1//===------------------------- atomic.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#ifndef _LIBCPP_HAS_NO_THREADS
11
12#include <climits>
13#include <atomic>
14#include <functional>
15
16#include <iostream>
17
18#ifdef __linux__
19
20#include <unistd.h>
21#include <linux/futex.h>
22#include <sys/syscall.h>
23
24#else // <- Add other operating systems here
25
26// Baseline needs no new headers
27
28#endif
29
30_LIBCPP_BEGIN_NAMESPACE_STD
31
32#ifdef __linux__
33
34static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
35                                              __cxx_contention_t __val)
36{
37    static constexpr timespec __timeout = { 2, 0 };
38    syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
39}
40
41static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
42                                              bool __notify_one)
43{
44    syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
45}
46
47#elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK)
48
49extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value,
50		uint32_t timeout); /* timeout is specified in microseconds */
51extern "C" int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value);
52
53#define UL_COMPARE_AND_WAIT				1
54#define ULF_WAKE_ALL					0x00000100
55
56static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
57                                              __cxx_contention_t __val)
58{
59    __ulock_wait(UL_COMPARE_AND_WAIT,
60                 const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0);
61}
62
63static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
64                                              bool __notify_one)
65{
66    __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL),
67                 const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
68}
69
70#else // <- Add other operating systems here
71
72// Baseline is just a timed backoff
73
74static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
75                                              __cxx_contention_t __val)
76{
77    __libcpp_thread_poll_with_backoff([=]() -> bool {
78        return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val);
79    }, __libcpp_timed_backoff_policy());
80}
81
82static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { }
83
84#endif // __linux__
85
86static constexpr size_t __libcpp_contention_table_size = (1 << 8);  /* < there's no magic in this number */
87
88struct alignas(64) /*  aim to avoid false sharing */ __libcpp_contention_table_entry
89{
90    __cxx_atomic_contention_t __contention_state;
91    __cxx_atomic_contention_t __platform_state;
92    inline constexpr __libcpp_contention_table_entry() :
93        __contention_state(0), __platform_state(0) { }
94};
95
96static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ];
97
98static hash<void const volatile*> __libcpp_contention_hasher;
99
100static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p)
101{
102    return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)];
103}
104
105/* Given an atomic to track contention and an atomic to actually wait on, which may be
106   the same atomic, we try to detect contention to avoid spuriously calling the platform. */
107
108static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state,
109                                       __cxx_atomic_contention_t const volatile* __platform_state,
110                                       bool __notify_one)
111{
112    if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst))
113        // We only call 'wake' if we consumed a contention bit here.
114        __libcpp_platform_wake_by_address(__platform_state, __notify_one);
115}
116static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state,
117                                                               __cxx_atomic_contention_t const volatile* __platform_state)
118{
119    // We will monitor this value.
120    return __cxx_atomic_load(__platform_state, memory_order_acquire);
121}
122static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state,
123                                     __cxx_atomic_contention_t const volatile* __platform_state,
124                                     __cxx_contention_t __old_value)
125{
126    __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
127    // We sleep as long as the monitored value hasn't changed.
128    __libcpp_platform_wait_on_address(__platform_state, __old_value);
129    __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
130}
131
132/* When the incoming atomic is the wrong size for the platform wait size, need to
133   launder the value sequence through an atomic from our table. */
134
135static void __libcpp_atomic_notify(void const volatile* __location)
136{
137    auto const __entry = __libcpp_contention_state(__location);
138    // The value sequence laundering happens on the next line below.
139    __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
140    __libcpp_contention_notify(&__entry->__contention_state,
141                               &__entry->__platform_state,
142                               false /* when laundering, we can't handle notify_one */);
143}
144_LIBCPP_EXPORTED_FROM_ABI
145void __cxx_atomic_notify_one(void const volatile* __location)
146    { __libcpp_atomic_notify(__location); }
147_LIBCPP_EXPORTED_FROM_ABI
148void __cxx_atomic_notify_all(void const volatile* __location)
149    { __libcpp_atomic_notify(__location); }
150_LIBCPP_EXPORTED_FROM_ABI
151__cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location)
152{
153    auto const __entry = __libcpp_contention_state(__location);
154    return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state);
155}
156_LIBCPP_EXPORTED_FROM_ABI
157void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value)
158{
159    auto const __entry = __libcpp_contention_state(__location);
160    __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value);
161}
162
163/* When the incoming atomic happens to be the platform wait size, we still need to use the
164   table for the contention detection, but we can use the atomic directly for the wait. */
165
166_LIBCPP_EXPORTED_FROM_ABI
167void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location)
168{
169    __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true);
170}
171_LIBCPP_EXPORTED_FROM_ABI
172void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location)
173{
174    __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false);
175}
176_LIBCPP_EXPORTED_FROM_ABI
177__cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location)
178{
179    return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location);
180}
181_LIBCPP_EXPORTED_FROM_ABI
182void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value)
183{
184    __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value);
185}
186
187_LIBCPP_END_NAMESPACE_STD
188
189#endif //_LIBCPP_HAS_NO_THREADS
190