diff --git a/libstdc++-v3/doc/xml/manual/parallel_mode.xml b/libstdc++-v3/doc/xml/manual/parallel_mode.xml
index fb439062fea..faea5a9e7f6 100644
--- a/libstdc++-v3/doc/xml/manual/parallel_mode.xml
+++ b/libstdc++-v3/doc/xml/manual/parallel_mode.xml
@@ -575,24 +575,36 @@ This means that the actual parallelization strategy is chosen at run-time.
 (Choosing the variants at compile-time will come soon.)
 </para>
 
+<para>
+For the following algorithms in general, we have
+<code>__gnu_parallel::parallel_tag</code> and
+<code>__gnu_parallel::default_parallel_tag</code>, in addition to
+<code>__gnu_parallel::sequential_tag</code>.
+<code>__gnu_parallel::default_parallel_tag</code> chooses the default 
+algorithm at compiletime, as does omitting the tag.
+<code>__gnu_parallel::parallel_tag</code> postpones the decision to runtime
+(see next section).
+For all tags, the number of threads desired for this call can optionally be
+passed to the respective tag's constructor.
+</para>
+
+<para>
+The <code>multiway_merge</code> algorithm comes with the additional choices,
+<code>__gnu_parallel::exact_tag</code> and
+<code>__gnu_parallel::sampling_tag</code>.
+Exact and sampling are the two available splitting strategies.
+</para>
+
 <para>
 For the <code>sort</code> and <code>stable_sort</code> algorithms, there are
-several possible choices, 
-<code>__gnu_parallel::parallel_tag</code>,
-<code>__gnu_parallel::default_parallel_tag</code>,
+several additional choices, namely
 <code>__gnu_parallel::multiway_mergesort_tag</code>,
 <code>__gnu_parallel::multiway_mergesort_exact_tag</code>, 
 <code>__gnu_parallel::multiway_mergesort_sampling_tag</code>,
-<code>__gnu_parallel::quicksort_tag</code>,
+<code>__gnu_parallel::quicksort_tag</code>, and
 <code>__gnu_parallel::balanced_quicksort_tag</code>.
-Multiway mergesort comes with two splitting strategies for merging, therefore
-the extra choice. If non is chosen, the default splitting strategy is selected.
-<code>__gnu_parallel::default_parallel_tag</code> chooses the default parallel
-sorting algorithm at runtime. <code>__gnu_parallel::parallel_tag</code>
-postpones the decision to runtime (see next section).
-The quicksort options cannot be used for <code>stable_sort</code>.
-For all tags, the number of threads desired for this call can optionally be
-passed to the tag's constructor.
+Multiway mergesort comes with the two splitting strategies for multi-way
+merging. The quicksort options cannot be used for <code>stable_sort</code>.
 </para>
 
 </sect3>
diff --git a/libstdc++-v3/include/parallel/merge.h b/libstdc++-v3/include/parallel/merge.h
index cabd5bd4de2..580b1479329 100644
--- a/libstdc++-v3/include/parallel/merge.h
+++ b/libstdc++-v3/include/parallel/merge.h
@@ -254,11 +254,11 @@ namespace __gnu_parallel
       RandomAccessIterator3
         target_end = parallel_multiway_merge
           < /* stable = */ true, /* sentinels = */ false>(
-            seqs, seqs + 2, target, comp,
+            seqs, seqs + 2, target,
             multiway_merge_exact_splitting
               < /* stable = */ true, iterator_pair*,
                 Comparator, difference_type1>,
-            max_length);
+            max_length, comp, omp_get_max_threads());
 
       return target_end;
     }
diff --git a/libstdc++-v3/include/parallel/multiway_merge.h b/libstdc++-v3/include/parallel/multiway_merge.h
index 37e99cdeb6a..3c75c70f3c4 100644
--- a/libstdc++-v3/include/parallel/multiway_merge.h
+++ b/libstdc++-v3/include/parallel/multiway_merge.h
@@ -297,7 +297,7 @@ template<template<typename RAI, typename C> class iterator,
       RandomAccessIteratorIterator seqs_begin,
       RandomAccessIteratorIterator seqs_end,
       RandomAccessIterator3 target,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     _GLIBCXX_CALL(length);
 
