Re: [libstdc++/61347] std::distance(list.first(),list.end()) in O(1)

[Jonathan Wakely <jwakely at redhat dot com>]

On 13/04/15 13:42 +0200, Marc Glisse wrote:
> this patch makes std::distance(list.first(),list.end()) a constant 
> time operation when optimizing, with no penalty for other calls. We 
> could do the test always (no __builtin_constant_p) but then it will 
> add a small runtime penalty for other calls, someone else can take 
> responsibility for that.

I like the way you've done it. No penalty for other calls is great
and IMHO it's OK that the optimisation only happens when optimising.

(Does it work even at -Og?)

> I could protect the whole overload with 
> #ifdef __OPTIMIZE__ (at -O0 the compiler does not remove the test 
> ++end==first as dead code), but I assume it is better to minimize 
> differences.

Agreed.

> There are other ways to specialize distance, but 
> overloading __distance seems to have the least drawbacks (most others 
> involve a new extra file). From an optimization POV, it would be a bit 
> better to avoid the while loop and instead call a separate function 
> that does it (say the regular __distance), it would make the function 
> smaller and thus easier to inline, but it is simpler this way and 
> works.

Is there any (dis)advantage to making one call the other, to avoid
duplicating the function bodies?

 template<typename _Tp>
   inline ptrdiff_t
   __distance(_GLIBCXX_STD_C::_List_iterator<_Tp> __first,
	       _GLIBCXX_STD_C::_List_iterator<_Tp> __last,
              input_iterator_tag __tag)
   {
     typedef _GLIBCXX_STD_C::_List_const_iterator<_Tp> _CIter;
     return std::__distance(_CIter(__first), _CIter(__last), __tag);
   }

> We could do something similar for std::set, but C++ will not let us 
> find the address of _Rb_tree_impl::_M_node_count from that of 
> _Rb_tree_impl::_M_header, except when _Key_compare is pod, which 
> luckily is an easily available information. Avoiding this complication 
> is why I insisted on wrapping the size of std::list in a 
> _List_node<size_t> for gcc-5, which may look a bit strange at first 
> sight.

Sadly, that node is going to look even stranger when I finish adding
support for C++11 allocators, as the type of node becomes dependent on
the allocator's pointer, which makes _List_node<size_t> much more
complicated :-(

I'll have to remember to add additional __distance overloads to handle
the new _List_ptr_iterator and _List_ptr_const_iterator types that
will be used for fancy pointers (although if I forget the optimisation
will still work for std::list<T, std::allocator<T>>, just not for the
vanishingly small number of people using allocators with fancy
pointers).

> Index: include/bits/stl_iterator_base_funcs.h
> ===================================================================
> --- include/bits/stl_iterator_base_funcs.h	(revision 222041)
> +++ include/bits/stl_iterator_base_funcs.h	(working copy)
> @@ -107,22 +107,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    *  n may be negative.
>    *
>    *  For random access iterators, this uses their @c + and @c - operations
>    *  and are constant time.  For other %iterator classes they are linear time.
>   */
>   template<typename _InputIterator>
>     inline typename iterator_traits<_InputIterator>::difference_type
>     distance(_InputIterator __first, _InputIterator __last)
>     {
>       // concept requirements -- taken care of in __distance
> -      return std::__distance(__first, __last,
> -			     std::__iterator_category(__first));
> +      return __distance(__first, __last, std::__iterator_category(__first));

This should still be a qualified call to std::__distance, otherwise the
compiler might end up instantiating things to figure out if there are
any associated namespaces, e.g. for vector<unique_ptr<T>>::iterator we
don't want to know T's base classes and rheir associated namespaces.

> +  // Detect when distance is used to compute the size of the whole list.
> +  template<typename _Tp>
> +    inline ptrdiff_t
> +    __distance(_GLIBCXX_STD_C::_List_iterator<_Tp> __first,
> +	       _GLIBCXX_STD_C::_List_iterator<_Tp> __last,
> +               input_iterator_tag)
> +    {
> +      typedef _GLIBCXX_STD_C::_List_node<size_t> _Sentinel;
> +      _GLIBCXX_STD_C::_List_iterator<_Tp> __beyond = __last;
> +      ++__beyond;
> +      bool __whole = __first == __beyond;
> +      if (__builtin_constant_p (__whole) && __whole)

This is clever :-)

This shouldn't interfere with any changes we might need to test before
backporting to the gcc-5-branch, so with the std:: qualification
restored on the call to __distance it's OK for trunk.

(I'll trust your judgment/masurements for whether it's worth making
the _List_iterator overload call the _List_const_iterator one.)