libcxx/include/__pstl/backends/libdispatch.h

//===----------------------------------------------------------------------===//
//
// 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_BACKENDS_LIBDISPATCH_H
#define _LIBCPP___PSTL_BACKENDS_LIBDISPATCH_H

#include <__algorithm/inplace_merge.h>
#include <__algorithm/lower_bound.h>
#include <__algorithm/max.h>
#include <__algorithm/merge.h>
#include <__algorithm/upper_bound.h>
#include <__atomic/atomic.h>
#include <__config>
#include <__exception/terminate.h>
#include <__iterator/iterator_traits.h>
#include <__iterator/move_iterator.h>
#include <__memory/allocator.h>
#include <__memory/construct_at.h>
#include <__memory/unique_ptr.h>
#include <__numeric/reduce.h>
#include <__pstl/configuration_fwd.h>
#include <__pstl/cpu_algos/cpu_traits.h>
#include <__utility/empty.h>
#include <__utility/exception_guard.h>
#include <__utility/move.h>
#include <__utility/pair.h>
#include <cstddef>
#include <new>
#include <optional>

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17

_LIBCPP_BEGIN_NAMESPACE_STD
namespace __pstl {

namespace __libdispatch {
// ::dispatch_apply is marked as __attribute__((nothrow)) because it doesn't let exceptions propagate, and neither do
// we.
// TODO: Do we want to add [[_Clang::__callback__(__func, __context, __)]]?
_LIBCPP_EXPORTED_FROM_ABI void
__dispatch_apply(size_t __chunk_count, void* __context, void (*__func)(void* __context, size_t __chunk)) noexcept;

template <class _Func>
_LIBCPP_HIDE_FROM_ABI void __dispatch_apply(size_t __chunk_count, _Func __func) noexcept {
  __libdispatch::__dispatch_apply(__chunk_count, &__func, [](void* __context, size_t __chunk) {
    (*static_cast<_Func*>(__context))(__chunk);
  });
}

struct __chunk_partitions {
  ptrdiff_t __chunk_count_; // includes the first chunk
  ptrdiff_t __chunk_size_;
  ptrdiff_t __first_chunk_size_;
};

[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI __chunk_partitions __partition_chunks(ptrdiff_t __size) noexcept;

template <class _RandomAccessIterator, class _Functor>
_LIBCPP_HIDE_FROM_ABI optional<__empty>
__dispatch_parallel_for(__chunk_partitions __partitions, _RandomAccessIterator __first, _Functor __func) {
  // Perform the chunked execution.
  __libdispatch::__dispatch_apply(__partitions.__chunk_count_, [&](size_t __chunk) {
    auto __this_chunk_size = __chunk == 0 ? __partitions.__first_chunk_size_ : __partitions.__chunk_size_;
    auto __index =
        __chunk == 0
            ? 0
            : (__chunk * __partitions.__chunk_size_) + (__partitions.__first_chunk_size_ - __partitions.__chunk_size_);
    __func(__first + __index, __first + __index + __this_chunk_size);
  });

  return __empty{};
}
} // namespace __libdispatch

template <>
struct __cpu_traits<__libdispatch_backend_tag> {
  template <class _RandomAccessIterator, class _Functor>
  _LIBCPP_HIDE_FROM_ABI static optional<__empty>
  __for_each(_RandomAccessIterator __first, _RandomAccessIterator __last, _Functor __func) {
    return __libdispatch::__dispatch_parallel_for(
        __libdispatch::__partition_chunks(__last - __first), std::move(__first), std::move(__func));
  }

  template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _RandomAccessIteratorOut>
  struct __merge_range {
    __merge_range(_RandomAccessIterator1 __mid1, _RandomAccessIterator2 __mid2, _RandomAccessIteratorOut __result)
        : __mid1_(__mid1), __mid2_(__mid2), __result_(__result) {}

    _RandomAccessIterator1 __mid1_;
    _RandomAccessIterator2 __mid2_;
    _RandomAccessIteratorOut __result_;
  };

  template <typename _RandomAccessIterator1,
            typename _RandomAccessIterator2,
            typename _RandomAccessIterator3,
            typename _Compare,
            typename _LeafMerge>
  _LIBCPP_HIDE_FROM_ABI static optional<__empty>
  __merge(_RandomAccessIterator1 __first1,
          _RandomAccessIterator1 __last1,
          _RandomAccessIterator2 __first2,
          _RandomAccessIterator2 __last2,
          _RandomAccessIterator3 __result,
          _Compare __comp,
          _LeafMerge __leaf_merge) noexcept {
    __libdispatch::__chunk_partitions __partitions =
        __libdispatch::__partition_chunks(std::max<ptrdiff_t>(__last1 - __first1, __last2 - __first2));

    if (__partitions.__chunk_count_ == 0)
      return __empty{};

    if (__partitions.__chunk_count_ == 1) {
      __leaf_merge(__first1, __last1, __first2, __last2, __result, __comp);
      return __empty{};
    }

    using __merge_range_t = __merge_range<_RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator3>;
    auto const __n_ranges = __partitions.__chunk_count_ + 1;

