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

[Patrick Palka]

On Thu, 4 Jan 2024, Patrick Palka wrote:

> On Sat, 23 Dec 2023, Ken Matsui 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;
> > +    };
> > +
> >    //
> >    // 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;
> 
> Is this fallback implementation faster than the current implementation?
> LGTM if so.

By the way, a related optimization might be to optimize __remove_cv_t
via the built-in (related since the current fallback implementation is
in terms of __remove_cv_t).  This should be safe since the alias
template isn't used in any function signature, so it's never mangled.

> 
> > +#endif
> > +
> >  template <typename _Tp>
> >    inline constexpr bool is_lvalue_reference_v = false;
> >  template <typename _Tp>
> > -- 
> > 2.43.0
> > 
> > 
>