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Optimize std::shuffle by using generator to get two values at once

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2016-10-14  Eelis van der Weegen  <eelis@eelis.net>

	* include/bits/stl_algo.h (shuffle): Extract two random numbers from
	each generator invocation when its range is large enough.

From-SVN: r241184
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Eelis van der Weegen 2016-10-14 19:40:32 +00:00 committed by Jonathan Wakely
parent 17739146f9
commit 38e34671fa
2 changed files with 46 additions and 0 deletions
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5
libstdc++-v3/ChangeLog
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@ -1,3 +1,8 @@
2016-10-14 Eelis van der Weegen <eelis@eelis.net>
* include/bits/stl_algo.h (shuffle): Extract two random numbers from
each generator invocation when its range is large enough.
2016-10-14 Jonathan Wakely <jwakely@redhat.com>
* testsuite/experimental/algorithm/sample.cc: Qualify calls to

41
libstdc++-v3/include/bits/stl_algo.h
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@ -3772,6 +3772,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typedef typename std::make_unsigned<_DistanceType>::type __ud_type;
typedef typename std::uniform_int_distribution<__ud_type> __distr_type;
typedef typename __distr_type::param_type __p_type;
typedef typename std::remove_reference<_UniformRandomNumberGenerator>::type _Gen;
typedef typename std::common_type<typename _Gen::result_type, __ud_type>::type __uc_type;
const __uc_type __urngrange = __g.max() - __g.min();
const __uc_type __urange = __uc_type(__last - __first);
if (__urngrange / __urange >= __urange)
// I.e. (__urngrange >= __urange * __urange) but without wrap issues.
{
_RandomAccessIterator __i = __first + 1;
// Since we know the range isn't empty, an even number of elements
// means an uneven number of elements /to swap/, in which case we
// do the first one up front:
if ((__urange % 2) == 0)
{
__distr_type __d{0, 1};
std::iter_swap(__i++, __first + __d(__g));
}
// Now we know that __last - __i is even, so we do the rest in pairs,
// using a single distribution invocation to produce swap positions
// for two successive elements at a time:
while (__i != __last)
{
const __uc_type __swap_range = __uc_type(__i - __first) + 1;
const __uc_type __comp_range = __swap_range * (__swap_range + 1);
std::uniform_int_distribution<__uc_type> __d{0, __comp_range - 1};
const __uc_type __pospos = __d(__g);
std::iter_swap(__i++, __first + (__pospos % __swap_range));
std::iter_swap(__i++, __first + (__pospos / __swap_range));
}
return;
}
__distr_type __d;
for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)