    // TODO: use __uninitialized_buffer
    auto __destroy = [=](__merge_range_t* __ptr) {
      std::destroy_n(__ptr, __n_ranges);
      std::allocator<__merge_range_t>().deallocate(__ptr, __n_ranges);
    };

    unique_ptr<__merge_range_t[], decltype(__destroy)> __ranges(
        [&]() -> __merge_range_t* {
#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
          try {
#  endif
            return std::allocator<__merge_range_t>().allocate(__n_ranges);
#  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
          } catch (const std::bad_alloc&) {
            return nullptr;
          }
#  endif
        }(),
        __destroy);

    if (!__ranges)
      return nullopt;

    // TODO: Improve the case where the smaller range is merged into just a few (or even one) chunks of the larger case
    __merge_range_t* __r = __ranges.get();
    std::__construct_at(__r++, __first1, __first2, __result);

    bool __iterate_first_range = __last1 - __first1 > __last2 - __first2;

    auto __compute_chunk = [&](size_t __chunk_size) -> __merge_range_t {
      auto [__mid1, __mid2] = [&] {
        if (__iterate_first_range) {
          auto __m1 = __first1 + __chunk_size;
          auto __m2 = std::lower_bound(__first2, __last2, __m1[-1], __comp);
          return std::make_pair(__m1, __m2);
        } else {
          auto __m2 = __first2 + __chunk_size;
          auto __m1 = std::lower_bound(__first1, __last1, __m2[-1], __comp);
          return std::make_pair(__m1, __m2);
        }
      }();

      __result += (__mid1 - __first1) + (__mid2 - __first2);
      __first1 = __mid1;
      __first2 = __mid2;
      return {std::move(__mid1), std::move(__mid2), __result};
    };

    // handle first chunk
    std::__construct_at(__r++, __compute_chunk(__partitions.__first_chunk_size_));

    // handle 2 -> N - 1 chunks
    for (ptrdiff_t __i = 0; __i != __partitions.__chunk_count_ - 2; ++__i)
      std::__construct_at(__r++, __compute_chunk(__partitions.__chunk_size_));

    // handle last chunk
    std::__construct_at(__r, __last1, __last2, __result);

    __libdispatch::__dispatch_apply(__partitions.__chunk_count_, [&](size_t __index) {
      auto __first_iters = __ranges[__index];
      auto __last_iters  = __ranges[__index + 1];
      __leaf_merge(
          __first_iters.__mid1_,
          __last_iters.__mid1_,
          __first_iters.__mid2_,
          __last_iters.__mid2_,
          __first_iters.__result_,
          __comp);
    });

    return __empty{};
  }

  template <class _RandomAccessIterator, class _Transform, class _Value, class _Combiner, class _Reduction>
  _LIBCPP_HIDE_FROM_ABI static optional<_Value> __transform_reduce(
      _RandomAccessIterator __first,
      _RandomAccessIterator __last,
      _Transform __transform,
      _Value __init,
      _Combiner __combiner,
      _Reduction __reduction) {
    if (__first == __last)
      return __init;

    auto __partitions = __libdispatch::__partition_chunks(__last - __first);

    auto __destroy = [__count = __partitions.__chunk_count_](_Value* __ptr) {
      std::destroy_n(__ptr, __count);
      std::allocator<_Value>().deallocate(__ptr, __count);
    };

    // TODO: use __uninitialized_buffer
    // TODO: allocate one element per worker instead of one element per chunk
    unique_ptr<_Value[], decltype(__destroy)> __values(
        std::allocator<_Value>().allocate(__partitions.__chunk_count_), __destroy);

    // __dispatch_apply is noexcept
    __libdispatch::__dispatch_apply(__partitions.__chunk_count_, [&](size_t __chunk) {
      auto __this_chunk_size = __chunk == 0 ? __partitions.__first_chunk_size_ : __partitions.__chunk_size_;
      auto __index           = __chunk == 0 ? 0
                                            : (__chunk * __partitions.__chunk_size_) +
                                        (__partitions.__first_chunk_size_ - __partitions.__chunk_size_);
      if (__this_chunk_size != 1) {
        std::__construct_at(
            __values.get() + __chunk,
            __reduction(__first + __index + 2,
                        __first + __index + __this_chunk_size,
                        __combiner(__transform(__first + __index), __transform(__first + __index + 1))));
      } else {
        std::__construct_at(__values.get() + __chunk, __transform(__first + __index));
      }
    });

