libcxx/include/__algorithm/pstl_backends/cpu_backends/find_if.h

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #ifndef _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_FIND_IF_H
  10 #define _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_FIND_IF_H
  11
  12 #include <__algorithm/find_if.h>
  13 #include <__algorithm/pstl_backends/cpu_backends/backend.h>
  14 #include <__atomic/atomic.h>
  15 #include <__config>
  16 #include <__functional/operations.h>
  17 #include <__iterator/concepts.h>
  18 #include <__iterator/iterator_traits.h>
  19 #include <__type_traits/is_execution_policy.h>
  20 #include <__utility/move.h>
  21 #include <__utility/pair.h>
  22 #include <cstddef>
  23 #include <optional>
  24
  25 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  26 #  pragma GCC system_header
  27 #endif
  28
  29 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
  30
  31 _LIBCPP_PUSH_MACROS
  32 #  include <__undef_macros>
  33
  34 _LIBCPP_BEGIN_NAMESPACE_STD
  35
  36 template <class _Index, class _Brick, class _Compare>
  37 _LIBCPP_HIDE_FROM_ABI optional<_Index>
  38 __parallel_find(_Index __first, _Index __last, _Brick __f, _Compare __comp, bool __b_first) {
  39   typedef typename std::iterator_traits<_Index>::difference_type _DifferenceType;
  40   const _DifferenceType __n      = __last - __first;
  41   _DifferenceType __initial_dist = __b_first ? __n : -1;
  42   std::atomic<_DifferenceType> __extremum(__initial_dist);
  43   // TODO: find out what is better here: parallel_for or parallel_reduce
  44   auto __res =
  45       __par_backend::__parallel_for(__first, __last, [__comp, __f, __first, &__extremum](_Index __i, _Index __j) {
  46         // See "Reducing Contention Through Priority Updates", PPoPP '13, for discussion of
  47         // why using a shared variable scales fairly well in this situation.
  48         if (__comp(__i - __first, __extremum)) {
  49           _Index __result = __f(__i, __j);
  50           // If not '__last' returned then we found what we want so put this to extremum
  51           if (__result != __j) {
  52             const _DifferenceType __k = __result - __first;
  53             for (_DifferenceType __old = __extremum; __comp(__k, __old); __old = __extremum) {
  54               __extremum.compare_exchange_weak(__old, __k);
  55             }
  56           }
  57         }
  58       });
  59   if (!__res)
  60     return nullopt;
  61   return __extremum.load() != __initial_dist ? __first + __extremum.load() : __last;
  62 }
  63
  64 template <class _Index, class _DifferenceType, class _Compare>
  65 _LIBCPP_HIDE_FROM_ABI _Index
  66 __simd_first(_Index __first, _DifferenceType __begin, _DifferenceType __end, _Compare __comp) noexcept {
  67   // Experiments show good block sizes like this
  68   const _DifferenceType __block_size                        = 8;
  69   alignas(__lane_size) _DifferenceType __lane[__block_size] = {0};
  70   while (__end - __begin >= __block_size) {
  71     _DifferenceType __found = 0;
  72     _PSTL_PRAGMA_SIMD_REDUCTION(| : __found) for (_DifferenceType __i = __begin; __i < __begin + __block_size; ++__i) {
  73       const _DifferenceType __t = __comp(__first, __i);
  74       __lane[__i - __begin]     = __t;
  75       __found |= __t;
  76     }
  77     if (__found) {
  78       _DifferenceType __i;
  79       // This will vectorize
  80       for (__i = 0; __i < __block_size; ++__i) {
  81         if (__lane[__i]) {
  82           break;
  83         }
  84       }
  85       return __first + __begin + __i;
  86     }
  87     __begin += __block_size;
  88   }
  89
  90   // Keep remainder scalar
  91   while (__begin != __end) {
  92     if (__comp(__first, __begin)) {
  93       return __first + __begin;
  94     }
  95     ++__begin;
  96   }
  97   return __first + __end;
  98 }
  99
 100 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 101 _LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
 102 __pstl_find_if(__cpu_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred) {
 103   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
 104                 __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
 105     return std::__parallel_find(
 106         __first,
 107         __last,
 108         [&__pred](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
 109           auto __res = std::__pstl_find_if<__remove_parallel_policy_t<_ExecutionPolicy>>(
 110               __cpu_backend_tag{}, __brick_first, __brick_last, __pred);
 111           _LIBCPP_ASSERT_INTERNAL(__res, "unseq/seq should never try to allocate!");
 112           return *std::move(__res);
 113         },
 114         less<>{},
 115         true);
 116   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
 117                        __has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
 118     using __diff_t = __iter_diff_t<_ForwardIterator>;
 119     return std::__simd_first(__first, __diff_t(0), __last - __first, [&__pred](_ForwardIterator __iter, __diff_t __i) {
 120       return __pred(__iter[__i]);
 121     });
 122   } else {
 123     return std::find_if(__first, __last, __pred);
 124   }
 125 }
 126
 127 _LIBCPP_END_NAMESPACE_STD
 128
 129 _LIBCPP_POP_MACROS
 130
 131 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 132
 133 #endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_CPU_BACKENDS_FIND_IF_H