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Performance tweaking rotate.

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rotate is a critical algorithm because it is often used by other algorithms,
both std and non-std.  The main thrust of this optimization is a specialized
algorithm when the 'distance' to be shifted is 1 (either left or right).  To my
surprise, this 'optimization' was not effective for types like std::string.
std::string favors rotate algorithms which only use swap.  But for types like
scalars, and especially when the sequence is random access, these new
specializations are a big win.  If it is a vector<size_t> for example, the
rotate is done via a memmove and can be several times faster than the gcd
algorithm.

I'm using is_trivially_move_assignable to distinguish between types like int and
types like string.  This is obviously an ad-hoc approximation, but I haven't
found a case where it doesn't give good results.

I've used a 'static if' (with is_trivially_move_assignable) in three places. 
Testing with both -Os and -O3 showed that clang eliminated all code not be
executed by the 'static if' (including the 'static if' itself).

llvm-svn: 161247
This commit is contained in:
Howard Hinnant 2012-08-03 18:01:20 +00:00
parent 46342fecd0
commit aca09de378
1 changed files with 84 additions and 27 deletions
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111
libcxx/include/algorithm
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@ -2103,12 +2103,31 @@ reverse_copy(_BidirectionalIterator __first, _BidirectionalIterator __last, _Out
template <class _ForwardIterator>
_ForwardIterator
__rotate(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last, false_type)
__rotate_left(_ForwardIterator __first, _ForwardIterator __last)
{
typedef typename iterator_traits<_ForwardIterator>::value_type value_type;
value_type __tmp = _VSTD::move(*__first);
_ForwardIterator __lm1 = _VSTD::move(_VSTD::next(__first), __last, __first);
*__lm1 = _VSTD::move(__tmp);
return __lm1;
}
template <class _BidirectionalIterator>
_BidirectionalIterator
__rotate_right(_BidirectionalIterator __first, _BidirectionalIterator __last)
{
typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
_BidirectionalIterator __lm1 = _VSTD::prev(__last);
value_type __tmp = _VSTD::move(*__lm1);
_BidirectionalIterator __fp1 = _VSTD::move_backward(__first, __lm1, __last);
*__first = _VSTD::move(__tmp);
return __fp1;
}
template <class _ForwardIterator>
_ForwardIterator
__rotate_forward(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last)
{
if (__first == __middle)
return __last;
if (__middle == __last)
return __first;
_ForwardIterator __i = __middle;
while (true)
{
@ -2156,15 +2175,11 @@ __gcd(_Integral __x, _Integral __y)
template<typename _RandomAccessIterator>
_RandomAccessIterator
__rotate(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last, true_type)
__rotate_gcd(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last)
{
typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
if (__first == __middle)
return __last;
if (__middle == __last)
return __first;
const difference_type __m1 = __middle - __first;
const difference_type __m2 = __last - __middle;
if (__m1 == __m2)
@ -2172,15 +2187,15 @@ __rotate(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomA
_VSTD::swap_ranges(__first, __middle, __middle);
return __middle;
}
const difference_type __g = __gcd(__m1, __m2);
const difference_type __g = _VSTD::__gcd(__m1, __m2);
for (_RandomAccessIterator __p = __first + __g; __p != __first;)
{
value_type __t(*--__p);
value_type __t(_VSTD::move(*--__p));
_RandomAccessIterator __p1 = __p;
_RandomAccessIterator __p2 = __p1 + __m1;
do
{
*__p1 = *__p2;
*__p1 = _VSTD::move(*__p2);
__p1 = __p2;
const difference_type __d = __last - __p2;
if (__m1 < __d)
@ -2188,30 +2203,72 @@ __rotate(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomA
else
__p2 = __first + (__m1 - __d);
} while (__p2 != __p);
*__p1 = __t;
*__p1 = _VSTD::move(__t);
}
return __first + __m2;
}
template <class _ForwardIterator>
inline _LIBCPP_INLINE_VISIBILITY
_ForwardIterator
__rotate(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last,
_VSTD::forward_iterator_tag)
{
typedef typename _VSTD::iterator_traits<_ForwardIterator>::value_type value_type;
if (_VSTD::is_trivially_move_assignable<value_type>::value)
{
if (_VSTD::next(__first) == __middle)
return _VSTD::__rotate_left(__first, __last);
}
return _VSTD::__rotate_forward(__first, __middle, __last);
}
template <class _BidirectionalIterator>
inline _LIBCPP_INLINE_VISIBILITY
_BidirectionalIterator
__rotate(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last,
_VSTD::bidirectional_iterator_tag)
{
typedef typename _VSTD::iterator_traits<_BidirectionalIterator>::value_type value_type;
if (_VSTD::is_trivially_move_assignable<value_type>::value)
{
if (_VSTD::next(__first) == __middle)
return _VSTD::__rotate_left(__first, __last);
if (_VSTD::next(__middle) == __last)
return _VSTD::__rotate_right(__first, __last);
}
return _VSTD::__rotate_forward(__first, __middle, __last);
}
template <class _RandomAccessIterator>
inline _LIBCPP_INLINE_VISIBILITY
_RandomAccessIterator
__rotate(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last,
_VSTD::random_access_iterator_tag)
{
typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type value_type;
if (_VSTD::is_trivially_move_assignable<value_type>::value)
{
if (_VSTD::next(__first) == __middle)
return _VSTD::__rotate_left(__first, __last);
if (_VSTD::next(__middle) == __last)
return _VSTD::__rotate_right(__first, __last);
return _VSTD::__rotate_gcd(__first, __middle, __last);
}
return _VSTD::__rotate_forward(__first, __middle, __last);
}
template <class _ForwardIterator>
inline _LIBCPP_INLINE_VISIBILITY
_ForwardIterator
rotate(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last)
{
if (__first == __middle)
return __last;
if (__middle == __last)
return __first;
return _VSTD::__rotate(__first, __middle, __last,
integral_constant
<
bool,
is_convertible
<
typename iterator_traits<_ForwardIterator>::iterator_category,
random_access_iterator_tag
>::value &&
is_trivially_copy_assignable
<
typename iterator_traits<_ForwardIterator>::value_type
>::value
>());
typename _VSTD::iterator_traits<_ForwardIterator>::iterator_category());
}
// rotate_copy