* [PATCH] libstdc++: Prefer const T to std::add_const_t<T>
@ 2022-07-07 16:54 Jonathan Wakely
2022-07-12 8:40 ` Jonathan Wakely
0 siblings, 1 reply; 2+ messages in thread
From: Jonathan Wakely @ 2022-07-07 16:54 UTC (permalink / raw)
To: libstdc++, gcc-patches
Does anybody see a problem with this change? The point is to avoid
unnecessary class template instantiations.
Tested aarch64-linux.
-- >8 --
For any typedef-name or template parameter, T, add_const_t<T> is
equivalent to T const, so we can avoid instantiating the std::add_const
class template and just say T const (or const T).
This isn't true for a non-typedef like int&, where int& const would be
ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
we know what that type is.
The only place we need to continue using std::add_const is in the
std::bind implementation where it's used as a template template
parameter to be applied as a metafunction elsewhere.
libstdc++-v3/ChangeLog:
* include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
add_const_t with const-qualifier.
* include/bits/utility.h (tuple_element<N, cv T>): Likewise for
all cv-qualifiers.
* include/std/type_traits (add_const, add_volatile): Replace
typedef-declaration with using-declaration.
(add_cv): Replace add_const and add_volatile with cv-qualifiers.
* include/std/variant (variant_alternative<N, cv T>): Replace
add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.
---
libstdc++-v3/include/bits/stl_iterator.h | 11 +++++------
libstdc++-v3/include/bits/utility.h | 6 +++---
libstdc++-v3/include/std/type_traits | 9 +++------
libstdc++-v3/include/std/variant | 6 +++---
4 files changed, 14 insertions(+), 18 deletions(-)
diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 12a89ab229f..049cb02a4c4 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -2536,19 +2536,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// of associative containers.
template<typename _InputIterator>
using __iter_key_t = remove_const_t<
- typename iterator_traits<_InputIterator>::value_type::first_type>;
+ typename iterator_traits<_InputIterator>::value_type::first_type>;
template<typename _InputIterator>
- using __iter_val_t =
- typename iterator_traits<_InputIterator>::value_type::second_type;
+ using __iter_val_t
+ = typename iterator_traits<_InputIterator>::value_type::second_type;
template<typename _T1, typename _T2>
struct pair;
template<typename _InputIterator>
- using __iter_to_alloc_t =
- pair<add_const_t<__iter_key_t<_InputIterator>>,
- __iter_val_t<_InputIterator>>;
+ using __iter_to_alloc_t
+ = pair<const __iter_key_t<_InputIterator>, __iter_val_t<_InputIterator>>;
#endif // __cpp_deduction_guides
_GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index e0e40309a6d..6a192e27836 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -86,19 +86,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<size_t __i, typename _Tp>
struct tuple_element<__i, const _Tp>
{
- typedef typename add_const<__tuple_element_t<__i, _Tp>>::type type;
+ using type = const __tuple_element_t<__i, _Tp>;
};
template<size_t __i, typename _Tp>
struct tuple_element<__i, volatile _Tp>
{
- typedef typename add_volatile<__tuple_element_t<__i, _Tp>>::type type;
+ using type = volatile __tuple_element_t<__i, _Tp>;
};
template<size_t __i, typename _Tp>
struct tuple_element<__i, const volatile _Tp>
{
- typedef typename add_cv<__tuple_element_t<__i, _Tp>>::type type;
+ using type = const volatile __tuple_element_t<__i, _Tp>;
};
#if __cplusplus >= 201402L
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 2572d8edd69..e5f58bc2e3f 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -1577,20 +1577,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/// add_const
template<typename _Tp>
struct add_const
- { typedef _Tp const type; };
+ { using type = _Tp const; };
/// add_volatile
template<typename _Tp>
struct add_volatile
- { typedef _Tp volatile type; };
+ { using type = _Tp volatile; };
/// add_cv
template<typename _Tp>
struct add_cv
- {
- typedef typename
- add_const<typename add_volatile<_Tp>::type>::type type;
- };
+ { using type = _Tp const volatile; };
#if __cplusplus > 201103L
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 5ff1e3edcdf..f8f15665433 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -107,15 +107,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<size_t _Np, typename _Variant>
struct variant_alternative<_Np, const _Variant>
- { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
+ { using type = const variant_alternative_t<_Np, _Variant>; };
template<size_t _Np, typename _Variant>
struct variant_alternative<_Np, volatile _Variant>
- { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
+ { using type = volatile variant_alternative_t<_Np, _Variant>; };
template<size_t _Np, typename _Variant>
struct variant_alternative<_Np, const volatile _Variant>
- { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
+ { using type = const volatile variant_alternative_t<_Np, _Variant>; };
inline constexpr size_t variant_npos = -1;
--
2.36.1
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] libstdc++: Prefer const T to std::add_const_t<T>
2022-07-07 16:54 [PATCH] libstdc++: Prefer const T to std::add_const_t<T> Jonathan Wakely
@ 2022-07-12 8:40 ` Jonathan Wakely
0 siblings, 0 replies; 2+ messages in thread
From: Jonathan Wakely @ 2022-07-12 8:40 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: libstdc++, gcc-patches
On Thu, 7 Jul 2022 at 17:55, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Does anybody see a problem with this change? The point is to avoid
> unnecessary class template instantiations.
