Coverage Report

Created: 2021-08-24 07:12

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