Enhance unordered container merge

diff --git a/libstdc++-v3/include/bits/hashtable.h b/libstdc++-v3/include/bits/hashtable.h
index 0e949d73614..6e2d4c10cfe 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -1076,7 +1076,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    {
 	      auto __pos = __i++;
 	      const key_type& __k = _ExtractKey{}(*__pos);
-	      __hash_code __code = this->_M_hash_code(__k);
+	      __hash_code __code
+		= this->_M_hash_code(__src.hash_function(), *__pos._M_cur);
 	      size_type __bkt = _M_bucket_index(__code);
 	      if (_M_find_node(__bkt, __k, __code) == nullptr)
 		{
@@ -1099,14 +1100,15 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      node_type>, "Node types are compatible");
 	  __glibcxx_assert(get_allocator() == __src.get_allocator());
 
+	  __node_ptr __hint = nullptr;
 	  this->reserve(size() + __src.size());
 	  for (auto __i = __src.cbegin(), __end = __src.cend(); __i != __end;)
 	    {
 	      auto __pos = __i++;
-	      const key_type& __k = _ExtractKey{}(*__pos);
-	      __hash_code __code = this->_M_hash_code(__k);
+	      __hash_code __code
+		= this->_M_hash_code(__src.hash_function(), *__pos._M_cur);
 	      auto __nh = __src.extract(__pos);
-	      _M_insert_multi_node(nullptr, __code, __nh._M_ptr);
+	      __hint = _M_insert_multi_node(__hint, __code, __nh._M_ptr)._M_cur;
 	      __nh._M_ptr = nullptr;
 	    }
 	}
diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index c0295b75963..95a1c45e634 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -1217,6 +1217,26 @@  namespace __detail
       friend struct _Local_iterator_base<_Key, _Value, _ExtractKey,
 					 _Hash, _RangeHash, _Unused, false>;
 
+      template<typename _H1, typename _H2, bool __with_cache>
+	struct _ReuseOrComputeHash
+	{
+	  std::size_t
+	  operator()(const _Hash_node_value<_Value, __with_cache>& __n) const
+	  { return _M_hash_code_base._M_hash_code(_ExtractKey{}(__n._M_v())); }
+
+	  const _Hash_code_base& _M_hash_code_base;
+	};
+
+      template<typename _Hn>
+	struct _ReuseOrComputeHash<_Hn, _Hn, true>
+	{
+	  _ReuseOrComputeHash(const _Hash_code_base&) { }
+
+	  std::size_t
+	  operator()(const _Hash_node_value<_Value, true>& __n) const
+	  { return __n._M_hash_code; }
+	};
+
     public:
       typedef _Hash					hasher;
 
@@ -1250,6 +1270,12 @@  namespace __detail
 	  return _M_hash()(__k);
 	}
 
+      template<typename _H2>
+	__hash_code
+	_M_hash_code(const _H2&,
+		const _Hash_node_value<_Value, __cache_hash_code>& __n) const
+	{ return _ReuseOrComputeHash<_Hash, _H2, __cache_hash_code>{ *this }(__n); }
+
       std::size_t
       _M_bucket_index(__hash_code __c, std::size_t __bkt_count) const
       { return _RangeHash{}(__c, __bkt_count); }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_multiset/modifiers/merge.cc b/libstdc++-v3/testsuite/23_containers/unordered_multiset/modifiers/merge.cc
index 1ed2ce234a1..07b8a344169 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_multiset/modifiers/merge.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_multiset/modifiers/merge.cc
@@ -17,6 +17,7 @@ 
 
 // { dg-do run { target c++17 } }
 
+#include <string>
 #include <unordered_set>
 #include <algorithm>
 #include <testsuite_hooks.h>
@@ -105,6 +106,26 @@  test04()
   VERIFY( c2.empty() );
 }
 
+void
+test05()
+{
+  const std::unordered_multiset<std::string> c0{ "abcd", "abcd", "efgh", "efgh", "ijkl", "ijkl" };
+  std::unordered_multiset<std::string> c1 = c0;
+  std::unordered_set<std::string> c2( c0.begin(), c0.end() );
+
+  c1.merge(c2);
+  VERIFY( c1.size() == (1.5 * c0.size()) );
+  for (auto& i : c1)
+    VERIFY( c1.count(i) == (1.5 * c0.count(i)) );
+  VERIFY( c2.empty() );
+
+  c1.clear();
+  c2.insert( c0.begin(), c0.end() );
+  c1.merge(std::move(c2));
+  VERIFY( c1.size() == (0.5 * c0.size()) );
+  VERIFY( c2.empty() );
+}
+
 int
 main()
 {
@@ -112,4 +133,5 @@  main()
   test02();
   test03();
   test04();
+  test05();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_set/modifiers/merge.cc b/libstdc++-v3/testsuite/23_containers/unordered_set/modifiers/merge.cc
index c9c8a60fd54..0e184b10c60 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_set/modifiers/merge.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_set/modifiers/merge.cc
@@ -17,6 +17,7 @@ 
 
 // { dg-do run { target c++17 } }
 
+#include <string>
 #include <unordered_set>
 #include <algorithm>
 #include <testsuite_hooks.h>
@@ -125,10 +126,52 @@  test03()
   VERIFY( c2.empty() );
 }
 
+void
+test04()
+{
+  const std::unordered_set<std::string> c0{ "abcd", "efgh", "ijkl", };
+  std::unordered_set<std::string> c1 = c0;
+  std::unordered_multiset<std::string> c2( c0.begin(), c0.end() );
+  c1.merge(c2);
+  VERIFY( c1 == c0 );
+  VERIFY( equal_elements(c2, c0) );
+
+  c1.clear();
+  c1.merge(c2);
+  VERIFY( c1 == c0 );
+  VERIFY( c2.empty() );
+
+  c2.merge(c1);
+  VERIFY( c1.empty() );
+  VERIFY( equal_elements(c2, c0) );
+
+  c1 = c0;
+  c2.merge(c1);
+  VERIFY( c1.empty() );
+  VERIFY( c2.size() == (2 * c0.size()) );
+  VERIFY( c2.count("abcd") == 2 );
+  VERIFY( c2.count("efgh") == 2 );
+  VERIFY( c2.count("ijkl") == 2 );
+
+  c1.merge(c2);
+  VERIFY( c1 == c0 );
+  VERIFY( equal_elements(c2, c0) );
+
+  c1.merge(std::move(c2));
+  VERIFY( c1 == c0 );
+  VERIFY( equal_elements(c2, c0) );
+
+  c1.clear();
+  c1.merge(std::move(c2));
+  VERIFY( c1 == c0 );
+  VERIFY( c2.empty() );
+}
+
 int
 main()
 {
   test01();
   test02();
   test03();
+  test04();
 }