|
|
// 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 <utility>
namespace absl {
namespace base_internal {
// In current versions of MSVC (as of July 2017), a std::atomic<T> where T is a
// pointer to function cannot be constant-initialized with an address constant
// expression. That is, the following code does not compile:
// void NoOp() {}
// constexpr std::atomic<void(*)()> ptr(NoOp);
//
// This is the only compiler we support that seems to have this issue. We
// conditionalize on MSVC here to use a fallback implementation. But we
// should revisit this occasionally. If MSVC fixes this compiler bug, we
// can then change this to be conditionalized on the value on _MSC_FULL_VER
// instead.
#ifdef _MSC_FULL_VER
#define ABSL_HAVE_FUNCTION_ADDRESS_CONSTANT_EXPRESSION 0
#else
#define ABSL_HAVE_FUNCTION_ADDRESS_CONSTANT_EXPRESSION 1
#endif
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_(DummyFunction) {}
// 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) {
assert(fn);
FnPtr expected = DummyFunction;
hook_.compare_exchange_strong(expected, fn, std::memory_order_acq_rel,
std::memory_order_acquire);
// If the compare and exchange failed, make sure that's because hook_ was
// already set to `fn` by an earlier call. Any other state reflects an API
// violation (calling Store() multiple times with different values).
//
// Avoid ABSL_RAW_CHECK, since raw logging depends on AtomicHook.
assert(expected == DummyFunction || expected == fn);
}
// 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 {
FnPtr hook = hook_.load(std::memory_order_acquire);
if (ABSL_HAVE_FUNCTION_ADDRESS_CONSTANT_EXPRESSION || hook) {
return hook(std::forward<CallArgs>(args)...);
} else {
return ReturnType();
}
}
// 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 = hook_.load(std::memory_order_acquire);
return (ptr == DummyFunction) ? nullptr : ptr;
}
private:
#if ABSL_HAVE_FUNCTION_ADDRESS_CONSTANT_EXPRESSION
static ReturnType DummyFunction(Args...) {
return ReturnType();
}
#else
static constexpr FnPtr DummyFunction = nullptr;
#endif
std::atomic<FnPtr> hook_;
};
#undef ABSL_HAVE_FUNCTION_ADDRESS_CONSTANT_EXPRESSION
} // namespace base_internal
} // namespace absl
#endif // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
|