@@ -416,7 +416,7 @@ template<template<typename RAI, typename C> class iterator,
   multiway_merge_4_variant(RandomAccessIteratorIterator seqs_begin,
                            RandomAccessIteratorIterator seqs_end,
                            RandomAccessIterator3 target,
-                           Comparator comp, _DifferenceTp length)
+                           _DifferenceTp length, Comparator comp)
   {
     _GLIBCXX_CALL(length);
     typedef _DifferenceTp difference_type;
@@ -540,8 +540,7 @@ template<typename LT,
   multiway_merge_loser_tree(RandomAccessIteratorIterator seqs_begin,
                             RandomAccessIteratorIterator seqs_end,
                             RandomAccessIterator3 target,
-                            Comparator comp,
-                            _DifferenceTp length)
+                            _DifferenceTp length, Comparator comp)
   {
     _GLIBCXX_CALL(length)
 
@@ -626,8 +625,8 @@ template<typename LT,
     const typename std::iterator_traits<typename std::iterator_traits<
       RandomAccessIteratorIterator>::value_type::first_type>::value_type&
         sentinel,
-    Comparator comp,
-    _DifferenceTp length)
+    _DifferenceTp length,
+    Comparator comp)
   {
     _GLIBCXX_CALL(length)
     typedef _DifferenceTp difference_type;
@@ -716,8 +715,8 @@ template<
     const typename std::iterator_traits<typename std::iterator_traits<
       RandomAccessIteratorIterator>::value_type::first_type>::value_type&
         sentinel,
-    Comparator comp,
-    _DifferenceTp length)
+    _DifferenceTp length,
+    Comparator comp)
   {
     _GLIBCXX_CALL(length)
 
@@ -740,7 +739,7 @@ template<
 
     target_end = multiway_merge_loser_tree_unguarded
         <UnguardedLoserTree>
-      (seqs_begin, seqs_end, target, sentinel, comp, length);
+      (seqs_begin, seqs_end, target, sentinel, length, comp);
 
 #if _GLIBCXX_ASSERTIONS
     _GLIBCXX_PARALLEL_ASSERT(target_end == target + length);
@@ -808,10 +807,10 @@ struct multiway_merge_3_variant_sentinel_switch
       RandomAccessIteratorIterator seqs_begin,
       RandomAccessIteratorIterator seqs_end,
       RandomAccessIterator3 target,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     return multiway_merge_3_variant<guarded_iterator>(
-        seqs_begin, seqs_end, target, comp, length);
+        seqs_begin, seqs_end, target, length, comp);
   }
 };
 
@@ -833,10 +832,10 @@ struct multiway_merge_3_variant_sentinel_switch
       RandomAccessIteratorIterator seqs_begin,
       RandomAccessIteratorIterator seqs_end,
       RandomAccessIterator3 target,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     return multiway_merge_3_variant<unguarded_iterator>(
-        seqs_begin, seqs_end, target, comp, length);
+        seqs_begin, seqs_end, target, length, comp);
   }
 };
 
@@ -857,10 +856,10 @@ struct multiway_merge_4_variant_sentinel_switch
       RandomAccessIteratorIterator seqs_begin,
       RandomAccessIteratorIterator seqs_end,
       RandomAccessIterator3 target,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     return multiway_merge_4_variant<guarded_iterator>(
-        seqs_begin, seqs_end, target, comp, length);
+        seqs_begin, seqs_end, target, length, comp);
   }
 };
 
@@ -882,10 +881,10 @@ struct multiway_merge_4_variant_sentinel_switch
       RandomAccessIteratorIterator seqs_begin,
       RandomAccessIteratorIterator seqs_end,
       RandomAccessIterator3 target,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     return multiway_merge_4_variant<unguarded_iterator>(
-        seqs_begin, seqs_end, target, comp, length);
+        seqs_begin, seqs_end, target, length, comp);
   }
 };
 
@@ -908,7 +907,7 @@ struct multiway_merge_k_variant_sentinel_switch
       const typename std::iterator_traits<typename std::iterator_traits<
         RandomAccessIteratorIterator>::value_type::first_type>::value_type&
           sentinel,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     typedef typename std::iterator_traits<RandomAccessIteratorIterator>
       ::value_type::first_type
@@ -921,7 +920,7 @@ struct multiway_merge_k_variant_sentinel_switch
             loser_tree_traits<value_type>::use_pointer
           , LoserTreePointerUnguarded<stable, value_type, Comparator>
           , LoserTreeUnguarded<stable, value_type, Comparator>
-        >::__type>(seqs_begin, seqs_end, target, sentinel, comp, length);
+        >::__type>(seqs_begin, seqs_end, target, sentinel, length, comp);
   }
 };
 
