Optimize emplace() for exactly a value_type or init_type argument (#227)

* Add structs to count special member functions

* Add failing emplace tests, that will pass after making the optimization

* Optimize emplace() to not allocate when we already have a value_type or init_type

* Fix newly failing cfoa tests

include/boost/unordered/detail/foa/concurrent_table.hpp

@@ -28,6 +28,7 @@
 #include <boost/unordered/detail/foa/tuple_rotate_right.hpp>
 #include <boost/unordered/detail/serialization_version.hpp>
 #include <boost/unordered/detail/static_assert.hpp>
+#include <boost/unordered/detail/type_traits.hpp>
 #include <cstddef>
 #include <functional>
 #include <initializer_list>
@@ -682,6 +683,14 @@ public:
     return construct_and_emplace(std::forward<Args>(args)...);
   }
 
+  /* Optimization for value_type and init_type, to avoid constructing twice */
+  template<typename Value>
+  BOOST_FORCEINLINE auto emplace(Value&& x)->typename std::enable_if<
+    detail::is_similar_to_any<Value,value_type,init_type>::value,bool>::type
+  {
+    return emplace_impl(std::forward<Value>(x));
+  }
+
   BOOST_FORCEINLINE bool
   insert(const init_type& x){return emplace_impl(x);}
 

include/boost/unordered/detail/foa/table.hpp

@@ -18,6 +18,7 @@
 #include <boost/core/serialization.hpp>
 #include <boost/unordered/detail/foa/core.hpp>
 #include <boost/unordered/detail/serialize_tracked_address.hpp>
+#include <boost/unordered/detail/type_traits.hpp>
 #include <cstddef>
 #include <iterator>
 #include <memory>
@@ -405,6 +406,16 @@ public:
     return emplace_impl(type_policy::move(x.value()));
   }
 
+  /* Optimization for value_type and init_type, to avoid constructing twice */
+  template <typename T>
+  BOOST_FORCEINLINE typename std::enable_if<
+    detail::is_similar_to_any<T, value_type, init_type>::value,
+    std::pair<iterator, bool> >::type
+  emplace(T&& x)
+  {
+    return emplace_impl(std::forward<T>(x));
+  }
+
   template<typename Key,typename... Args>
   BOOST_FORCEINLINE std::pair<iterator,bool> try_emplace(
     Key&& x,Args&&... args)

include/boost/unordered/detail/type_traits.hpp

@@ -147,6 +147,23 @@ namespace boost {
           !std::is_convertible<Key, const_iterator>::value;
       };
 
+      template <class T>
+      using remove_cvref_t =
+        typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+
+      template <class T, class U>
+      using is_similar = std::is_same<remove_cvref_t<T>, remove_cvref_t<U> >;
+
+      template <class, class...> struct is_similar_to_any : std::false_type
+      {
+      };
+      template <class T, class U, class... Us>
+      struct is_similar_to_any<T, U, Us...>
+          : std::conditional<is_similar<T, U>::value, is_similar<T, U>,
+              is_similar_to_any<T, Us...> >::type
+      {
+      };
+
 #if BOOST_UNORDERED_TEMPLATE_DEDUCTION_GUIDES
       // https://eel.is/c++draft/container.requirements#container.alloc.reqmts-34
       // https://eel.is/c++draft/container.requirements#unord.req.general-243