Re: [PATCH] PR libstdc++/86127 avoid unnecessary allocator conversions

[Jonathan Wakely <jwakely at redhat dot com>]

On 14/06/18 07:45 +0200, François Dumont wrote:
> You're still using __gnu_cxx::__alloc_traits for 
> _Fwd_list_base::_Node_alloc_traits to get the _S_always_equal().
>
> So you might perhaps cleanup this part of the ChangeLog entry.

Oops.

> Or make this true by making noexcept qualification dependent on the 
> underlying constructors like I did for rb_tree:
>
>       _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a)
>       noexcept( noexcept(
>     _Rb_tree(std::declval<_Rb_tree&&>(), std::declval<_Node_allocator&&>(),
>          std::declval<typename _Alloc_traits::is_always_equal>())) )

It's also needed in forward_list.tcc for _S_propagate_on_copy_assign
and _S_always_equal which is why I kept it. But I fogot to update the
ChangeLog after deciding to keep it.


> On 13/06/2018 17:13, Jonathan Wakely wrote:
> > There is no need to use an allocator of the correct value_type when
> > calling allocator_traits::construct and allocator_traits::destroy. The
> > existing node allocator can be used, instead of constructing a new
> > allocator object every time.
> >
> > There's also no benefit to using __gnu_cxx::__alloc_traits instead of
> > std::allocator_traits to get the pointer and const_pointer types.
> > std::forward_list is only available for C++11 and later, when
> > std::allocator_traits is available too.
> >
> >     PR libstdc++/86127
> >     * include/bits/forward_list.h (_Fwd_list_base::_Tp_alloc_type): 
> > Remove
> >     unused typedef.
> >     (_Fwd_list_base::_Node_alloc_traits): Use allocator_traits instead of
> >     __gnu_cxx::__alloc_traits.
> >     (_Fwd_list_base::_M_create_node, _Fwd_list_base::_M_erase_after):
> >     Use node allocator to create and destroy elements.
> >     (forward_list::_Tp_alloc_type): Remove unused typedef.
> >     (forward_list::_Alloc_traits): Use allocator_traits instead of
> >     __gnu_cxx::__alloc_traits.
> >
> > Tested powerpc64le-linux, committed to trunk.