@@ -945,7 +944,7 @@ struct multiway_merge_k_variant_sentinel_switch
       const typename std::iterator_traits<typename std::iterator_traits<
         RandomAccessIteratorIterator>::value_type::first_type>::value_type&
           sentinel,
-      Comparator comp, _DifferenceTp length)
+      _DifferenceTp length, Comparator comp)
   {
     typedef typename std::iterator_traits<RandomAccessIteratorIterator>
       ::value_type::first_type
@@ -958,7 +957,7 @@ struct multiway_merge_k_variant_sentinel_switch
             loser_tree_traits<value_type>::use_pointer
           , LoserTreePointer<stable, value_type, Comparator>
           , LoserTree<stable, value_type, Comparator>
-        >::__type >(seqs_begin, seqs_end, target, comp, length);
+        >::__type >(seqs_begin, seqs_end, target, length, comp);
   }
 };
 
@@ -990,7 +989,7 @@ template<
     const typename std::iterator_traits<typename std::iterator_traits<
       RandomAccessIteratorIterator>::value_type::first_type>::value_type&
         sentinel,
-    Comparator comp, _DifferenceTp length)
+    _DifferenceTp length, Comparator comp)
   {
     _GLIBCXX_CALL(length)
 
@@ -1043,7 +1042,7 @@ template<
           , RandomAccessIteratorIterator
           , RandomAccessIterator3
           , _DifferenceTp
-          , Comparator>()(seqs_begin, seqs_end, target, comp, length);
+          , Comparator>()(seqs_begin, seqs_end, target, length, comp);
         break;
       case 4:
         return_target = multiway_merge_4_variant_sentinel_switch<
@@ -1051,7 +1050,7 @@ template<
           , RandomAccessIteratorIterator
           , RandomAccessIterator3
           , _DifferenceTp
-          , Comparator>()(seqs_begin, seqs_end, target, comp, length);
+          , Comparator>()(seqs_begin, seqs_end, target, length, comp);
         break;
       default:
           return_target = multiway_merge_k_variant_sentinel_switch<
@@ -1060,8 +1059,7 @@ template<
             , RandomAccessIteratorIterator
             , RandomAccessIterator3
             , _DifferenceTp
-            , Comparator>()
-                (seqs_begin, seqs_end, target, sentinel, comp, length);
+            , Comparator>()(seqs_begin, seqs_end, target, sentinel, length, comp);
           break;
       }
 #if _GLIBCXX_ASSERTIONS
@@ -1108,8 +1106,7 @@ template<
 void multiway_merge_sampling_splitting(
     RandomAccessIteratorIterator seqs_begin,
     RandomAccessIteratorIterator seqs_end,
-    Comparator comp, difference_type length,
-    difference_type total_length,
+    difference_type length, difference_type total_length, Comparator comp,
     std::vector<std::pair<difference_type, difference_type> > *pieces)
 {
   typedef typename std::iterator_traits<RandomAccessIteratorIterator>
@@ -1190,9 +1187,7 @@ template<
 void multiway_merge_exact_splitting(
     RandomAccessIteratorIterator seqs_begin,
     RandomAccessIteratorIterator seqs_end,
-    Comparator comp,
-    difference_type length,
-    difference_type total_length,
+    difference_type length, difference_type total_length, Comparator comp,
     std::vector<std::pair<difference_type, difference_type> > *pieces)
 {
   typedef typename std::iterator_traits<RandomAccessIteratorIterator>
@@ -1297,9 +1292,10 @@ template<
   parallel_multiway_merge(RandomAccessIteratorIterator seqs_begin,
                           RandomAccessIteratorIterator seqs_end,
                           RandomAccessIterator3 target,
-                          Comparator comp,
                           Splitter splitter,
-                          _DifferenceTp length)
+                          _DifferenceTp length,
+                          Comparator comp,
+                          thread_index_t num_threads)
     {
 #if _GLIBCXX_ASSERTIONS
       _GLIBCXX_PARALLEL_ASSERT(seqs_end - seqs_begin > 1);
@@ -1347,8 +1343,8 @@ template<
 
       std::vector<std::pair<difference_type, difference_type> >* pieces;
 
-      thread_index_t num_threads = static_cast<thread_index_t>
-        (std::min<difference_type>(get_max_threads(), total_length));
+      num_threads = static_cast<thread_index_t>
+        (std::min<difference_type>(num_threads, total_length));
 
 #     pragma omp parallel num_threads (num_threads)
         {
@@ -1365,8 +1361,8 @@ template<
                   __gnu_parallel::_Settings::get().merge_oversampling *
                     num_threads;
 