    return std::reduce(
        std::make_move_iterator(__values.get()),
        std::make_move_iterator(__values.get() + __partitions.__chunk_count_),
        std::move(__init),
        __combiner);
  }

  template <class _RandomAccessIterator, class _Comp, class _LeafSort>
  _LIBCPP_HIDE_FROM_ABI static optional<__empty>
  __stable_sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Comp __comp, _LeafSort __leaf_sort) {
    const auto __size = __last - __first;
    auto __partitions = __libdispatch::__partition_chunks(__size);

    if (__partitions.__chunk_count_ == 0)
      return __empty{};

    if (__partitions.__chunk_count_ == 1) {
      __leaf_sort(__first, __last, __comp);
      return __empty{};
    }

    using _Value = __iter_value_type<_RandomAccessIterator>;

    auto __destroy = [__size](_Value* __ptr) {
      std::destroy_n(__ptr, __size);
      std::allocator<_Value>().deallocate(__ptr, __size);
    };

    // TODO: use __uninitialized_buffer
    unique_ptr<_Value[], decltype(__destroy)> __values(std::allocator<_Value>().allocate(__size), __destroy);

    // Initialize all elements to a moved-from state
    // TODO: Don't do this - this can be done in the first merge - see https://llvm.org/PR63928
    std::__construct_at(__values.get(), std::move(*__first));
    for (__iter_diff_t<_RandomAccessIterator> __i = 1; __i != __size; ++__i) {
      std::__construct_at(__values.get() + __i, std::move(__values.get()[__i - 1]));
    }
    *__first = std::move(__values.get()[__size - 1]);

    __libdispatch::__dispatch_parallel_for(
        __partitions,
        __first,
        [&__leaf_sort, &__comp](_RandomAccessIterator __chunk_first, _RandomAccessIterator __chunk_last) {
          __leaf_sort(std::move(__chunk_first), std::move(__chunk_last), __comp);
        });

    bool __objects_are_in_buffer = false;
    do {
      const auto __old_chunk_size = __partitions.__chunk_size_;
      if (__partitions.__chunk_count_ % 2 == 1) {
        auto __inplace_merge_chunks = [&__comp, &__partitions](auto __first_chunk_begin) {
          std::inplace_merge(
              __first_chunk_begin,
              __first_chunk_begin + __partitions.__first_chunk_size_,
              __first_chunk_begin + __partitions.__first_chunk_size_ + __partitions.__chunk_size_,
              __comp);
        };
        if (__objects_are_in_buffer)
          __inplace_merge_chunks(__values.get());
        else
          __inplace_merge_chunks(__first);
        __partitions.__first_chunk_size_ += 2 * __partitions.__chunk_size_;
      } else {
        __partitions.__first_chunk_size_ += __partitions.__chunk_size_;
      }

      __partitions.__chunk_size_ *= 2;
      __partitions.__chunk_count_ /= 2;

      auto __merge_chunks = [__partitions, __old_chunk_size, &__comp](auto __from_first, auto __to_first) {
        __libdispatch::__dispatch_parallel_for(
            __partitions,
            __from_first,
            [__old_chunk_size, &__from_first, &__to_first, &__comp](auto __chunk_first, auto __chunk_last) {
              std::merge(std::make_move_iterator(__chunk_first),
                         std::make_move_iterator(__chunk_last - __old_chunk_size),
                         std::make_move_iterator(__chunk_last - __old_chunk_size),
                         std::make_move_iterator(__chunk_last),
                         __to_first + (__chunk_first - __from_first),
                         __comp);
            });
      };

      if (__objects_are_in_buffer)
        __merge_chunks(__values.get(), __first);
      else
        __merge_chunks(__first, __values.get());
      __objects_are_in_buffer = !__objects_are_in_buffer;
    } while (__partitions.__chunk_count_ > 1);

    if (__objects_are_in_buffer) {
      std::move(__values.get(), __values.get() + __size, __first);
    }

    return __empty{};
  }

  _LIBCPP_HIDE_FROM_ABI static void __cancel_execution() {}

  static constexpr size_t __lane_size = 64;
};

} // namespace __pstl
_LIBCPP_END_NAMESPACE_STD

#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17

_LIBCPP_POP_MACROS

// Implement PSTL algorithms based on the __cpu_traits specialized above
#include <__pstl/cpu_algos/any_of.h>
#include <__pstl/cpu_algos/fill.h>
#include <__pstl/cpu_algos/find_if.h>
#include <__pstl/cpu_algos/for_each.h>
#include <__pstl/cpu_algos/merge.h>
#include <__pstl/cpu_algos/stable_sort.h>
#include <__pstl/cpu_algos/transform.h>
#include <__pstl/cpu_algos/transform_reduce.h>

#endif // _LIBCPP___PSTL_BACKENDS_LIBDISPATCH_H