Re: niter_base simplification

[Jonathan Wakely <jwakely at redhat dot com>]

On 22/04/15 22:10 +0200, François Dumont wrote:
> Hello
>
>    I don't know if I am missing something but I think __niter_base 
> could be simplified to remove usage of _Iter_base. Additionally I 
> overload it to also remove __normal_iterator layer even if behind a 
> reverse_iterator or move_iterator, might help compiler to optimize 
> code, no ? If not, might allow other algo optimization in the 
> future...
>
>    I prefered to provide a __make_reverse_iterator to allow the 
> latter in C++11 and not only in C++14. Is it fine to do it this way or 
> do you prefer to simply get rid of all this part ?

It's fine to add __make_reverse_iterator but see my comment below.

>    * include/bits/cpp_type_traits.h (__gnu_cxx::__normal_iterator): 
> Delete.

You're removing __is_normal_iterator not __normal_iterator.

>    * include/bits/stl_algobase.h (std::__niter_base): Adapt.
>    * include/bits/stl_iterator.h (__make_reverse_iterator): New in C++11.
>    (std::__niter_base): Overloads for std::reverse_iterator,
>    __gnu_cxx::__normal_iterator and std::move_iterator.
>
> Tested under Linux x86_64. I checked that std::copy still ends up 
> calling __builtin_memmove when used on vector iterators.
>
> François

> diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
> index 0bcb133..73eea6b 100644
> --- a/libstdc++-v3/include/bits/stl_algobase.h
> +++ b/libstdc++-v3/include/bits/stl_algobase.h
> @@ -270,17 +270,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       return __a;
>     }
>
> -  // If _Iterator is a __normal_iterator return its base (a plain pointer,
> -  // normally) otherwise return it untouched.  See copy, fill, ... 
> +  // Fallback implementation of the function used to remove the
> +  // __normal_iterator wrapper. See copy, fill, ...

It's a bit strange to have a function with no other overloads visible
described as a fallback. It would be good to say that the other
definition is in bits/stl_iterator.h

>   template<typename _Iterator>
> -    struct _Niter_base
> -    : _Iter_base<_Iterator, __is_normal_iterator<_Iterator>::__value>
> -    { };
> -
> -  template<typename _Iterator>
> -    inline typename _Niter_base<_Iterator>::iterator_type
> +    inline _Iterator
>     __niter_base(_Iterator __it)
> -    { return std::_Niter_base<_Iterator>::_S_base(__it); }
> +    { return __it; }
>
>   // Likewise, for move_iterator.

This comment no longer makes sense, because you've removed the comment
on _Niter_base that it referred to. Please restore the original text
of the _Niter_base comment for _Miter_base.

(Alternatively, could the same simplification be made for
__miter_base? Do we need _Miter_base<> or just two overloads of
__miter_base()?)


>   template<typename _Iterator>
> diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
> index 4a9189e..3aad9f3 100644
> --- a/libstdc++-v3/include/bits/stl_iterator.h
> +++ b/libstdc++-v3/include/bits/stl_iterator.h
> @@ -390,7 +390,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     { return __y.base() - __x.base(); }
>   //@}
>
> -#if __cplusplus > 201103L
> +#if __cplusplus == 201103L
> +  template<typename _Iterator>
> +    inline reverse_iterator<_Iterator>
> +    __make_reverse_iterator(_Iterator __i)
> +    { return reverse_iterator<_Iterator>(__i); }
> +
> +# define _GLIBCXX_MAKE_REVERSE_ITERATOR(_Iter) \
> +  std::__make_reverse_iterator(_Iter)
> +#elif __cplusplus > 201103L
> #define __cpp_lib_make_reverse_iterator 201402
>
>   // _GLIBCXX_RESOLVE_LIB_DEFECTS
> @@ -400,6 +408,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     inline reverse_iterator<_Iterator>
>     make_reverse_iterator(_Iterator __i)
>     { return reverse_iterator<_Iterator>(__i); }
> +
> +# define _GLIBCXX_MAKE_REVERSE_ITERATOR(_Iter) \
> +  std::make_reverse_iterator(_Iter)
> +#endif
> +
> +#if __cplusplus >= 201103L
> +  template<typename _Iterator>
> +    auto
> +    __niter_base(reverse_iterator<_Iterator> __it)
> +    -> decltype(_GLIBCXX_MAKE_REVERSE_ITERATOR(__niter_base(__it.base())))
> +    { return _GLIBCXX_MAKE_REVERSE_ITERATOR(__niter_base(__it.base())); }
> #endif

It might be simpler to just add __make_reverse_iterator for >= 201103L
and then always use std::__make_reverse_iterator instead of a macro.

That's similar to what we do for std:__addressof and std:addressof.