array profile mode (was: Re: [patch] libstdc++/65352 fix ubsan errors in std::array<T, 0>)

[François Dumont <frs dot dumont at gmail dot com>]

On 28/05/2015 16:46, Jonathan Wakely wrote:
> On 28/05/15 12:53 +0100, Jonathan Wakely wrote:
> > Unsurprisingly ubsan doesn't like referencing a null pointer.
> >
> > With this change __array_traits::_S_ref is only used to access an
> > element, which is invalid for std::array<T, 0> anyway.
> >
> > Tested powerpc64le-linux, committed to trunk.
>
> I forgot the debug and profile modes, fixed like so.
>
> 1) Why do we even have _profile::array? What's it for?

    There is no profiling code in array, indeed. This is so just for 
consistency with Debug mode. As soon as one of the alternative mode is 
activated normal implementation is put within __cxx1998 namespace and 
you need to provide an alternative implementation, potentially 
referencing the latter.

I tried once to do:

namespace std
{
namespace __profile
{
  template<typename _Tp, std::size_t _Nm>
    using array = _GLIBCXX_STD_C::array<_Tp, _Nm>;

}
}

but it doesn't work as we can't expose a template alias to user code in 
place of a struct.

So we could:
1. Have a special management of array in normal mode, not put it in 
__cxx1998 when only profile mode is activated.
2. Use inheritance on the normal implementation to reduce amount of 
duplicated code.

I will have a try in coming days.

François