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

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 497744e..1537793 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -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

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index 0538a79..db99cb8 100644 (file)
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/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)