's not what we want.

Yes it is. Did you mean to give S a copy constructor, copy assignment
operator, or destructor instead?

> "Trivially copyable"
> is a reasonably common abstraction (if in doubt we could even define it in
> the ABI), and captures the idea that we need well (namely that a bit-copy
> is enough).
>
>
> Ciao,
> Michael.
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