-              splitter(ne_seqs, ne_seqs + k, comp, length, total_length,
-                       pieces);
+              splitter(ne_seqs, ne_seqs + k, length, total_length,
+                       comp, pieces);
             } //single
 
           thread_index_t iam = omp_get_thread_num();
@@ -1389,7 +1385,7 @@ template<
           if(length > target_position)
             sequential_multiway_merge<stable, sentinels>(
               chunks, chunks + k, target + target_position,
-              *(seqs_begin->second), comp, length - target_position);
+               *(seqs_begin->second), length - target_position, comp);
 
           delete[] chunks;
         } // parallel
@@ -1481,6 +1477,7 @@ template<
  *
  * @return end iterator of output sequence
  */
+// multiway_merge
 // public interface
 template<
     typename RandomAccessIteratorPairIterator
@@ -1491,49 +1488,7 @@ RandomAccessIteratorOut
 multiway_merge(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length)
-{
-  typedef _DifferenceTp difference_type;
-  _GLIBCXX_CALL(seqs_end - seqs_begin)
-
-  // catch special case: no sequences
-  if (seqs_begin == seqs_end)
-    return target;
-
-  // Execute merge; maybe parallel, depending on the number of merged
-  // elements and the number of sequences and global thresholds in
-  // Settings.
-  if ((seqs_end - seqs_begin > 1) &&
-        _GLIBCXX_PARALLEL_CONDITION(
-        ((seqs_end - seqs_begin) >=
-        __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
-        && ((sequence_index_t)length >=
-        __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
-    return parallel_multiway_merge
-      </* stable = */ false, /* sentinels = */ false>
-        (seqs_begin, seqs_end, target, comp,
-        multiway_merge_sampling_splitting</* stable = */ false,
-          typename std::iterator_traits<RandomAccessIteratorPairIterator>
-            ::value_type*, Comparator, _DifferenceTp>,
-        static_cast<difference_type>(length));
-  else
-    return sequential_multiway_merge
-      </* stable = */false, /* sentinels = */ false>(
-        seqs_begin, seqs_end,
-        target, *(seqs_begin->second), comp, length);
-}
-
-// public interface
-template<
-    typename RandomAccessIteratorPairIterator
-  , typename RandomAccessIteratorOut
-  , typename _DifferenceTp
-  , typename Comparator>
-RandomAccessIteratorOut
-multiway_merge(RandomAccessIteratorPairIterator seqs_begin
-    , RandomAccessIteratorPairIterator seqs_end
-    , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
+    , _DifferenceTp length, Comparator comp
     , __gnu_parallel::sequential_tag)
 {
   typedef _DifferenceTp difference_type;
@@ -1546,10 +1501,10 @@ multiway_merge(RandomAccessIteratorPairIterator seqs_begin
   // Execute multiway merge *sequentially*.
   return sequential_multiway_merge
     </* stable = */ false, /* sentinels = */ false>
-      (seqs_begin, seqs_end, target, *(seqs_begin->second), comp, length);
+      (seqs_begin, seqs_end, target, *(seqs_begin->second), length, comp);
 }
 
-//public interface
+// public interface
 template<
     typename RandomAccessIteratorPairIterator
   , typename RandomAccessIteratorOut
@@ -1559,8 +1514,50 @@ RandomAccessIteratorOut
 multiway_merge(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
-    , __gnu_parallel::exact_tag)
+    , _DifferenceTp length, Comparator comp
+    , __gnu_parallel::exact_tag tag)
+{
+    typedef _DifferenceTp difference_type;
+    _GLIBCXX_CALL(seqs_end - seqs_begin)
+
+    // catch special case: no sequences
+    if (seqs_begin == seqs_end)
+      return target;
+
+    // Execute merge; maybe parallel, depending on the number of merged
+    // elements and the number of sequences and global thresholds in
+    // Settings.
+    if ((seqs_end - seqs_begin > 1) &&
+          _GLIBCXX_PARALLEL_CONDITION(
+          ((seqs_end - seqs_begin) >=
+             __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
+          && ((sequence_index_t)length >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
+      return parallel_multiway_merge
+                    </* stable = */ false, /* sentinels = */ false>(
+          seqs_begin, seqs_end, target,
+          multiway_merge_exact_splitting</* stable = */ false,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
+    else
+      return sequential_multiway_merge
+                      </* stable = */ false, /* sentinels = */ false>(
+          seqs_begin, seqs_end, target, *(seqs_begin->second), length, comp);
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+multiway_merge(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , __gnu_parallel::sampling_tag tag)
 {
     typedef _DifferenceTp difference_type;
     _GLIBCXX_CALL(seqs_end - seqs_begin)
@@ -1581,16 +1578,16 @@ multiway_merge(RandomAccessIteratorPairIterator seqs_begin
       return parallel_multiway_merge
                     </* stable = */ false, /* sentinels = */ false>(
           seqs_begin, seqs_end,
-          target, comp,
+          target,
           multiway_merge_exact_splitting</* stable = */ false,
             typename std::iterator_traits<RandomAccessIteratorPairIterator>
               ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
     else
       return sequential_multiway_merge
                       </* stable = */ false, /* sentinels = */ false>(
           seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
+          target, *(seqs_begin->second), length, comp);
 }
 
