[1/2] libstdc++: Optimize is_void and is_null_pointer
Commit Message
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(-)
Comments
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
@@ -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.