libstdc++: Avoid unnecessary allocations in std::map insertions [PR92300]

Inserting a pair<Key, Value> into a map<Key, Value> will allocate a new
node and construct a pair<const Key, Value> in the node, then check if
the Key is already present in the map. That is because pair<Key, Value>
is not the same type as the map's value_type. But it only differs in the
const-qualification on the Key, and so we should be able to do the
lookup directly, without allocating a new node. This avoids allocating
and then deallocating a node for the case where the key is already found
and nothing gets inserted.

We can take this optimization further and lookup the key directly for a
pair<Key, X>, pair<const Key, X>, pair<Key&, X> etc. for any X. A strict
reading of the standard says we can only do this when we know the
allocator won't do anything funky with the value when constructing a
pair<const Key, Value> from a slightly different type. Inserting that
type only requires the value_type to be Cpp17EmplaceInsertable into the
container, and that doesn't have any requirement that the value is
unchanged (unlike Cpp17CopyInsertable and Cpp17MoveInsertable). For that
reason, the optimization is only done for maps using std::allocator.

A similar optimization can be done for map.emplace(key, value) where the
first argument is similar to the key_type and so can be looked up
without allocating a new node and constructing a key_type.

Finally, both of the insert and emplace cases can use the same
optimization when key_type is a scalar type and some other scalar is
being passed as the insert/emplace argument. Converting from one scalar
type to another won't have surprising value-altering behaviour, and has
no side effects (unlike e.g. constructing a std::string from a const
char* argument, which might allocate).

We don't need to do this for std::multimap, because we always insert the
new node even if the key is already present. So there's no benefit to
doing the lookup before allocating the new node.

libstdc++-v3/ChangeLog:

	PR libstdc++/92300
	* include/bits/stl_map.h (insert(Pair&&), emplace(Args&&...)):
	Check whether the arguments can be looked up directly without
	constructing a temporary node first.
	* include/bits/stl_pair.h (__is_pair): Move to here, from ...
	* include/bits/uses_allocator_args.h (__is_pair): ... here.
	* testsuite/23_containers/map/modifiers/emplace/92300.cc: New test.
	* testsuite/23_containers/map/modifiers/insert/92300.cc: New test.

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

@@ -154,6 +154,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       typedef __gnu_cxx::__alloc_traits<_Pair_alloc_type> _Alloc_traits;
 
+#if __cplusplus >= 201703L
+      template<typename _Up, typename _Vp = remove_reference_t<_Up>>
+	static constexpr bool __usable_key
+	  = __or_v<is_same<const _Vp, const _Key>,
+		   __and_<is_scalar<_Vp>, is_scalar<_Key>>>;
+#endif
+
     public:
       // many of these are specified differently in ISO, but the following are
       // "functionally equivalent"
@@ -574,7 +581,27 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       template<typename... _Args>
 	std::pair<iterator, bool>
 	emplace(_Args&&... __args)
-	{ return _M_t._M_emplace_unique(std::forward<_Args>(__args)...); }
+	{
+#if __cplusplus >= 201703L
+	  if constexpr (sizeof...(_Args) == 2)
+	    if constexpr (is_same_v<allocator_type, allocator<value_type>>)
+	      {
+		auto&& [__a, __v] = pair<_Args&...>(__args...);
+		if constexpr (__usable_key<decltype(__a)>)
+		  {
+		    const key_type& __k = __a;
+		    iterator __i = lower_bound(__k);
+		    if (__i == end() || key_comp()(__k, (*__i).first))
+		      {
+			__i = emplace_hint(__i, std::forward<_Args>(__args)...);
+			return {__i, true};
+		      }
+		    return {__i, false};
+		  }
+	      }
+#endif
+	  return _M_t._M_emplace_unique(std::forward<_Args>(__args)...);
+	}
 