 // public interface
@@ -1600,42 +1597,34 @@ template<
   , typename _DifferenceTp
   , typename Comparator>
 RandomAccessIteratorOut
-stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
+multiway_merge(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length)
+    , _DifferenceTp length, Comparator comp
+    , parallel_tag tag = parallel_tag(0))
 {
-    typedef _DifferenceTp difference_type;
-    _GLIBCXX_CALL(seqs_end - seqs_begin)
-
-    // catch special case: no sequences
-    if (seqs_begin == seqs_end)
-      return target;
-
-    // Execute merge; maybe parallel, depending on the number of merged
-    // elements and the number of sequences and global thresholds in
-    // Settings.
-    if ((seqs_end - seqs_begin > 1) &&
-          _GLIBCXX_PARALLEL_CONDITION(
-          ((seqs_end - seqs_begin) >=
-            __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
-          && ((sequence_index_t)length >=
-            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
-      return parallel_multiway_merge
-        </* stable = */ true, /* sentinels = */ false>(
-          seqs_begin, seqs_end,
-          target, comp,
-          multiway_merge_sampling_splitting</* stable = */ true,
-            typename std::iterator_traits<RandomAccessIteratorPairIterator>
-              ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
-    else
-      return sequential_multiway_merge
-        </* stable = */ true, /* sentinels = */ false>(
-          seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
+  return multiway_merge(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
 }
 
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+multiway_merge(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , default_parallel_tag tag)
+{
+  return multiway_merge(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
+}
+
+// stable_multiway_merge
 // public interface
 template<
     typename RandomAccessIteratorPairIterator
@@ -1646,7 +1635,7 @@ RandomAccessIteratorOut
 stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
+    , _DifferenceTp length, Comparator comp
     , __gnu_parallel::sequential_tag)
 {
     typedef _DifferenceTp difference_type;
@@ -1659,7 +1648,7 @@ stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
     // Execute multiway merge *sequentially*.
     return sequential_multiway_merge
       </* stable = */ true, /* sentinels = */ false>
-        (seqs_begin, seqs_end, target, *(seqs_begin->second), comp, length);
+        (seqs_begin, seqs_end, target, *(seqs_begin->second), length, comp);
 }
 
 // public interface
@@ -1672,8 +1661,8 @@ RandomAccessIteratorOut
 stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
-    , __gnu_parallel::exact_tag)
+    , _DifferenceTp length, Comparator comp
+    , __gnu_parallel::exact_tag tag)
 {
     typedef _DifferenceTp difference_type;
     _GLIBCXX_CALL(seqs_end - seqs_begin)
@@ -1694,17 +1683,95 @@ stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
       return parallel_multiway_merge
         </* stable = */ true, /* sentinels = */ false>(
           seqs_begin, seqs_end,
-          target, comp, 
-          multiway_merge_exact_splitting
-            </* stable = */ true,
-              typename std::iterator_traits<RandomAccessIteratorPairIterator>
-                ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
+          target,
+          multiway_merge_exact_splitting</* stable = */ true,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
     else
       return sequential_multiway_merge</* stable = */ true,
         /* sentinels = */ false>(
           seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
+          target, *(seqs_begin->second), length, comp);
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , sampling_tag tag)
+{
+    typedef _DifferenceTp difference_type;
+    _GLIBCXX_CALL(seqs_end - seqs_begin)
+
+    // catch special case: no sequences
+    if (seqs_begin == seqs_end)
+      return target;
+
+    // Execute merge; maybe parallel, depending on the number of merged
+    // elements and the number of sequences and global thresholds in
+    // Settings.
+    if ((seqs_end - seqs_begin > 1) &&
+          _GLIBCXX_PARALLEL_CONDITION(
+          ((seqs_end - seqs_begin) >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
+          && ((sequence_index_t)length >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
+      return parallel_multiway_merge
+        </* stable = */ true, /* sentinels = */ false>(
+          seqs_begin, seqs_end,
+          target,
+          multiway_merge_sampling_splitting</* stable = */ true,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
+    else
+      return sequential_multiway_merge
+        </* stable = */ true, /* sentinels = */ false>(
+          seqs_begin, seqs_end,
+          target, *(seqs_begin->second), length, comp);
+}
+
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , parallel_tag tag = parallel_tag(0))
+{
+  return stable_multiway_merge(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , default_parallel_tag tag)
+{
+  return stable_multiway_merge(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
 }
 
