Coverage Report

Created: 2022-07-16 07:03

/Users/buildslave/jenkins/workspace/coverage/llvm-project/libcxx/src/atomic.cpp
Line
Count
Source (jump to first uncovered line)
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 <__config>
10
#ifndef _LIBCPP_HAS_NO_THREADS
11
12
#include <atomic>
13
#include <climits>
14
#include <functional>
15
#include <thread>
16
17
#ifdef __linux__
18
19
#include <unistd.h>
20
#include <linux/futex.h>
21
#include <sys/syscall.h>
22
23
// libc++ uses SYS_futex as a universal syscall name. However, on 32 bit architectures
24
// with a 64 bit time_t, we need to specify SYS_futex_time64.
25
#if !defined(SYS_futex) && defined(SYS_futex_time64)
26
# define SYS_futex SYS_futex_time64
27
#endif
28
29
#else // <- Add other operating systems here
30
31
// Baseline needs no new headers
32
33
#endif
34
35
_LIBCPP_BEGIN_NAMESPACE_STD
36
37
#ifdef __linux__
38
39
static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
40
                                              __cxx_contention_t __val)
41
{
42
    static constexpr timespec __timeout = { 2, 0 };
43
    syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
44
}
45
46
static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
47
                                              bool __notify_one)
48
{
49
    syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
50
}
51
52
#elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK)
53
54
extern "C" int __ulock_wait(uint32_t operation, void *addr, uint64_t value,
55
    uint32_t timeout); /* timeout is specified in microseconds */
56
extern "C" int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value);
57
58
#define UL_COMPARE_AND_WAIT       1
59
#define ULF_WAKE_ALL          0x00000100
60
61
static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
62
                                              __cxx_contention_t __val)
63
{
64
    __ulock_wait(UL_COMPARE_AND_WAIT,
65
                 const_cast<__cxx_atomic_contention_t*>(__ptr), __val, 0);
66
}
67
68
static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
69
                                              bool __notify_one)
70
{
71
    __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL),
72
                 const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
73
}
74
75
#else // <- Add other operating systems here
76
77
// Baseline is just a timed backoff
78
79
static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
80
                                              __cxx_contention_t __val)
81
0
{
82
0
    __libcpp_thread_poll_with_backoff([=]() -> bool {
83
0
        return !__cxx_nonatomic_compare_equal(__cxx_atomic_load(__ptr, memory_order_relaxed), __val);
84
0
    }, __libcpp_timed_backoff_policy());
85
0
}
86
87
0
static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile*, bool) { }
88
89
#endif // __linux__
90
91
static constexpr size_t __libcpp_contention_table_size = (1 << 8);  /* < there's no magic in this number */
92
93
struct alignas(64) /*  aim to avoid false sharing */ __libcpp_contention_table_entry
94
{
95
    __cxx_atomic_contention_t __contention_state;
96
    __cxx_atomic_contention_t __platform_state;
97
    inline constexpr __libcpp_contention_table_entry() :
98
0
        __contention_state(0), __platform_state(0) { }
99
};
100
101
static __libcpp_contention_table_entry __libcpp_contention_table[ __libcpp_contention_table_size ];
102
103
static hash<void const volatile*> __libcpp_contention_hasher;
104
105
static __libcpp_contention_table_entry* __libcpp_contention_state(void const volatile * p)
106
0
{
107
0
    return &__libcpp_contention_table[__libcpp_contention_hasher(p) & (__libcpp_contention_table_size - 1)];
108
0
}
109
110
/* Given an atomic to track contention and an atomic to actually wait on, which may be
111
   the same atomic, we try to detect contention to avoid spuriously calling the platform. */
112
113
static void __libcpp_contention_notify(__cxx_atomic_contention_t volatile* __contention_state,
114
                                       __cxx_atomic_contention_t const volatile* __platform_state,
115
                                       bool __notify_one)
116
0
{
117
0
    if(0 != __cxx_atomic_load(__contention_state, memory_order_seq_cst))
118
        // We only call 'wake' if we consumed a contention bit here.
