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List: cfe-commits
Subject: Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct
From: Richard Smith via cfe-commits <cfe-commits () lists ! llvm ! org>
Date: 2016-02-21 6:48:46
Message-ID: CAOfiQqnQtJLNoa_aXtxeOiPQAJ6F7Grut14S9OyoUdvdAnUZuQ () mail ! gmail ! com
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On 20 Feb 2016 10:01 p.m., "H.J. Lu" <hjl.tools@gmail.com> wrote:
>
> On Sat, Feb 20, 2016 at 9:47 PM, Richard Smith <richard@metafoo.co.uk>
wrote:
> > On 20 Feb 2016 6:54 p.m., "H.J. Lu" <hjl.tools@gmail.com> wrote:
> >>
> >> On Sat, Feb 20, 2016 at 4:57 PM, Matthijs van Duin
> >> <matthijsvanduin@gmail.com> wrote:
> >> > On 20 February 2016 at 23:35, H.J. Lu <hjl.tools@gmail.com> wrote:
> >> >> Can a compiler tell if a copy constructor or destructor is trivial
> >> >> from the class declaration without function body?
> >> >
> >> > Yes, the mere presence of the declaration suffices to render it
> >> > non-trivial (unless explicitly declared "= default" like I did with
> >> > the default constructor, in which case there's no function body).
> >>
> >> How about this?
> >>
> >> An empty type is a type where it and all of its subobjects
(recursively)
> >> are of class, structure, union, or array type. An empty type may only
> >> have static member functions, default constructor, default copy
> >> constructor, default copy assignment operator or default destructor.
> >
> > No, that's the wrong rule still. Please leave the C++ rule here to the
C++
> > ABI rather than trying to reinvent it. Whether a type is empty is
completely
> > orthogonal to whether it must be passed through memory for C++ ABI /
> > semantics reasons.
>
> What is the correct wording? The last one:
>
> An empty type is a type where it and all of its subobjects (recursively)
> are of class, structure, union, or array type.
>
> doesn't cover "trivially-copyable".
That's correct. Whether a type is trivially copyable is unrelated to
whether it is empty.
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<p dir="ltr">On 20 Feb 2016 10:01 p.m., "H.J. Lu" <<a \
href="mailto:hjl.tools@gmail.com">hjl.tools@gmail.com</a>> wrote:<br> ><br>
> On Sat, Feb 20, 2016 at 9:47 PM, Richard Smith <<a \
href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>> wrote:<br> > \
> On 20 Feb 2016 6:54 p.m., "H.J. Lu" <<a \
href="mailto:hjl.tools@gmail.com">hjl.tools@gmail.com</a>> wrote:<br> > \
>><br> > >> On Sat, Feb 20, 2016 at 4:57 PM, Matthijs van Duin<br>
> >> <<a \
href="mailto:matthijsvanduin@gmail.com">matthijsvanduin@gmail.com</a>> wrote:<br> \
> >> > On 20 February 2016 at 23:35, H.J. Lu <<a \
href="mailto:hjl.tools@gmail.com">hjl.tools@gmail.com</a>> wrote:<br> > \
>> >> Can a compiler tell if a copy constructor or destructor is \
trivial<br> > >> >> from the class declaration without function \
body?<br> > >> ><br>
> >> > Yes, the mere presence of the declaration suffices to render \
it<br> > >> > non-trivial (unless explicitly declared "= \
default" like I did with<br> > >> > the default constructor, in \
which case there's no function body).<br> > >><br>
> >> How about this?<br>
> >><br>
> >> An empty type is a type where it and all of its subobjects \
(recursively)<br> > >> are of class, structure, union, or array type. An \
empty type may only<br> > >> have static member functions, default \
constructor, default copy<br> > >> constructor, default copy assignment \
operator or default destructor.<br> > ><br>
> > No, that's the wrong rule still. Please leave the C++ rule here to the \
C++<br> > > ABI rather than trying to reinvent it. Whether a type is empty is \
completely<br> > > orthogonal to whether it must be passed through memory for \
C++ ABI /<br> > > semantics reasons.<br>
><br>
> What is the correct wording? The last one:<br>
><br>
> An empty type is a type where it and all of its subobjects (recursively)<br>
> are of class, structure, union, or array type.<br>
><br>
> doesn't cover "trivially-copyable".</p>
<p dir="ltr">That's correct. Whether a type is trivially copyable is unrelated to \
whether it is empty.<br> </p>
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