libstdc++: [_Hashtable] Fix hash code cache usage when stateful hash functor

It is wrong to reuse a cached hash code from another container when this code depends
on the state of the container's Hash functor.

Add checks that Hash functor is stateless before reusing the cached hash code.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h
	(_Hash_code_base::_M_copy_code, _Hash_code_base::_M_store_code): Remove.
	* include/bits/hashtable.h (_M_hash_code_ext): New.
	(_M_merge_multi(_Hashtable&)): Use latter.
	(_M_copy_code): New.
	(_M_assign): Use latter.
	(_M_bucket_index_ex): New.
	(_M_equals): Use latter.
	(_M_store_code): New.
	(_M_src_hash_code): Remove key_type parameter.
	* testsuite/23_containers/unordered_map/modifiers/merge.cc (test10): New
	test case.

3 files changed, 98 insertions, 37 deletions

diff --git a/libstdc++-v3/include/bits/hashtable.h b/libstdc++-v3/include/bits/hashtable.h
index d6d76a7..246e62a 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -808,6 +808,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _M_bucket_index(__hash_code __c) const
       { return __hash_code_base::_M_bucket_index(__c, _M_bucket_count); }
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
+      // Get hash code for a node that comes from another _Hashtable.
+      // Reuse a cached hash code if the hash function is stateless,
+      // otherwise recalculate it using our own hash function.
+      __hash_code
+      _M_hash_code_ext(const __node_value_type& __from) const
+      {
+	if constexpr (__and_<__hash_cached, is_empty<_Hash>>::value)
+	  return __from._M_hash_code;
+	else
+	  return this->_M_hash_code(_ExtractKey{}(__from._M_v()));
+      }
+
+      // Like _M_bucket_index but when the node is coming from another
+      // container instance.
+      size_type
+      _M_bucket_index_ext(const __node_value_type& __from) const
+      { return _RangeHash{}(_M_hash_code_ext(__from), _M_bucket_count); }
+
+      void
+      _M_copy_code(__node_value_type& __to,
+		   const __node_value_type& __from) const
+      {
+	if constexpr (__hash_cached::value)
+	  __to._M_hash_code = _M_hash_code_ext(__from);
+      }
+
+      void
+      _M_store_code(__node_value_type& __to, __hash_code __code) const
+      {
+	if constexpr (__hash_cached::value)
+	  __to._M_hash_code = __code;
+      }
+#pragma GCC diagnostic pop
+
       // Find and insert helper functions and types
 
       // Find the node before the one matching the criteria.
@@ -1210,15 +1246,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // with a hash function that might not match this->hash_function().
       template<typename _H2>
 	__hash_code
-	_M_src_hash_code(const _H2&, const key_type& __k,
-			 const __node_value_type& __src_n) const
+	_M_src_hash_code(const _H2&, const __node_value_type& __src_n) const
 	{
-	  if constexpr (std::is_same_v<_H2, _Hash>)
-	    if constexpr (std::is_empty_v<_Hash>)
-	      // If the node has a cached hash code, it's OK to use it.
-	      return this->_M_hash_code(__src_n);
-
-	  return this->_M_hash_code(__k);
+	  if constexpr (__and_<__hash_cached,
+			is_same<_H2, _Hash>, is_empty<_Hash>>::value)
+	    // If the node has a cached hash code, it's OK to use it.
+	    return __src_n._M_hash_code;
+	  else
+	    return this->_M_hash_code(_ExtractKey{}(__src_n._M_v()));
 	}
 
     public:
@@ -1327,9 +1362,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	do
 	  {
 	    const auto& __node = static_cast<__node_type&>(*__prev->_M_nxt);
-	    const key_type& __k = _ExtractKey{}(__node._M_v());
-	    // Hash code from this->hash_function():
-	    auto __code = _M_src_hash_code(__src.hash_function(), __k, __node);
+	    // Hash code from this:
+	    auto __code = _M_hash_code_ext(__node);
 	    // Bucket index in __src, using code from __src.hash_function():
 	    size_type __src_bkt = __src._M_bucket_index(__node);
 	    auto __nh = __src._M_extract_node(__src_bkt, __prev);
@@ -1355,9 +1389,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  for (auto __i = __src.cbegin(), __end = __src.cend(); __i != __end;)
 	    {
 	      auto __pos = __i++;
-	      const key_type& __k = _ExtractKey{}(*__pos);
 	      __hash_code __code
-		= _M_src_hash_code(__src.hash_function(), __k, *__pos._M_cur);
+		= _M_src_hash_code(__src.hash_function(), *__pos._M_cur);
 	      auto __nh = __src.extract(__pos);
 	      __hint = _M_insert_multi_node(__hint, __code, __nh._M_ptr)._M_cur;
 	      __nh.release();
@@ -1587,7 +1620,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__node_ptr __ht_n = __ht._M_begin();
 	__node_ptr __this_n
 	  = __node_gen(static_cast<_FromVal>(__ht_n->_M_v()));
-	this->_M_copy_code(*__this_n, *__ht_n);
+	_M_copy_code(*__this_n, *__ht_n);
 	_M_update_bbegin(__this_n);
 
