1//===------------------------- barrier.cpp ---------------------------------===//
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 <__config>
10
11#ifndef _LIBCPP_HAS_NO_THREADS
12
13#include <barrier>
14#include <thread>
15
16_LIBCPP_BEGIN_NAMESPACE_STD
17
18#if !defined(_LIBCPP_HAS_NO_TREE_BARRIER) && (_LIBCPP_STD_VER > 11)
19
20class __barrier_algorithm_base {
21public:
22    struct alignas(64) /* naturally-align the heap state */ __state_t
23    {
24        struct {
25            __atomic_base<__barrier_phase_t> __phase = ATOMIC_VAR_INIT(0);
26        } __tickets[64];
27    };
28
29    ptrdiff_t&         __expected;
30    unique_ptr<char[]> __state_allocation;
31    __state_t*         __state;
32
33    _LIBCPP_HIDDEN
34    __barrier_algorithm_base(ptrdiff_t& __expected)
35        : __expected(__expected)
36    {
37        size_t const __count = (__expected + 1) >> 1;
38        size_t const __size = sizeof(__state_t) * __count;
39        size_t __allocation_size = __size + alignof(__state_t);
40        __state_allocation = unique_ptr<char[]>(new char[__allocation_size]);
41        void* __allocation = __state_allocation.get();
42        void* const __state_ = align(alignof(__state_t), __size, __allocation, __allocation_size);
43        __state = new (__state_) __barrier_algorithm_base::__state_t[__count];
44    }
45    _LIBCPP_HIDDEN
46    bool __arrive(__barrier_phase_t __old_phase)
47    {
48        __barrier_phase_t const __half_step = __old_phase + 1,
49                                __full_step = __old_phase + 2;
50        size_t __current_expected = __expected,
51            __current = hash<thread::id>()(this_thread::get_id()) % ((__expected + 1) >> 1);
52        for(int __round = 0;; ++__round) {
53            if(__current_expected <= 1)
54                return true;
55            size_t const __end_node = ((__current_expected + 1) >> 1),
56                         __last_node = __end_node - 1;
57            for(;;++__current) {
58                if(__current == __end_node)
59                __current = 0;
60                __barrier_phase_t expect = __old_phase;
61                if(__current == __last_node && (__current_expected & 1))
62                {
63                    if(__state[__current].__tickets[__round].__phase.compare_exchange_strong(expect, __full_step, memory_order_acq_rel))
64                        break;    // I'm 1 in 1, go to next __round
65                }
66                else if(__state[__current].__tickets[__round].__phase.compare_exchange_strong(expect, __half_step, memory_order_acq_rel))
67                {
68                    return false; // I'm 1 in 2, done with arrival
69                }
70                else if(expect == __half_step)
71                {
72                    if(__state[__current].__tickets[__round].__phase.compare_exchange_strong(expect, __full_step, memory_order_acq_rel))
73                        break;    // I'm 2 in 2, go to next __round
74                }
75            }
76            __current_expected = __last_node + 1;
77            __current >>= 1;
78        }
79    }
80};
81
82_LIBCPP_EXPORTED_FROM_ABI
83__barrier_algorithm_base * __construct_barrier_algorithm_base(ptrdiff_t& __expected)
84{
85    return new __barrier_algorithm_base(__expected);
86}
87_LIBCPP_EXPORTED_FROM_ABI
88bool __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier,
89                                     __barrier_phase_t __old_phase)
90{
91    return __barrier->__arrive(__old_phase);
92}
93_LIBCPP_EXPORTED_FROM_ABI
94void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier)
95{
96    delete __barrier;
97}
98
99#endif //!defined(_LIBCPP_HAS_NO_TREE_BARRIER) && (_LIBCPP_STD_VER >= 11)
100
101_LIBCPP_END_NAMESPACE_STD
102
103#endif //_LIBCPP_HAS_NO_THREADS
104