[gcc r14-6630] libstdc++: Optimize std::is_function compilation performance

[gcc r14-6630] libstdc++: Optimize std::is_function compilation performance

[Ken Matsui]

https://gcc.gnu.org/g:cdd4387c5b6aeace2e58318691a5880d29157db9

commit r14-6630-gcdd4387c5b6aeace2e58318691a5880d29157db9
Author: Ken Matsui <kmatsui@gcc.gnu.org>
Date:   Sat Jun 10 21:01:39 2023 -0700

    libstdc++: Optimize std::is_function compilation performance
    
    This patch optimizes the compilation performance of std::is_function
    by dispatching to the new __is_function built-in trait.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (is_function): Use __is_function
            built-in trait.
            (is_function_v): Likewise. Optimize its implementation.  Move
            this under is_const_v as this depends on is_const_v.
    
    Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/std/type_traits | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index db880d87f60..b6d0441129b 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -637,6 +637,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { };
 
   /// is_function
+#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_function)
+  template<typename _Tp>
+    struct is_function
+    : public __bool_constant<__is_function(_Tp)>
+    { };
+#else
   template<typename _Tp>
     struct is_function
     : public __bool_constant<!is_const<const _Tp>::value> { };
@@ -648,6 +654,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     struct is_function<_Tp&&>
     : public false_type { };
+#endif
 
 #ifdef __cpp_lib_is_null_pointer // C++ >= 11
   /// is_null_pointer (LWG 2247).
@@ -3255,8 +3262,7 @@ template <typename _Tp>
   inline constexpr bool is_union_v = __is_union(_Tp);
 template <typename _Tp>
   inline constexpr bool is_class_v = __is_class(_Tp);
-template <typename _Tp>
-  inline constexpr bool is_function_v = is_function<_Tp>::value;
+// is_function_v is defined below, after is_const_v.
 
 #if _GLIBCXX_USE_BUILTIN_TRAIT(__is_reference)
 template <typename _Tp>
@@ -3293,6 +3299,19 @@ template <typename _Tp>
   inline constexpr bool is_const_v = false;
 template <typename _Tp>
   inline constexpr bool is_const_v<const _Tp> = true;
+
+#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_function)
+template <typename _Tp>
+  inline constexpr bool is_function_v = __is_function(_Tp);
+#else
+template <typename _Tp>
+  inline constexpr bool is_function_v = !is_const_v<const _Tp>;
+template <typename _Tp>
+  inline constexpr bool is_function_v<_Tp&> = false;
+template <typename _Tp>
+  inline constexpr bool is_function_v<_Tp&&> = false;
+#endif
+
 template <typename _Tp>
   inline constexpr bool is_volatile_v = false;
 template <typename _Tp>