 /**
@@ -1782,6 +1849,7 @@ stable_multiway_merge(RandomAccessIteratorPairIterator seqs_begin
  *
  * @return end iterator of output sequence
  */
+// multiway_merge_sentinels
 // public interface
 template<
     typename RandomAccessIteratorPairIterator
@@ -1792,7 +1860,79 @@ RandomAccessIteratorOut
 multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length)
+    , _DifferenceTp length, Comparator comp
+    , __gnu_parallel::sequential_tag)
+{
+    typedef _DifferenceTp difference_type;
+    _GLIBCXX_CALL(seqs_end - seqs_begin)
+
+    // catch special case: no sequences
+    if (seqs_begin == seqs_end)
+      return target;
+
+    // Execute multiway merge *sequentially*.
+    return sequential_multiway_merge
+      </* stable = */ false, /* sentinels = */ true>
+        (seqs_begin, seqs_end,
+         target, *(seqs_begin->second), length, comp);
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , __gnu_parallel::exact_tag tag)
+{
+    typedef _DifferenceTp difference_type;
+    _GLIBCXX_CALL(seqs_end - seqs_begin)
+
+    // catch special case: no sequences
+    if (seqs_begin == seqs_end)
+      return target;
+
+    // Execute merge; maybe parallel, depending on the number of merged
+    // elements and the number of sequences and global thresholds in
+    // Settings.
+    if ((seqs_end - seqs_begin > 1) &&
+          _GLIBCXX_PARALLEL_CONDITION(
+          ((seqs_end - seqs_begin) >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
+          && ((sequence_index_t)length >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
+      return parallel_multiway_merge
+        </* stable = */ false, /* sentinels = */ true>(
+          seqs_begin, seqs_end,
+          target,
+          multiway_merge_exact_splitting</* stable = */ false,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
+    else
+      return sequential_multiway_merge
+        </* stable = */ false, /* sentinels = */ true>(
+          seqs_begin, seqs_end,
+          target, *(seqs_begin->second), length, comp);
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , sampling_tag tag)
 {
     typedef _DifferenceTp difference_type;
     _GLIBCXX_CALL(seqs_end - seqs_begin)
@@ -1812,43 +1952,16 @@ multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
             __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
       return parallel_multiway_merge
         </* stable = */ false, /* sentinels = */ true>
-          (seqs_begin, seqs_end, target, comp,
-          multiway_merge_sampling_splitting
-            </* stable = */ false,
-              typename std::iterator_traits<RandomAccessIteratorPairIterator>
-                ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
+          (seqs_begin, seqs_end, target,
+          multiway_merge_sampling_splitting</* stable = */ false,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
     else
       return sequential_multiway_merge
         </* stable = */false, /* sentinels = */ true>(
           seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
-}
-
-//public interface
-template<
-    typename RandomAccessIteratorPairIterator
-  , typename RandomAccessIteratorOut
-  , typename _DifferenceTp
-  , typename Comparator>
-RandomAccessIteratorOut
-multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
-    , RandomAccessIteratorPairIterator seqs_end
-    , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
-    , __gnu_parallel::sequential_tag)
-{
-    typedef _DifferenceTp difference_type;
-    _GLIBCXX_CALL(seqs_end - seqs_begin)
-
-    // catch special case: no sequences
-    if (seqs_begin == seqs_end)
-      return target;
-
-    // Execute multiway merge *sequentially*.
-    return sequential_multiway_merge
-      </* stable = */ false, /* sentinels = */ true>
-        (seqs_begin, seqs_end, target, *(seqs_begin->second), comp, length);
+          target, *(seqs_begin->second), length, comp);
 }
 
