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

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

[Patrick Palka]

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

[PATCH 2/2] libstdc++: Optimize is_reference

[Patrick Palka]

Instead of defining is_reference in terms of is_lvalue_reference
and is_rvalue_reference, just define it directly.

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
This reduces memory usage of join.cc by 1%.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_reference): Make the primary
	template derive from false_type.  Define two partial
	specializations that derive from true_type.
---
 libstdc++-v3/include/std/type_traits | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index b83e7257a9f..94e73eafd2f 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -611,8 +611,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// is_reference
   template<typename _Tp>
     struct is_reference
-    : public __or_<is_lvalue_reference<_Tp>,
-                   is_rvalue_reference<_Tp>>::type
+    : public false_type
+    { };
+
+  template<typename _Tp>
+    struct is_reference<_Tp&>
+    : public true_type
+    { };
+
+  template<typename _Tp>
+    struct is_reference<_Tp&&>
+    : public true_type
     { };
 
   /// is_arithmetic
-- 
2.37.3.518.g79f2338b37

Re: [PATCH 2/2] libstdc++: Optimize is_reference

[Jonathan Wakely]

On Wed, 7 Sept 2022 at 01:46, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Instead of defining is_reference in terms of is_lvalue_reference
> and is_rvalue_reference, just define it directly.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

Yes, thanks (I already did this for the std::is_reference_v variable
template, but for some reason left this equivalent change in the local
branch where I was doing the traits refactoring).

> This reduces memory usage of join.cc by 1%.

Now that many of the variable templates have been optimized to avoid
instantiating class templates, I wonder if the <ranges> code (and
anything else that's only defined for C++17 or later) would benefit
from using foo_v<T> && bar_v<T> instead of __and_<foo<T>, bar<T>>.
With your improvements to __and_ maybe it doesn't make so much
difference.


>
> libstdc++-v3/ChangeLog:
>
>         * include/std/type_traits (is_reference): Make the primary
>         template derive from false_type.  Define two partial
>         specializations that derive from true_type.
> ---
>  libstdc++-v3/include/std/type_traits | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index b83e7257a9f..94e73eafd2f 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -611,8 +611,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    /// is_reference
>    template<typename _Tp>
>      struct is_reference
> -    : public __or_<is_lvalue_reference<_Tp>,
> -                   is_rvalue_reference<_Tp>>::type
> +    : public false_type
> +    { };
> +
> +  template<typename _Tp>
> +    struct is_reference<_Tp&>
> +    : public true_type
> +    { };
> +
> +  template<typename _Tp>
> +    struct is_reference<_Tp&&>
> +    : public true_type
>      { };
>
>    /// is_arithmetic
> --
> 2.37.3.518.g79f2338b37
>

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

[Jonathan Wakely]

On Wed, 7 Sept 2022 at 01:46, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> 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?

Yes, and we should do the same for is_void_v and  is_null_pointer_v.
Even though the class templates they instantiate will be cheaper now,
they can still avoid any class template instantiation.

> The is_void change alone reduces memory usage for join.cc by
> almost 1%.

Also in my queue of trait refactoring, but I haven't tested if these
are worth doing yet:

template<typename T>
  struct is_object : is_const<const T>::type { };
template<>
  struct is_object<void> : false_type { };
template<>
  struct is_object<const void> : false_type { };
// and so on for volatile and const volatile

And we could define is_object_v as simply is_const_v<const T> && is_void_v<T>.

Improving is_object should benefit <ranges> and also the is_scalar
change proposed at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96710#c2