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


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autogenerated on Wed Jun 19 2019 19:19:56