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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/110512] C++20 random access iterators run
 sequentially with PSTL
Date: Tue, 09 Jan 2024 23:15:32 +0000
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X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110512

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The proposed change will compile very slowly, something like this would be
better:

--- a/libstdc++-v3/include/pstl/execution_impl.h
+++ b/libstdc++-v3/include/pstl/execution_impl.h
@@ -19,13 +19,24 @@ namespace __pstl
 {
 namespace __internal
 {
+#if __glibcxx_concepts
+template<typename _Iter>
+  concept __is_random_access_iter
+    =3D std::is_base_of_v<std::random_access_iterator_tag,
+                       std::__iter_category_t<_Iter>>
+      || std::random_access_iterator<_Iter>;

+template <typename... _IteratorTypes>
+  using __are_random_access_iterators
+    =3D std::bool_constant<(__is_random_access_iter<_IteratorTypes> && ...=
)>;
+#else
 template <typename _IteratorTag, typename... _IteratorTypes>
 using __are_iterators_of =3D std::conjunction<
     std::is_base_of<_IteratorTag, typename
std::iterator_traits<std::decay_t<_IteratorTypes>>::iterator_category>...>;

 template <typename... _IteratorTypes>
 using __are_random_access_iterators =3D
__are_iterators_of<std::random_access_iterator_tag, _IteratorTypes...>;
+#endif

 struct __serial_backend_tag
 {



This uses the libstdc++ helper __iter_category_t but since we no longer nee=
d to
sync with upstream, that seems fine.=