From: Jonathan Wakely <jwakely@redhat.com>
To: Marc Glisse <marc.glisse@inria.fr>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: [libstdc++/61347] std::distance(list.first(),list.end()) in O(1)
Date: Mon, 13 Apr 2015 13:40:00 -0000 [thread overview]
Message-ID: <20150413134025.GI9755@redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1504131033550.13447@stedding.saclay.inria.fr>
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.)
next prev parent reply other threads:[~2015-04-13 13:40 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-13 11:42 Marc Glisse
2015-04-13 13:40 ` Jonathan Wakely [this message]
2015-04-13 14:15 ` Marc Glisse
2015-04-13 15:11 ` Jonathan Wakely
2015-04-14 8:24 ` Marc Glisse
2015-04-14 9:32 ` Jonathan Wakely
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