[llvm] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Louis Dionne via llvm-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___ALGORITHM_PSTL_BACKENDS_OPENMP_BACKEND_OMP_OFFLOAD_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_OPENMP_BACKEND_OMP_OFFLOAD_H
+
+#include <__assert>
+#include <__config>
+#include <__functional/operations.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/wrap_iter.h>
+#include <__memory/addressof.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/is_pointer.h>
+#include <__type_traits/is_same.h>
+#include <__utility/move.h>
+#include <cstddef>
+
+// is_same
+
+// __libcpp_is_contiguous_iterator
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace __par_backend {
+inline namespace __omp_gpu_backend {
+
+// Functions for eaxtracting the pase pointers
+
+// In the general case we do not need to extract it. This is for instance the
+// case for pointers.
+template <typename _Tp>
+_LIBCPP_HIDE_FROM_ABI inline auto __omp_extract_base_ptr(_Tp p) noexcept {
+  return p;
+}
+
+// For vectors and arrays, etc, we need to extract the underlying base pointer.
+template <typename _Tp>
+_LIBCPP_HIDE_FROM_ABI inline _Tp __omp_extract_base_ptr(std::__wrap_iter<_Tp> w) noexcept {
+  std::pointer_traits<std::__wrap_iter<_Tp>> PT;
+  return PT.to_address(w);
+}
+
+//===----------------------------------------------------------------------===//
+// The following four functions differentiates between contiguous iterators and
+// non-contiguous iterators. That allows to use the same implementations for
+// reference and value iterators
+//===----------------------------------------------------------------------===//
+
+template <class _Iterator, class _DifferenceType>
+_LIBCPP_HIDE_FROM_ABI void
+__omp_map_to([[maybe_unused]] const _Iterator p, [[maybe_unused]] const _DifferenceType len) noexcept {
+  if constexpr (__libcpp_is_contiguous_iterator<_Iterator>::value) {
+#  pragma omp target enter data map(to : p[0 : len])
+  } else {
+#  pragma omp target enter data map(to : p)
+  }
+}
+
+template <class _Iterator, class _DifferenceType>
+_LIBCPP_HIDE_FROM_ABI void
+__omp_map_from([[maybe_unused]] const _Iterator p, [[maybe_unused]] const _DifferenceType len) noexcept {
+  if constexpr (__libcpp_is_contiguous_iterator<_Iterator>::value) {
+#  pragma omp target exit data map(from : p[0 : len])
+  } else {
+#  pragma omp target exit data map(release : p)
+  }
+}
+
+template <class _Iterator, class _DifferenceType>
+_LIBCPP_HIDE_FROM_ABI void
+__omp_map_alloc([[maybe_unused]] const _Iterator p, [[maybe_unused]] const _DifferenceType len) noexcept {
+  if constexpr (__libcpp_is_contiguous_iterator<_Iterator>::value) {
+#  pragma omp target enter data map(alloc : p[0 : len])
+  } else {
+#  pragma omp target enter data map(to : p)
+  }
+}
+
+template <class _Iterator, class _DifferenceType>
+_LIBCPP_HIDE_FROM_ABI void
+__omp_map_free([[maybe_unused]] const _Iterator p, [[maybe_unused]] const _DifferenceType len) noexcept {
+  if constexpr (__libcpp_is_contiguous_iterator<_Iterator>::value) {
+#  pragma omp target exit data map(release : p[0 : len])
+  } else {
+#  pragma omp target exit data map(release : p)
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// Templates for one iterator
+//===----------------------------------------------------------------------===//
+
+// Applying function or lambda in a loop
+
+template <class _Iterator, class _DifferenceType, class _Function>
+_LIBCPP_HIDE_FROM_ABI _Iterator __omp_parallel_for_simd_1(
+    _Iterator __first, _DifferenceType __n, _Function __f, [[maybe_unused]] const int __device = 0) noexcept {
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ldionne wrote:

`__device` is always 0 right now, so I would remove it.

https://github.com/llvm/llvm-project/pull/66968