[clang-tools-extra] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Louis Dionne via cfe-commits]

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@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_FIND_IF_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_OPENMP_BACKEND_FIND_IF_H
+
+#include <__algorithm/find_if.h>
+#include <__algorithm/pstl_backends/cpu_backends/backend.h>
+#include <__algorithm/pstl_backends/openmp/backend.h>
+#include <__config>
+#include <optional>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace __par_backend {
+inline namespace __omp_gpu_backend {
+
+template <class _Tp, class _DifferenceType, class _Predicate>
+_LIBCPP_HIDE_FROM_ABI _DifferenceType
+__omp_parallel_for_min_idx(_Tp* __first, _DifferenceType __n, _Predicate __pred) noexcept {
+  __omp_gpu_backend::__omp_map_to(__first, __n);
+  _DifferenceType idx = __n;
+#  pragma omp target teams distribute parallel for simd reduction(min : idx)
+  for (_DifferenceType __i = 0; __i < __n; ++__i) {
+    if (__pred(*(__first + __i))) {
+      idx = (__i < idx) ? __i : idx;
+    }
+  }
+  __omp_gpu_backend::__omp_map_free(__first, __n);
+  return idx;
+}
+
+// Extracting the underlying pointer
+
+template <class _ForwardIterator, class _DifferenceType, class _Predicate>
+_LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
+__parallel_for_min_idx(_ForwardIterator __first, _DifferenceType __n, _Predicate __pred) noexcept {
+  return __first +
+         __omp_gpu_backend::__omp_parallel_for_min_idx(__omp_gpu_backend::__omp_extract_base_ptr(__first), __n, __pred);
+}
+
+} // namespace __omp_gpu_backend
+} // namespace __par_backend
+
+template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
+_LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
+__pstl_find_if(__omp_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred) {
+  if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
+                __is_parallel_execution_policy_v<_ExecutionPolicy> &&
+                __libcpp_is_contiguous_iterator<_ForwardIterator>::value) {
+    return __par_backend::__parallel_for_min_idx(__first, __last - __first, __pred);
----------------
ldionne wrote:

```suggestion
    return std::__rewrap_iter(__first, __par_backend::__omp_parallel_for_min_idx(std::__unwrap_iter(__first), __last - __first, __pred));
```

And then you can get rid of `__parallel_for_min_idx` since it's such a small function.

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