llvm-project

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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include <barrier>
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#include <thread>
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_LIBCPP_BEGIN_NAMESPACE_STD
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#if !defined(_LIBCPP_HAS_NO_TREE_BARRIER)
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class __barrier_algorithm_base {
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public:
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  struct alignas(64) /* naturally-align the heap state */ __state_t {
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    struct {
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      __atomic_base<__barrier_phase_t> __phase{0};
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    } __tickets[64];
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  };
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  ptrdiff_t& __expected_;
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  unique_ptr<__state_t[]> __state_;
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  _LIBCPP_HIDDEN __barrier_algorithm_base(ptrdiff_t& __expected) : __expected_(__expected) {
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    size_t const __count = (__expected + 1) >> 1;
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    __state_             = unique_ptr<__state_t[]>(new __state_t[__count]);
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  }
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  _LIBCPP_HIDDEN bool __arrive(__barrier_phase_t __old_phase) {
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    __barrier_phase_t const __half_step = __old_phase + 1, __full_step = __old_phase + 2;
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    size_t __current_expected = __expected_,
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           __current          = hash<thread::id>()(this_thread::get_id()) % ((__expected_ + 1) >> 1);
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    for (int __round = 0;; ++__round) {
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      if (__current_expected <= 1)
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        return true;
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      size_t const __end_node = ((__current_expected + 1) >> 1), __last_node = __end_node - 1;
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      for (;; ++__current) {
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        if (__current == __end_node)
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          __current = 0;
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        __barrier_phase_t expect = __old_phase;
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        if (__current == __last_node && (__current_expected & 1)) {
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          if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
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                  expect, __full_step, memory_order_acq_rel))
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            break; // I'm 1 in 1, go to next __round
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        } else if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
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                       expect, __half_step, memory_order_acq_rel)) {
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          return false; // I'm 1 in 2, done with arrival
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        } else if (expect == __half_step) {
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          if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
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                  expect, __full_step, memory_order_acq_rel))
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            break; // I'm 2 in 2, go to next __round
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        }
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      }
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      __current_expected = __last_node + 1;
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      __current >>= 1;
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    }
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  }
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};
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_LIBCPP_EXPORTED_FROM_ABI __barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected) {
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  return new __barrier_algorithm_base(__expected);
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}
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_LIBCPP_EXPORTED_FROM_ABI bool
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__arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier, __barrier_phase_t __old_phase) noexcept {
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  return __barrier->__arrive(__old_phase);
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}
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_LIBCPP_EXPORTED_FROM_ABI void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier) noexcept {
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  delete __barrier;
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}
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#endif // !defined(_LIBCPP_HAS_NO_TREE_BARRIER)
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_LIBCPP_END_NAMESPACE_STD
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