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.

From-SVN: r241184

libstdc++-v3/ChangeLog

+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

libstdc++-v3/include/bits/stl_algo.h

@@ -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)