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 e&: expect, d: __full_step, m: 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 e&: expect, d: __half_step, m: 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 e&: expect, d: __full_step, m: 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
72