Re: [PATCH v26 03/13] libstdc++: Optimize std::is_pointer compilation performance

[Andrew Pinski <pinskia@gmail.com>]

On Thu, Jun 13, 2024 at 5:32 AM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 11/05/24 02:01 -0700, 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 | 31 ++++++++++++++-
> > libstdc++-v3/include/std/type_traits        | 44 +++++++++++++++++----
> > 2 files changed, 66 insertions(+), 9 deletions(-)
> >
> >diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
> >index 59f1a1875eb..210a9ea00da 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>
>
> I was worried that reusing __is_pointer this way would cause a problem
> with Clang, because it has an __is_pointer built-in and the code above
> causes a warning:
>
> isp.cc:2:12: warning: keyword '__is_pointer' will be made available as an identifier for the remainder of the translation unit [-Wkeyword-compat]
>      2 |     struct __is_pointer
>        |            ^
>
> I thought this warning meant it was only available as an identifier.
> But in fact it becomes available as both an identifier and as the
> built-in.
>
> This is what I tested:
>
> template<typename T, bool IsPtr = __is_pointer(T)>
> struct __is_pointer
> {
>    enum { value = IsPtr };
> };
> static_assert( __is_pointer<int*>::value, "" );
>
> template<typename T>
> struct is_pointer
> {
>    static constexpr bool value = __is_pointer(T);
> };
> static_assert( is_pointer<int*>::value, "" );
>
> So the is_pointer template can still use the built-in even though the
> name '__is_pointer' has been declared as an identifier.
>
> So Clang matches GCC and it works fine. Good!

Actually clang does not work the same as GCC in the end; it is much
more inconsistent in what it allows.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115497 (and
https://github.com/llvm/llvm-project/issues/95598). Where sometimes
the usage works as a trait after the use of it as an identifier and
sometimes it fails.

Thanks,
Andrew Pinski

>
> >+    {
> >+      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>
>
> None of the other traits in bits/cpp_type_traits.h is true for
> cv-qualified types, so these partial specializations make __is_pointer
> the odd one out. Are they necessary?
>
> >+    {
> >+      enum { __value = 1 };
> >+      typedef __true_type __type;
> >+    };
> >+
> >+  template<typename _Tp>
> >+    struct __is_pointer<_Tp* volatile>
> >+    {
> >+      enum { __value = 1 };
> >+      typedef __true_type __type;
> >+    };
> >+
> >+  template<typename _Tp>
> >+    struct __is_pointer<_Tp* const volatile>
> >+    {
> >+      enum { __value = 1 };
> >+      typedef __true_type __type;
> >+    };
> >+#endif
> >+
> >   //
> >   // An arithmetic type is an integer type or a floating point type
> >   //
> >@@ -387,7 +416,7 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
> >
> >   //
> >   // A scalar type is an arithmetic type or a pointer type
> >-  //
> >+  //
> >   template<typename _Tp>
> >     struct __is_scalar
> >     : public __traitor<__is_arithmetic<_Tp>, __is_pointer<_Tp> >
> >diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> >index 748fa186881..ea013b4b7bc 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>
> >@@ -3268,8 +3282,22 @@ 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>
> >--
> >2.44.0
> >
>