>
> Tested aarch64-linux.
Pushed to trunk now.
>
> -- >8 --
>
> For any typedef-name or template parameter, T, add_const_t<T> is
> equivalent to T const, so we can avoid instantiating the std::add_const
> class template and just say T const (or const T).
>
> This isn't true for a non-typedef like int&, where int& const would be
> ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
> we know what that type is.
>
> The only place we need to continue using std::add_const is in the
> std::bind implementation where it's used as a template template
> parameter to be applied as a metafunction elsewhere.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
> add_const_t with const-qualifier.
> * include/bits/utility.h (tuple_element<N, cv T>): Likewise for
> all cv-qualifiers.
> * include/std/type_traits (add_const, add_volatile): Replace
> typedef-declaration with using-declaration.
> (add_cv): Replace add_const and add_volatile with cv-qualifiers.
> * include/std/variant (variant_alternative<N, cv T>): Replace
> add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.
> ---
> libstdc++-v3/include/bits/stl_iterator.h | 11 +++++------
> libstdc++-v3/include/bits/utility.h | 6 +++---
> libstdc++-v3/include/std/type_traits | 9 +++------
> libstdc++-v3/include/std/variant | 6 +++---
> 4 files changed, 14 insertions(+), 18 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
> index 12a89ab229f..049cb02a4c4 100644
> --- a/libstdc++-v3/include/bits/stl_iterator.h
> +++ b/libstdc++-v3/include/bits/stl_iterator.h
> @@ -2536,19 +2536,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // of associative containers.
> template<typename _InputIterator>
> using __iter_key_t = remove_const_t<
> - typename iterator_traits<_InputIterator>::value_type::first_type>;
> + typename iterator_traits<_InputIterator>::value_type::first_type>;
>
> template<typename _InputIterator>
> - using __iter_val_t =
> - typename iterator_traits<_InputIterator>::value_type::second_type;
> + using __iter_val_t
> + = typename iterator_traits<_InputIterator>::value_type::second_type;
>
> template<typename _T1, typename _T2>
> struct pair;
>
> template<typename _InputIterator>
> - using __iter_to_alloc_t =
> - pair<add_const_t<__iter_key_t<_InputIterator>>,
> - __iter_val_t<_InputIterator>>;
> + using __iter_to_alloc_t
> + = pair<const __iter_key_t<_InputIterator>, __iter_val_t<_InputIterator>>;
> #endif // __cpp_deduction_guides
>
> _GLIBCXX_END_NAMESPACE_VERSION
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
> index e0e40309a6d..6a192e27836 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -86,19 +86,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<size_t __i, typename _Tp>
> struct tuple_element<__i, const _Tp>
> {
> - typedef typename add_const<__tuple_element_t<__i, _Tp>>::type type;
> + using type = const __tuple_element_t<__i, _Tp>;
> };
>
> template<size_t __i, typename _Tp>
> struct tuple_element<__i, volatile _Tp>
> {
> - typedef typename add_volatile<__tuple_element_t<__i, _Tp>>::type type;
> + using type = volatile __tuple_element_t<__i, _Tp>;
> };
>
> template<size_t __i, typename _Tp>
> struct tuple_element<__i, const volatile _Tp>
> {
> - typedef typename add_cv<__tuple_element_t<__i, _Tp>>::type type;
> + using type = const volatile __tuple_element_t<__i, _Tp>;
> };
>
> #if __cplusplus >= 201402L
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 2572d8edd69..e5f58bc2e3f 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -1577,20 +1577,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> /// add_const
> template<typename _Tp>
> struct add_const
> - { typedef _Tp const type; };
> + { using type = _Tp const; };
>
> /// add_volatile
> template<typename _Tp>
> struct add_volatile
> - { typedef _Tp volatile type; };
> + { using type = _Tp volatile; };
>
> /// add_cv
> template<typename _Tp>
> struct add_cv
> - {
> - typedef typename
> - add_const<typename add_volatile<_Tp>::type>::type type;
> - };
> + { using type = _Tp const volatile; };
>
> #if __cplusplus > 201103L
>
> diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> index 5ff1e3edcdf..f8f15665433 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -107,15 +107,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> template<size_t _Np, typename _Variant>
> struct variant_alternative<_Np, const _Variant>
> - { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
> + { using type = const variant_alternative_t<_Np, _Variant>; };
>
> template<size_t _Np, typename _Variant>
> struct variant_alternative<_Np, volatile _Variant>
> - { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
> + { using type = volatile variant_alternative_t<_Np, _Variant>; };
>
> template<size_t _Np, typename _Variant>
> struct variant_alternative<_Np, const volatile _Variant>
> - { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
> + { using type = const volatile variant_alternative_t<_Np, _Variant>; };
>
> inline constexpr size_t variant_npos = -1;
>
> --
> 2.36.1
>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2022-07-12 8:40 ` Jonathan Wakely
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