[libcxx-commits] [libcxx] [libc++][pstl] Introduce a "statpart" backend (PR #230540)

[Louis Dionne via libcxx-commits]

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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___PSTL_STATPART_H
+#define _LIBCPP___PSTL_STATPART_H
+
+#include <__algorithm/find_end.h>
+#include <__algorithm/find_if.h>
+#include <__algorithm/for_each.h>
+#include <__algorithm/is_heap_until.h>
+#include <__algorithm/lower_bound.h>
+#include <__algorithm/merge.h>
+#include <__algorithm/min.h>
+#include <__algorithm/min_element.h>
+#include <__algorithm/minmax_element.h>
+#include <__algorithm/mismatch.h>
+#include <__algorithm/move.h>
+#include <__algorithm/reverse.h>
+#include <__algorithm/search.h>
+#include <__algorithm/search_n.h>
+#include <__algorithm/stable_sort.h>
+#include <__algorithm/swap_ranges.h>
+#include <__algorithm/transform.h>
+#include <__algorithm/upper_bound.h>
+#include <__atomic/atomic.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/move_iterator.h>
+#include <__iterator/next.h>
+#include <__iterator/reverse_iterator.h>
+#include <__memory/allocator.h>
+#include <__memory/construct_at.h>
+#include <__memory/destroy.h>
+#include <__memory/uninitialized_algorithms.h>
+#include <__memory/unique_ptr.h>
+#include <__numeric/reduce.h>
+#include <__numeric/transform_reduce.h>
+#include <__optional/nullopt_t.h>
+#include <__optional/optional.h>
+#include <__pstl/backend_fwd.h>
+#include <__pstl/cpu_algos/cpu_traits.h>
+#include <__pstl/cpu_algos/find_if.h>
+#include <__thread/thread.h>
+#include <__type_traits/is_assignable.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_execution_policy.h>
+#include <__type_traits/is_trivially_copyable.h>
+#include <__utility/convert_to_integral.h>
+#include <__utility/empty.h>
+#include <__utility/move.h>
+#include <__utility/pair.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+_LIBCPP_BEGIN_EXPLICIT_ABI_ANNOTATIONS
+namespace __pstl {
+namespace __statpart {
+
+// The backbone of the backend: applies the function to each of the indices [0, __chunk_count] in parallel.
+// The actual number of parallel workers and the order of application is unspecified.
+_LIBCPP_EXPORTED_FROM_ABI void
+__apply(size_t __chunk_count, void* __context, void (*__func)(void* __context, size_t __chunk)) noexcept;
+
+// The maximum number of partitions to be processed per each CPU
+constexpr inline size_t __max_partitions_per_cpu = 8;
+
+// The minimum number of elements to sort serially in a single partition
+constexpr inline size_t __min_sort_partition_size = 256;
+
+enum class __partitioning_result {
+  // returned when the partitioning result is not nullopt
+  __ok,
+  // returned when there is not enough element to produce a single partition or the final number of partitions is less
+  // than 2, i.e. there's no reason to process in parallel
+  __not_enough_elements,
+  // later: returned memory couldn't be allocated for the stashed forward iterators
+  __bad_alloc
+};
+
+template <class _Iterator>
+struct _LIBCPP_HIDE_FROM_ABI __iterator_range {
+  _Iterator __first;
+  _Iterator __last;
+};
+
+// __iterator_partitions<Iter> holds a finite per-determined set of partitions and allows to query each iterator range
----------------
ldionne wrote:

```suggestion
// __iterator_partitions<Iter> holds a finite pre-determined set of partitions and allows to query each iterator range
```

https://github.com/llvm/llvm-project/pull/230540