 	// Then deal with other nodes.
@@ -1596,7 +1629,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  {
 	    __this_n = __node_gen(static_cast<_FromVal>(__ht_n->_M_v()));
 	    __prev_n->_M_nxt = __this_n;
-	    this->_M_copy_code(*__this_n, *__ht_n);
+	    _M_copy_code(*__this_n, *__ht_n);
 	    size_type __bkt = _M_bucket_index(*__this_n);
 	    if (!_M_buckets[__bkt])
 	      _M_buckets[__bkt] = __prev_n;
@@ -2437,7 +2470,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
 
       __rehash_guard._M_guarded_obj = nullptr;
-      this->_M_store_code(*__node, __code);
+      _M_store_code(*__node, __code);
 
       // Always insert at the beginning of the bucket.
       _M_insert_bucket_begin(__bkt, __node);
@@ -2464,7 +2497,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_M_rehash(__do_rehash.second, false_type{});
 
       __rehash_guard._M_guarded_obj = nullptr;
-      this->_M_store_code(*__node, __code);
+      _M_store_code(*__node, __code);
       const key_type& __k = _ExtractKey{}(__node->_M_v());
       size_type __bkt = _M_bucket_index(__code);
 
@@ -2851,7 +2884,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       if constexpr (__unique_keys::value)
 	for (auto __x_n = _M_begin(); __x_n; __x_n = __x_n->_M_next())
 	  {
-	    std::size_t __ybkt = __other._M_bucket_index(*__x_n);
+	    std::size_t __ybkt = __other._M_bucket_index_ext(*__x_n);
 	    auto __prev_n = __other._M_buckets[__ybkt];
 	    if (!__prev_n)
 	      return false;
@@ -2878,7 +2911,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		 __x_n_end = __x_n_end->_M_next())
 	      ++__x_count;
 
-	    std::size_t __ybkt = __other._M_bucket_index(*__x_n);
+	    std::size_t __ybkt = __other._M_bucket_index_ext(*__x_n);
 	    auto __y_prev_n = __other._M_buckets[__ybkt];
 	    if (!__y_prev_n)
 	      return false;
diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index 76fbc0d..048855d 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -1105,24 +1105,6 @@ namespace __detail
       _M_bucket_index(const _Hash_node_value<_Value, true>& __n,
 		      size_t __bkt_count) const noexcept
       { return _RangeHash{}(__n._M_hash_code, __bkt_count); }
-
-      void
-      _M_store_code(_Hash_node_code_cache<false>&, __hash_code) const
-      { }
-
-      void
-      _M_copy_code(_Hash_node_code_cache<false>&,
-		   const _Hash_node_code_cache<false>&) const
-      { }
-
-      void
-      _M_store_code(_Hash_node_code_cache<true>& __n, __hash_code __c) const
-      { __n._M_hash_code = __c; }
-
-      void
-      _M_copy_code(_Hash_node_code_cache<true>& __to,
-		   const _Hash_node_code_cache<true>& __from) const
-      { __to._M_hash_code = __from._M_hash_code; }
     };
 
   /// Partial specialization used when nodes contain a cached hash code.
diff --git a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
index 10b6146..010181f 100644
--- a/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
+++ b/libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/merge.cc
@@ -417,6 +417,51 @@ test09()
   VERIFY( c2.size() == 3 );
 }
 
+struct slow_stateful_hash
+{
+  size_t seed = 0;
+
+  auto operator()(const int& i) const noexcept
+  { return std::hash<int>()(i) + seed; }
+};
+
+namespace std
+{
+  template<>
+    struct __is_fast_hash<slow_stateful_hash> : public std::false_type
+    { };
+}
+
+void
+test10()
+{
+  using map_type = std::unordered_map<int, int, slow_stateful_hash>;
+  map_type c1({ {1, 1}, {3, 3}, {5, 5} }, 0, slow_stateful_hash{1});
+  map_type c2({ {2, 2}, {4, 4}, {6, 6} }, 0, slow_stateful_hash{2});
+  const auto c3 = c2;
+
+  c1.merge(c2);
+  VERIFY( c1.size() == 6 );
+  VERIFY( c2.empty() );
+
+  c2 = c3;
+  c1.clear();
+  c1.merge(std::move(c2));
+  VERIFY( c1 == c3 );
+  VERIFY( c2.empty() );
+
+  c2.merge(std::move(c1));
+  VERIFY( c1.empty() );
+  VERIFY( c2 == c3 );
+
+  c2.merge(c1);
+  VERIFY( c1.empty() );
+  VERIFY( c2 == c3 );
+
+  c2.merge(c2);
+  VERIFY( c2 == c3 );
+}
+
 int
 main()
 {
@@ -429,4 +474,5 @@ main()
   test07();
   test08();
   test09();
+  test10();
 }