[cxx-abi-dev] Passing an empty class by value

[John McCall]

> On Dec 11, 2015, at 2:41 AM, Gabriel Dos Reis <gdr at integrable-solutions.net> wrote:
> 
> Passing nothing at all is makes the most sense for a vigorous C++ support.  Passing a 1-byte that nothing can possibly read and makes sense of is an unfortunate sub-optimal codegen.

I completely agree that psABI authors should specify that empty types are passed as nothing.

John.

> 
> On Thu, Dec 10, 2015 at 11:05 PM, Marc Glisse <marc.glisse at inria.fr <mailto:marc.glisse at inria.fr>> wrote:
> On Fri, 11 Dec 2015, Nelson, Clark wrote:
> 
> It has come to my attention that GCC and clang generate incompatible code
> for passing an argument of an empty class type.
> 
> clang seems to completely ignore arguments and parameters of empty class
> type -- which seems to make a certain amount of sense.
> 
> OTOH, as far as I understand it, GCC effectively treats an empty class
> equivalently to a class containing a single member with some character
> type -- which also seems pretty reasonable.
> 
> Should the C++ ABI come down on one side or the other of this question?
> 
> See A-5 in https://mentorembedded.github.io/cxx-abi/cxx-closed.html <https://mentorembedded.github.io/cxx-abi/cxx-closed.html> for some historical discussion.
> 
> This is really the sort of question a psABI should settle. But of course
> the C language doesn't actually support a structure with no members, so
> it's not too surprising if a psABI doesn't nail down what should happen
> for this.
> 
> Last time I raised this, psABI was also suggested:
> http://sourcerytools.com/pipermail/cxx-abi-dev/2013-November/002627.html <http://sourcerytools.com/pipermail/cxx-abi-dev/2013-November/002627.html>
> 
> -- 
> Marc Glisse
> 
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