[PATCH v4 08/12] libstdc++: Optimize std::add_lvalue_reference compilation performance

[Ken Matsui <kmatsui@gcc.gnu.org>]

This patch optimizes the compilation performance of
std::add_lvalue_reference by dispatching to the new
__add_lvalue_reference built-in trait.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (add_lvalue_reference): Use
	__add_lvalue_reference built-in trait.
	(__add_lvalue_reference_helper): Likewise.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
---
 libstdc++-v3/include/std/type_traits | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 2e1cc1c1d5f..1f4e6db72f4 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -1129,6 +1129,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 
   /// @cond undocumented
+#if _GLIBCXX_USE_BUILTIN_TRAIT(__add_lvalue_reference)
+  template<typename _Tp>
+    struct __add_lvalue_reference_helper
+    { using type = __add_lvalue_reference(_Tp); };
+#else
   template<typename _Tp, typename = void>
     struct __add_lvalue_reference_helper
     { using type = _Tp; };
@@ -1136,6 +1141,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     struct __add_lvalue_reference_helper<_Tp, __void_t<_Tp&>>
     { using type = _Tp&; };
+#endif
 
   template<typename _Tp>
     using __add_lval_ref_t = typename __add_lvalue_reference_helper<_Tp>::type;
@@ -1703,9 +1709,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
 
   /// add_lvalue_reference
+#if _GLIBCXX_USE_BUILTIN_TRAIT(__add_lvalue_reference)
+  template<typename _Tp>
+    struct add_lvalue_reference
+    { using type = __add_lvalue_reference(_Tp); };
+#else
   template<typename _Tp>
     struct add_lvalue_reference
     { using type = __add_lval_ref_t<_Tp>; };
+#endif
 
   /// add_rvalue_reference
   template<typename _Tp>
-- 
2.43.0