119
0
        __libcpp_platform_wake_by_address(__platform_state, __notify_one);
120
0
}
121
static __cxx_contention_t __libcpp_contention_monitor_for_wait(__cxx_atomic_contention_t volatile* __contention_state,
122
                                                               __cxx_atomic_contention_t const volatile* __platform_state)
123
0
{
124
    // We will monitor this value.
125
0
    return __cxx_atomic_load(__platform_state, memory_order_acquire);
126
0
}
127
static void __libcpp_contention_wait(__cxx_atomic_contention_t volatile* __contention_state,
128
                                     __cxx_atomic_contention_t const volatile* __platform_state,
129
                                     __cxx_contention_t __old_value)
130
0
{
131
0
    __cxx_atomic_fetch_add(__contention_state, __cxx_contention_t(1), memory_order_seq_cst);
132
    // We sleep as long as the monitored value hasn't changed.
133
0
    __libcpp_platform_wait_on_address(__platform_state, __old_value);
134
0
    __cxx_atomic_fetch_sub(__contention_state, __cxx_contention_t(1), memory_order_release);
135
0
}
136
137
/* When the incoming atomic is the wrong size for the platform wait size, need to
138
   launder the value sequence through an atomic from our table. */
139
140
static void __libcpp_atomic_notify(void const volatile* __location)
141
0
{
142
0
    auto const __entry = __libcpp_contention_state(__location);
143
    // The value sequence laundering happens on the next line below.
144
0
    __cxx_atomic_fetch_add(&__entry->__platform_state, __cxx_contention_t(1), memory_order_release);
145
0
    __libcpp_contention_notify(&__entry->__contention_state,
146
0
                               &__entry->__platform_state,
147
0
                               false /* when laundering, we can't handle notify_one */);
148
0
}
149
_LIBCPP_EXPORTED_FROM_ABI
150
void __cxx_atomic_notify_one(void const volatile* __location)
151
0
    { __libcpp_atomic_notify(__location); }
152
_LIBCPP_EXPORTED_FROM_ABI
153
void __cxx_atomic_notify_all(void const volatile* __location)
154
0
    { __libcpp_atomic_notify(__location); }
155
_LIBCPP_EXPORTED_FROM_ABI
156
__cxx_contention_t __libcpp_atomic_monitor(void const volatile* __location)
157
0
{
158
0
    auto const __entry = __libcpp_contention_state(__location);
159
0
    return __libcpp_contention_monitor_for_wait(&__entry->__contention_state, &__entry->__platform_state);
160
0
}
161
_LIBCPP_EXPORTED_FROM_ABI
162
void __libcpp_atomic_wait(void const volatile* __location, __cxx_contention_t __old_value)
163
0
{
164
0
    auto const __entry = __libcpp_contention_state(__location);
165
0
    __libcpp_contention_wait(&__entry->__contention_state, &__entry->__platform_state, __old_value);
166
0
}
167
168
/* When the incoming atomic happens to be the platform wait size, we still need to use the
169
   table for the contention detection, but we can use the atomic directly for the wait. */
170
171
_LIBCPP_EXPORTED_FROM_ABI
172
void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile* __location)
173
0
{
174
0
    __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, true);
175
0
}
176
_LIBCPP_EXPORTED_FROM_ABI
177
void __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile* __location)
178
0
{
179
0
    __libcpp_contention_notify(&__libcpp_contention_state(__location)->__contention_state, __location, false);
180
0
}
181
_LIBCPP_EXPORTED_FROM_ABI
182
__cxx_contention_t __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile* __location)
183
0
{
184
0
    return __libcpp_contention_monitor_for_wait(&__libcpp_contention_state(__location)->__contention_state, __location);
185
0
}
186
_LIBCPP_EXPORTED_FROM_ABI
187
void __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile* __location, __cxx_contention_t __old_value)
188
0
{
189
0
    __libcpp_contention_wait(&__libcpp_contention_state(__location)->__contention_state, __location, __old_value);
190
0
}
191
192
_LIBCPP_END_NAMESPACE_STD
193
194
#endif //_LIBCPP_HAS_NO_THREADS