[PATCH 1/2] libstdc++: Optimize is_void and is_null_pointer

[Patrick Palka via Gcc-patches]

Instead of defining these in terms of a helper class template
and the relatively expensive __remove_cv_t, just declare four
explicit specializations of the main template, one for each choice
of cv-quals.

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
The is_void change alone reduces memory usage for join.cc by
almost 1%.

libstdc++-v3/ChangeLog:

        * include/std/type_traits (__is_void_helper): Remove.
        (is_void): Make the primary template derive from false_type,
        and define four explicit specializations that derive from
        true_type.
        (__is_null_pointer_helper, is_null_pointer): Likewise.
---
 libstdc++-v3/include/std/type_traits | 48 ++++++++++++++++++----------
 1 file changed, 31 insertions(+), 17 deletions(-)

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index e4d167939d9..b83e7257a9f 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -289,23 +289,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // __remove_cv_t (std::remove_cv_t for C++11).
   template<typename _Tp>
     using __remove_cv_t = typename remove_cv<_Tp>::type;
+  /// @endcond
 
   // Primary type categories.
 
-  template<typename>
-    struct __is_void_helper
+  /// is_void
+  template<typename _Tp>
+    struct is_void
     : public false_type { };
 
   template<>
-    struct __is_void_helper<void>
+    struct is_void<void>
     : public true_type { };
-  /// @endcond
 
-  /// is_void
-  template<typename _Tp>
-    struct is_void
-    : public __is_void_helper<__remove_cv_t<_Tp>>::type
-    { };
+  template<>
+    struct is_void<const void>
+    : public true_type { };
+
+  template<>
+    struct is_void<volatile void>
+    : public true_type { };
+
+  template<>
+    struct is_void<const volatile void>
+    : public true_type { };
 
   /// @cond undocumented
   template<typename>
@@ -571,19 +578,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #define __cpp_lib_is_null_pointer 201309L
 
-  template<typename>
-    struct __is_null_pointer_helper
+  /// is_null_pointer (LWG 2247).
+  template<typename _Tp>
+    struct is_null_pointer
     : public false_type { };
 
   template<>
-    struct __is_null_pointer_helper<std::nullptr_t>
+    struct is_null_pointer<std::nullptr_t>
     : public true_type { };
 
-  /// is_null_pointer (LWG 2247).
-  template<typename _Tp>
-    struct is_null_pointer
-    : public __is_null_pointer_helper<__remove_cv_t<_Tp>>::type
-    { };
+  template<>
+    struct is_null_pointer<const std::nullptr_t>
+    : public true_type { };
+
+  template<>
+    struct is_null_pointer<volatile std::nullptr_t>
+    : public true_type { };
+
+  template<>
+    struct is_null_pointer<const volatile std::nullptr_t>
+    : public true_type { };
 
   /// __is_nullptr_t (deprecated extension).
   /// @deprecated Non-standard. Use `is_null_pointer` instead.
-- 
2.37.3.518.g79f2338b37