       /**
        *  @brief Attempts to build and insert a std::pair into the %map.
@@ -814,7 +841,25 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	__enable_if_t<is_constructible<value_type, _Pair>::value,
 		      pair<iterator, bool>>
 	insert(_Pair&& __x)
-	{ return _M_t._M_emplace_unique(std::forward<_Pair>(__x)); }
+	{
+#if __cplusplus >= 201703L
+	  using _P2 = remove_reference_t<_Pair>;
+	  if constexpr (__is_pair<_P2>)
+	    if constexpr (is_same_v<allocator_type, allocator<value_type>>)
+	      if constexpr (__usable_key<typename _P2::first_type>)
+		{
+		  const key_type& __k = __x.first;
+		  iterator __i = lower_bound(__k);
+		  if (__i == end() || key_comp()(__k, (*__i).first))
+		    {
+		      __i = emplace_hint(__i, std::forward<_Pair>(__x));
+		      return {__i, true};
+		    }
+		  return {__i, false};
+		}
+#endif
+	  return _M_t._M_emplace_unique(std::forward<_Pair>(__x));
+	}
 #endif
       /// @}
 

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

@@ -777,6 +777,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _Tp1, typename _Tp2>
     inline constexpr size_t tuple_size_v<const pair<_Tp1, _Tp2>> = 2;
+
+  template<typename _Tp>
+    inline constexpr bool __is_pair = false;
+
+  template<typename _Tp, typename _Up>
+    inline constexpr bool __is_pair<pair<_Tp, _Up>> = true;
+
+  template<typename _Tp, typename _Up>
+    inline constexpr bool __is_pair<const pair<_Tp, _Up>> = true;
 #endif
 
   /// @cond undocumented

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

@@ -56,12 +56,6 @@
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
-  template<typename _Tp>
-    inline constexpr bool __is_pair = false;
-  template<typename _Tp, typename _Up>
-    inline constexpr bool __is_pair<pair<_Tp, _Up>> = true;
-  template<typename _Tp, typename _Up>
-    inline constexpr bool __is_pair<const pair<_Tp, _Up>> = true;
 
   template<typename _Tp>
     concept _Std_pair = __is_pair<_Tp>;

libstdc++-v3/testsuite/23_containers/map/modifiers/emplace/92300.cc

@@ -0,0 +1,36 @@
+// { dg-do run { target c++17 } }
+
+#include <map>
+#include <cstdlib>
+
+bool oom = false;
+
+void* operator new(std::size_t n)
+{
+  if (oom)
+    throw std::bad_alloc();
+  return std::malloc(n);
+}
+
+void operator delete(void* p)
+{
+  std::free(p);
+}
+
+void operator delete(void* p, std::size_t)
+{
+  std::free(p);
+}
+
+int main()
+{
+  std::map<int, int> m;
+  int i = 0;
+  (void) m[i];
+  oom = true;
+  m.emplace(i, 1);
+  m.emplace(i, 2L);
+  const int c = 3;
+  m.emplace(i, c);
+  m.emplace((long)i, 4);
+}

libstdc++-v3/testsuite/23_containers/map/modifiers/insert/92300.cc

@@ -0,0 +1,38 @@
+// { dg-do run { target c++17 } }
+
+#include <map>
+#include <cstdlib>
+
+bool oom = false;
+
+void* operator new(std::size_t n)
+{
+  if (oom)
+    throw std::bad_alloc();
+  return std::malloc(n);
+}
+
+void operator delete(void* p)
+{
+  std::free(p);
+}
+
+void operator delete(void* p, std::size_t)
+{
+  std::free(p);
+}
+
+int main()
+{
+  using std::pair;
+  std::map<int, int> m;
+  int i = 0;
+  (void) m[i];
+  oom = true;
+  m.insert({i, 1});			    // insert(value_type&&)
+  m.insert(pair<int, int>(i, 2));	    // insert(Pair&&)
+  m.insert(pair<int&, int>(i, 3));	    // insert(Pair&&)
+  m.insert(pair<int, long>(i, 4L));	    // insert(Pair&&)
+  m.insert(pair<const int, long>(i, 5L));   // insert(Pair&&)
+  m.insert(pair<const int&, long>(i, 6L));  // insert(Pair&&)
+}