 // public interface
@@ -1861,41 +1974,31 @@ RandomAccessIteratorOut
 multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
-    , __gnu_parallel::exact_tag)
+    , _DifferenceTp length, Comparator comp
+    , parallel_tag tag = parallel_tag(0))
 {
-    typedef _DifferenceTp difference_type;
-    _GLIBCXX_CALL(seqs_end - seqs_begin)
-
-    // catch special case: no sequences
-    if (seqs_begin == seqs_end)
-      return target;
-
-    // Execute merge; maybe parallel, depending on the number of merged
-    // elements and the number of sequences and global thresholds in
-    // Settings.
-    if ((seqs_end - seqs_begin > 1) &&
-          _GLIBCXX_PARALLEL_CONDITION(
-          ((seqs_end - seqs_begin) >=
-            __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
-          && ((sequence_index_t)length >=
-            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
-      return parallel_multiway_merge
-        </* stable = */ false, /* sentinels = */ true>(
-          seqs_begin, seqs_end,
-          target, comp,
-          multiway_merge_exact_splitting
-            </* stable = */ false,
-              typename std::iterator_traits<RandomAccessIteratorPairIterator>
-                ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
-    else
-      return sequential_multiway_merge
-        </* stable = */ false, /* sentinels = */ true>(
-          seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
+  return multiway_merge_sentinels(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
 }
 
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , default_parallel_tag tag)
+{
+  return multiway_merge_sentinels(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
+}
+
+// stable_multiway_merge_sentinels
 // public interface
 template<
     typename RandomAccessIteratorPairIterator
@@ -1906,51 +2009,7 @@ RandomAccessIteratorOut
 stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length)
-{
-    typedef _DifferenceTp difference_type;
-    _GLIBCXX_CALL(seqs_end - seqs_begin)
-
-    // catch special case: no sequences
-    if (seqs_begin == seqs_end)
-      return target;
-
-    // Execute merge; maybe parallel, depending on the number of merged
-    // elements and the number of sequences and global thresholds in
-    // Settings.
-    if ((seqs_end - seqs_begin > 1) &&
-          _GLIBCXX_PARALLEL_CONDITION(
-          ((seqs_end - seqs_begin) >=
-            __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
-          && ((sequence_index_t)length >=
-            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
-      return parallel_multiway_merge
-        </* stable = */ true, /* sentinels = */ true>(
-          seqs_begin, seqs_end,
-          target, comp,
-          multiway_merge_sampling_splitting
-            </* stable = */ true,
-              typename std::iterator_traits<RandomAccessIteratorPairIterator>
-                ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
-    else
-      return sequential_multiway_merge
-        </* stable = */ true, /* sentinels = */ true>(
-          seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
-}
-
-// public interface
-template<
-    typename RandomAccessIteratorPairIterator
-  , typename RandomAccessIteratorOut
-  , typename _DifferenceTp
-  , typename Comparator>
-RandomAccessIteratorOut
-stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
-    , RandomAccessIteratorPairIterator seqs_end
-    , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
+    , _DifferenceTp length, Comparator comp
     , __gnu_parallel::sequential_tag)
 {
     typedef _DifferenceTp difference_type;
@@ -1963,7 +2022,7 @@ stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
     // Execute multiway merge *sequentially*.
     return sequential_multiway_merge
       </* stable = */ true, /* sentinels = */ true>
-        (seqs_begin, seqs_end, target, *(seqs_begin->second), comp, length);
+        (seqs_begin, seqs_end, target, *(seqs_begin->second), length, comp);
 }
 
