On Thu, 11 Jan 2024, Jonathan Wakely wrote:
> Tested x86_64-linux and aarch64-linux, with TBB 2020.3 only.
>
> Reviews requested.
>
> -- >8 --
>
> This is a step towards implementing the C++23 change P2408R5, "Ranges
> iterators as inputs to non-Ranges algorithms". C++20 random access
> iterators which do not meet the C==17RandomAccessIterator requirements
> will now be recognized by the PSTL algorithms.
IIUC P2408R5 only relaxes the iterator requirements on non-mutating
algorithms, but presumably this patch relaxes the requirements for all
parallel algorithms? Perhaps that's safe here, not sure..
Besides that LGTM.
>
> We can also optimize the C++17 implementation by using std::__or_, and
> use std::__remove_cvref_t and std::__iter_category_t for readability.
> This diverges from the upstream PSTL, but since libc++ is no longer
> using that upstream (so we're the only consumer of this code) I think
> it's reasonable to use libstdc++ extensions in localized places like
> this. Rebasing this small header on upstream should not be difficult.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/110512
> * include/pstl/execution_impl.h (__are_random_access_iterators):
> Recognize C++20 random access iterators, and use more efficient
> implementations.
> * testsuite/25_algorithms/pstl/110512.cc: New test.
> ---
> libstdc++-v3/include/pstl/execution_impl.h | 21 ++++++++++---
> .../testsuite/25_algorithms/pstl/110512.cc | 31 +++++++++++++++++++
> 2 files changed, 47 insertions(+), 5 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/25_algorithms/pstl/110512.cc
>
> diff --git a/libstdc++-v3/include/pstl/execution_impl.h
> b/libstdc++-v3/include/pstl/execution_impl.h
> index 64f6cc4357a..c84061848b9 100644
> --- a/libstdc++-v3/include/pstl/execution_impl.h
> +++ b/libstdc++-v3/include/pstl/execution_impl.h
> @@ -19,13 +19,24 @@ namespace __pstl
> {
> namespace __internal
> {
> -
> -template <typename _IteratorTag, typename... _IteratorTypes>
> -using __are_iterators_of = std::conjunction<
> - std::is_base_of<_IteratorTag, typename
> std::iterator_traits<std::decay_t<_IteratorTypes>>::iterator_category>...>;
> +#if __glibcxx_concepts
> +template<typename _Iter>
> + concept __is_random_access_iter
> + = 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 =
> __are_iterators_of<std::random_access_iterator_tag, _IteratorTypes...>;
> + using __are_random_access_iterators
> + =
> std::bool_constant<(__is_random_access_iter<std::remove_cvref_t<_IteratorTypes>>
> && ...)>;
> +#else
> +template <typename... _IteratorTypes>
> +using __are_random_access_iterators
> + = std::__and_<
> + std::is_base_of<std::random_access_iterator_tag,
> +
> std::__iter_category_t<std::__remove_cvref_t<_IteratorTypes>>>...
> + >;
> +#endif
>
> struct __serial_backend_tag
> {
> diff --git a/libstdc++-v3/testsuite/25_algorithms/pstl/110512.cc
> b/libstdc++-v3/testsuite/25_algorithms/pstl/110512.cc
> new file mode 100644
> index 00000000000..188c7c915e5
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/pstl/110512.cc
> @@ -0,0 +1,31 @@
> +// { dg-do compile { target c++17 } }
> +
> +// Bug 110512 - C++20 random access iterators run sequentially with PSTL
> +
> +#include <algorithm>
> +#include <execution>
> +#include <ranges>
> +#include <testsuite_iterators.h>
> +
> +using InputIter = __gnu_test::input_iterator_wrapper<int>;
> +using FwdIter = __gnu_test::forward_iterator_wrapper<long>;
> +using RAIter = __gnu_test::random_access_iterator_wrapper<float>;
> +
> +template<typename... Iter>
> +constexpr bool all_random_access
> + = __pstl::__internal::__are_random_access_iterators<Iter...>::value;
> +
> +using __pstl::__internal::__are_random_access_iterators;
> +static_assert( all_random_access<RAIter> );
> +static_assert( all_random_access<int*, RAIter, const long*> );
> +static_assert( ! all_random_access<RAIter, FwdIter> );
> +static_assert( ! all_random_access<FwdIter, InputIter, RAIter> );
> +
> +#if __cpp_lib_ranges
> +using IotaIter = std::ranges::iterator_t<std::ranges::iota_view<int, int>>;
> +static_assert( std::random_access_iterator<IotaIter> );
> +static_assert( all_random_access<IotaIter> );
> +static_assert( all_random_access<IotaIter, RAIter> );
> +static_assert( all_random_access<RAIter, IotaIter> );
> +static_assert( ! all_random_access<RAIter, IotaIter, FwdIter> );
> +#endif
> --
> 2.43.0
>
>
