Re: [PATCH v2 6/8] libstdc++: Optimize std::is_pointer compilation performance

[Jonathan Wakely <jwakely@redhat.com>]

On Sat, 23 Dec 2023 at 22:07, Ken Matsui <kmatsui@gcc.gnu.org> wrote:
>
> This patch optimizes the compilation performance of std::is_pointer
> by dispatching to the new __is_pointer built-in trait.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/cpp_type_traits.h (__is_pointer): Use
>         __is_pointer built-in trait.  Optimize its implementation.
>         * include/std/type_traits (is_pointer): Likewise.
>         (is_pointer_v): Likewise.
>
> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
> ---
>  libstdc++-v3/include/bits/cpp_type_traits.h | 29 ++++++++++++++
>  libstdc++-v3/include/std/type_traits        | 44 +++++++++++++++++----
>  2 files changed, 65 insertions(+), 8 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
> index 4312f32a4e0..c348df97f72 100644
> --- a/libstdc++-v3/include/bits/cpp_type_traits.h
> +++ b/libstdc++-v3/include/bits/cpp_type_traits.h
> @@ -363,6 +363,13 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
>    //
>    // Pointer types
>    //
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_pointer)
> +  template<typename _Tp, bool _IsPtr = __is_pointer(_Tp)>
> +    struct __is_pointer : __truth_type<_IsPtr>
> +    {
> +      enum { __value = _IsPtr };
> +    };
> +#else
>    template<typename _Tp>
>      struct __is_pointer
>      {
> @@ -377,6 +384,28 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
>        typedef __true_type __type;
>      };
>
> +  template<typename _Tp>
> +    struct __is_pointer<_Tp* const>
> +    {
> +      enum { __value = 1 };
> +      typedef __true_type __type;
> +    };
> +
> +  template<typename _Tp>
> +    struct __is_pointer<_Tp* volatile>
> +    {
> +      enum { __value = 1 };
> +      typedef __true_type __type;
> +    };
> +#endif
> +
> +  template<typename _Tp>
> +    struct __is_pointer<_Tp* const volatile>
> +    {
> +      enum { __value = 1 };
> +      typedef __true_type __type;
> +    };

Why is this partial specialization outside the #else group?


> +
>    //
>    // An arithmetic type is an integer type or a floating point type
>    //
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 30b0778e58a..d53911b2fa0 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -542,19 +542,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      : public true_type { };
>  #endif
>
> -  template<typename>
> -    struct __is_pointer_helper
> +  /// is_pointer
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_pointer)
> +  template<typename _Tp>
> +    struct is_pointer
> +    : public __bool_constant<__is_pointer(_Tp)>
> +    { };
> +#else
> +  template<typename _Tp>
> +    struct is_pointer
>      : public false_type { };
>
>    template<typename _Tp>
> -    struct __is_pointer_helper<_Tp*>
> +    struct is_pointer<_Tp*>
>      : public true_type { };
>
> -  /// is_pointer
>    template<typename _Tp>
> -    struct is_pointer
> -    : public __is_pointer_helper<__remove_cv_t<_Tp>>::type
> -    { };
> +    struct is_pointer<_Tp* const>
> +    : public true_type { };
> +
> +  template<typename _Tp>
> +    struct is_pointer<_Tp* volatile>
> +    : public true_type { };
> +
> +  template<typename _Tp>
> +    struct is_pointer<_Tp* const volatile>
> +    : public true_type { };
> +#endif
>
>    /// is_lvalue_reference
>    template<typename>
> @@ -3252,8 +3266,22 @@ template <typename _Tp, size_t _Num>
>    inline constexpr bool is_array_v<_Tp[_Num]> = true;
>  #endif
>
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_pointer)
> +template <typename _Tp>
> +  inline constexpr bool is_pointer_v = __is_pointer(_Tp);
> +#else
>  template <typename _Tp>
> -  inline constexpr bool is_pointer_v = is_pointer<_Tp>::value;
> +  inline constexpr bool is_pointer_v = false;
> +template <typename _Tp>
> +  inline constexpr bool is_pointer_v<_Tp*> = true;
> +template <typename _Tp>
> +  inline constexpr bool is_pointer_v<_Tp* const> = true;
> +template <typename _Tp>
> +  inline constexpr bool is_pointer_v<_Tp* volatile> = true;
> +template <typename _Tp>
> +  inline constexpr bool is_pointer_v<_Tp* const volatile> = true;
> +#endif
> +
>  template <typename _Tp>
>    inline constexpr bool is_lvalue_reference_v = false;
>  template <typename _Tp>
> --
> 2.43.0
>