 // public interface
@@ -1976,8 +2035,8 @@ RandomAccessIteratorOut
 stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
     , RandomAccessIteratorPairIterator seqs_end
     , RandomAccessIteratorOut target
-    , Comparator comp, _DifferenceTp length
-    , __gnu_parallel::exact_tag)
+    , _DifferenceTp length, Comparator comp
+    , __gnu_parallel::exact_tag tag)
 {
     typedef _DifferenceTp difference_type;
     _GLIBCXX_CALL(seqs_end - seqs_begin)
@@ -1998,17 +2057,93 @@ stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
       return parallel_multiway_merge
         </* stable = */ true, /* sentinels = */ true>(
           seqs_begin, seqs_end,
-          target, comp, 
-          multiway_merge_exact_splitting
-            </* stable = */ true,
-              typename std::iterator_traits<RandomAccessIteratorPairIterator>
-                ::value_type*, Comparator, _DifferenceTp>,
-          static_cast<difference_type>(length));
+          target,
+          multiway_merge_exact_splitting</* stable = */ true,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
+    else
+      return sequential_multiway_merge
+        </* stable = */ true, /* sentinels = */ true>(
+          seqs_begin, seqs_end, target, *(seqs_begin->second), length, comp);
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , sampling_tag tag)
+{
+    typedef _DifferenceTp difference_type;
+    _GLIBCXX_CALL(seqs_end - seqs_begin)
+
+    // catch special case: no sequences
+    if (seqs_begin == seqs_end)
+      return target;
+
+    // Execute merge; maybe parallel, depending on the number of merged
+    // elements and the number of sequences and global thresholds in
+    // Settings.
+    if ((seqs_end - seqs_begin > 1) &&
+          _GLIBCXX_PARALLEL_CONDITION(
+          ((seqs_end - seqs_begin) >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_k)
+          && ((sequence_index_t)length >=
+            __gnu_parallel::_Settings::get().multiway_merge_minimal_n)))
+      return parallel_multiway_merge
+        </* stable = */ true, /* sentinels = */ true>(
+          seqs_begin, seqs_end,
+          target,
+          multiway_merge_sampling_splitting</* stable = */ true,
+            typename std::iterator_traits<RandomAccessIteratorPairIterator>
+              ::value_type*, Comparator, _DifferenceTp>,
+          static_cast<difference_type>(length), comp, tag.get_num_threads());
     else
       return sequential_multiway_merge
         </* stable = */ true, /* sentinels = */ true>(
           seqs_begin, seqs_end,
-          target, *(seqs_begin->second), comp, length);
+          target, *(seqs_begin->second), length, comp);
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , parallel_tag tag = parallel_tag(0))
+{
+  return stable_multiway_merge_sentinels(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
+}
+
+// public interface
+template<
+    typename RandomAccessIteratorPairIterator
+  , typename RandomAccessIteratorOut
+  , typename _DifferenceTp
+  , typename Comparator>
+RandomAccessIteratorOut
+stable_multiway_merge_sentinels(RandomAccessIteratorPairIterator seqs_begin
+    , RandomAccessIteratorPairIterator seqs_end
+    , RandomAccessIteratorOut target
+    , _DifferenceTp length, Comparator comp
+    , default_parallel_tag tag)
+{
+  return stable_multiway_merge_sentinels(seqs_begin, seqs_end, target, length, comp,
+                         exact_tag(tag.get_num_threads()));
 }
 
 }; // namespace __gnu_parallel
diff --git a/libstdc++-v3/include/parallel/multiway_mergesort.h b/libstdc++-v3/include/parallel/multiway_mergesort.h
index 3791a144d53..9d9733ad05f 100644
--- a/libstdc++-v3/include/parallel/multiway_mergesort.h
+++ b/libstdc++-v3/include/parallel/multiway_mergesort.h
@@ -289,8 +289,8 @@ template<typename SeqRandomAccessIterator, typename RandomAccessIterator,
                       Comparator& comp,
                       DiffType length_am) const
     {
-      stable_multiway_merge(seqs_begin, seqs_end, target, comp,
-                       length_am, sequential_tag());
+      stable_multiway_merge(seqs_begin, seqs_end, target, length_am, comp,
+                       sequential_tag());
     }
   };
 
@@ -306,8 +306,8 @@ template<typename SeqRandomAccessIterator, typename RandomAccessIterator,
                       Comparator& comp,
                       DiffType length_am) const
     {
-      multiway_merge(seqs_begin, seqs_end, target, comp,
-                       length_am, sequential_tag());
+      multiway_merge(seqs_begin, seqs_end, target, length_am, comp,
+                       sequential_tag());
     }
   };
 
diff --git a/libstdc++-v3/include/parallel/tags.h b/libstdc++-v3/include/parallel/tags.h
index cf937af4952..0ca0994e3ab 100644
--- a/libstdc++-v3/include/parallel/tags.h
+++ b/libstdc++-v3/include/parallel/tags.h
@@ -47,9 +47,6 @@ namespace __gnu_parallel
   /** @brief Forces sequential execution at compile time. */
   struct sequential_tag { };
 
-  /** @brief Forces exact splitting in multiway merge at compile time. */
-  struct exact_tag { };
-
   /** @brief Recommends parallel execution at compile time,
    *  optionally using a user-specified number of threads. */
   struct parallel_tag
@@ -119,6 +116,25 @@ namespace __gnu_parallel
   struct find_tag { };
 
 
+  /** @brief Forces parallel merging
+   *  with exact splitting, at compile time. */
+  struct exact_tag : public parallel_tag
+  {
+      exact_tag() { }
+      exact_tag(thread_index_t num_threads)
+          : parallel_tag(num_threads) { }
+  };
+
+  /** @brief Forces parallel merging
+   *  with exact splitting, at compile time. */
+  struct sampling_tag : public parallel_tag
+  {
+      sampling_tag() { }
+      sampling_tag(thread_index_t num_threads)
+          : parallel_tag(num_threads) { }
+  };
+
+
   /** @brief Forces parallel sorting using multiway mergesort
    *  at compile time. */
   struct multiway_mergesort_tag : public parallel_tag