1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
|
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
#define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
#include <atomic>
#include <cassert>
#include <cstdint>
#include <utility>
#ifdef _MSC_FULL_VER
#define ABSL_HAVE_WORKING_ATOMIC_POINTER 0
#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 0
#else
#define ABSL_HAVE_WORKING_ATOMIC_POINTER 1
#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 1
#endif
namespace absl {
namespace base_internal {
template <typename T>
class AtomicHook;
// `AtomicHook` is a helper class, templatized on a raw function pointer type,
// for implementing Abseil customization hooks. It is a callable object that
// dispatches to the registered hook. Objects of type `AtomicHook` must have
// static or thread storage duration.
//
// A default constructed object performs a no-op (and returns a default
// constructed object) if no hook has been registered.
//
// Hooks can be pre-registered via constant initialization, for example,
// `ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);`
// and then changed at runtime via a call to `Store()`.
//
// Reads and writes guarantee memory_order_acquire/memory_order_release
// semantics.
template <typename ReturnType, typename... Args>
class AtomicHook<ReturnType (*)(Args...)> {
public:
using FnPtr = ReturnType (*)(Args...);
// Constructs an object that by default performs a no-op (and
// returns a default constructed object) when no hook as been registered.
constexpr AtomicHook() : AtomicHook(DummyFunction) {}
// Constructs an object that by default dispatches to/returns the
// pre-registered default_fn when no hook has been registered at runtime.
#if ABSL_HAVE_WORKING_ATOMIC_POINTER && ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
explicit constexpr AtomicHook(FnPtr default_fn)
: hook_(default_fn), default_fn_(default_fn) {}
#else
// On MSVC, this function sometimes executes after dynamic initialization =(.
// If a non-zero `hook_` has been installed by a dynamic initializer, we want
// to preserve it. If not, `hook_` will be zero initialized and we have no
// need to set it to `kUninitialized`.
// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
explicit constexpr AtomicHook(FnPtr default_fn)
: /* hook_(deliberately omitted), */ default_fn_(default_fn) {
static_assert(kUninitialized == 0, "here we rely on zero-initialization");
}
#endif
// Stores the provided function pointer as the value for this hook.
//
// This is intended to be called once. Multiple calls are legal only if the
// same function pointer is provided for each call. The store is implemented
// as a memory_order_release operation, and read accesses are implemented as
// memory_order_acquire.
void Store(FnPtr fn) {
bool success = DoStore(fn);
static_cast<void>(success);
assert(success);
}
// Invokes the registered callback. If no callback has yet been registered, a
// default-constructed object of the appropriate type is returned instead.
template <typename... CallArgs>
ReturnType operator()(CallArgs&&... args) const {
return DoLoad()(std::forward<CallArgs>(args)...);
}
// Returns the registered callback, or nullptr if none has been registered.
// Useful if client code needs to conditionalize behavior based on whether a
// callback was registered.
//
// Note that atomic_hook.Load()() and atomic_hook() have different semantics:
// operator()() will perform a no-op if no callback was registered, while
// Load()() will dereference a null function pointer. Prefer operator()() to
// Load()() unless you must conditionalize behavior on whether a hook was
// registered.
FnPtr Load() const {
FnPtr ptr = DoLoad();
return (ptr == DummyFunction) ? nullptr : ptr;
}
private:
static ReturnType DummyFunction(Args...) {
return ReturnType();
}
// Current versions of MSVC (as of September 2017) have a broken
// implementation of std::atomic<T*>: Its constructor attempts to do the
// equivalent of a reinterpret_cast in a constexpr context, which is not
// allowed.
//
// This causes an issue when building with LLVM under Windows. To avoid this,
// we use a less-efficient, intptr_t-based implementation on Windows.
#if ABSL_HAVE_WORKING_ATOMIC_POINTER
// Return the stored value, or DummyFunction if no value has been stored.
FnPtr DoLoad() const { return hook_.load(std::memory_order_acquire); }
// Store the given value. Returns false if a different value was already
// stored to this object.
bool DoStore(FnPtr fn) {
assert(fn);
FnPtr expected = default_fn_;
const bool store_succeeded = hook_.compare_exchange_strong(
expected, fn, std::memory_order_acq_rel, std::memory_order_acquire);
const bool same_value_already_stored = (expected == fn);
return store_succeeded || same_value_already_stored;
}
std::atomic<FnPtr> hook_;
#else // !ABSL_HAVE_WORKING_ATOMIC_POINTER
// Use a sentinel value unlikely to be the address of an actual function.
static constexpr intptr_t kUninitialized = 0;
static_assert(sizeof(intptr_t) >= sizeof(FnPtr),
"intptr_t can't contain a function pointer");
FnPtr DoLoad() const {
const intptr_t value = hook_.load(std::memory_order_acquire);
if (value == kUninitialized) {
return default_fn_;
}
return reinterpret_cast<FnPtr>(value);
}
bool DoStore(FnPtr fn) {
assert(fn);
const auto value = reinterpret_cast<intptr_t>(fn);
intptr_t expected = kUninitialized;
const bool store_succeeded = hook_.compare_exchange_strong(
expected, value, std::memory_order_acq_rel, std::memory_order_acquire);
const bool same_value_already_stored = (expected == value);
return store_succeeded || same_value_already_stored;
}
std::atomic<intptr_t> hook_;
#endif
const FnPtr default_fn_;
};
#undef ABSL_HAVE_WORKING_ATOMIC_POINTER
#undef ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
} // namespace base_internal
} // namespace absl
#endif // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
|
