commit 1ddf9566764a85da4826628098b352bd30ba2bbc
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Fri Oct 14 20:27:13 2016 +0100
Optimize std::shuffle by using generator to get two values at once
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.
@@ -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)