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#include <barrier>
10#include <thread>
11
12_LIBCPP_BEGIN_NAMESPACE_STD
13
14class __barrier_algorithm_base {
15public:
16  struct alignas(64) /* naturally-align the heap state */ __state_t {
17    struct {
18      atomic<__barrier_phase_t> __phase{0};
19    } __tickets[64];
20  };
21
22  ptrdiff_t& __expected_;
23  unique_ptr<__state_t[]> __state_;
24
25  _LIBCPP_HIDDEN __barrier_algorithm_base(ptrdiff_t& __expected) : __expected_(__expected) {
26    size_t const __count = (__expected + 1) >> 1;
27    __state_             = unique_ptr<__state_t[]>(new __state_t[__count]);
28  }
29  _LIBCPP_HIDDEN bool __arrive(__barrier_phase_t __old_phase) {
30    __barrier_phase_t const __half_step = __old_phase + 1, __full_step = __old_phase + 2;
31    size_t __current_expected = __expected_,
32           __current          = hash<thread::id>()(this_thread::get_id()) % ((__expected_ + 1) >> 1);
33    for (int __round = 0;; ++__round) {
34      if (__current_expected <= 1)
35        return true;
36      size_t const __end_node = ((__current_expected + 1) >> 1), __last_node = __end_node - 1;
37      for (;; ++__current) {
38        if (__current == __end_node)
39          __current = 0;
40        __barrier_phase_t expect = __old_phase;
41        if (__current == __last_node && (__current_expected & 1)) {
42          if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
43                  expect, __full_step, memory_order_acq_rel))
44            break; // I'm 1 in 1, go to next __round
45        } else if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
46                       expect, __half_step, memory_order_acq_rel)) {
47          return false; // I'm 1 in 2, done with arrival
48        } else if (expect == __half_step) {
49          if (__state_[__current].__tickets[__round].__phase.compare_exchange_strong(
50                  expect, __full_step, memory_order_acq_rel))
51            break; // I'm 2 in 2, go to next __round
52        }
53      }
54      __current_expected = __last_node + 1;
55      __current >>= 1;
56    }
57  }
58};
59
60_LIBCPP_EXPORTED_FROM_ABI __barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected) {
61  return new __barrier_algorithm_base(__expected);
62}
63_LIBCPP_EXPORTED_FROM_ABI bool
64__arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier, __barrier_phase_t __old_phase) noexcept {
65  return __barrier->__arrive(__old_phase);
66}
67_LIBCPP_EXPORTED_FROM_ABI void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier) noexcept {
68  delete __barrier;
69}
70
71_LIBCPP_END_NAMESPACE_STD