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Diffstat (limited to 'Firestore/third_party/abseil-cpp/absl/base/internal/atomic_hook.h')
-rw-r--r-- | Firestore/third_party/abseil-cpp/absl/base/internal/atomic_hook.h | 150 |
1 files changed, 150 insertions, 0 deletions
diff --git a/Firestore/third_party/abseil-cpp/absl/base/internal/atomic_hook.h b/Firestore/third_party/abseil-cpp/absl/base/internal/atomic_hook.h new file mode 100644 index 0000000..47d4013 --- /dev/null +++ b/Firestore/third_party/abseil-cpp/absl/base/internal/atomic_hook.h @@ -0,0 +1,150 @@ +// 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 +// +// http://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> + +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, or performs a no-op (and returns a default +// constructed object) if no hook has been registered. +// +// 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...); + + constexpr AtomicHook() : hook_(kInitialValue) {} + + // 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. + +#ifdef _MSC_FULL_VER +#define ABSL_HAVE_WORKING_ATOMIC_POINTER 0 +#else +#define ABSL_HAVE_WORKING_ATOMIC_POINTER 1 +#endif + +#if ABSL_HAVE_WORKING_ATOMIC_POINTER + static constexpr FnPtr kInitialValue = &DummyFunction; + + // 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 = DummyFunction; + hook_.compare_exchange_strong(expected, fn, std::memory_order_acq_rel, + std::memory_order_acquire); + const bool store_succeeded = (expected == DummyFunction); + 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 kInitialValue = 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 == 0) { + return DummyFunction; + } + return reinterpret_cast<FnPtr>(value); + } + + bool DoStore(FnPtr fn) { + assert(fn); + const auto value = reinterpret_cast<intptr_t>(fn); + intptr_t expected = 0; + hook_.compare_exchange_strong(expected, value, std::memory_order_acq_rel, + std::memory_order_acquire); + const bool store_succeeded = (expected == 0); + const bool same_value_already_stored = (expected == value); + return store_succeeded || same_value_already_stored; + } + + std::atomic<intptr_t> hook_; +#endif +}; + +#undef ABSL_HAVE_WORKING_ATOMIC_POINTER + +} // namespace base_internal +} // namespace absl + +#endif // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_ |