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Diffstat (limited to 'third_party/abseil_cpp/absl/base/call_once.h')
| -rw-r--r-- | third_party/abseil_cpp/absl/base/call_once.h | 226 |
1 files changed, 0 insertions, 226 deletions
diff --git a/third_party/abseil_cpp/absl/base/call_once.h b/third_party/abseil_cpp/absl/base/call_once.h deleted file mode 100644 index bc5ec93704..0000000000 --- a/third_party/abseil_cpp/absl/base/call_once.h +++ /dev/null @@ -1,226 +0,0 @@ -// 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. -// -// ----------------------------------------------------------------------------- -// File: call_once.h -// ----------------------------------------------------------------------------- -// -// This header file provides an Abseil version of `std::call_once` for invoking -// a given function at most once, across all threads. This Abseil version is -// faster than the C++11 version and incorporates the C++17 argument-passing -// fix, so that (for example) non-const references may be passed to the invoked -// function. - -#ifndef ABSL_BASE_CALL_ONCE_H_ -#define ABSL_BASE_CALL_ONCE_H_ - -#include <algorithm> -#include <atomic> -#include <cstdint> -#include <type_traits> -#include <utility> - -#include "absl/base/internal/invoke.h" -#include "absl/base/internal/low_level_scheduling.h" -#include "absl/base/internal/raw_logging.h" -#include "absl/base/internal/scheduling_mode.h" -#include "absl/base/internal/spinlock_wait.h" -#include "absl/base/macros.h" -#include "absl/base/optimization.h" -#include "absl/base/port.h" - -namespace absl { -ABSL_NAMESPACE_BEGIN - -class once_flag; - -namespace base_internal { -std::atomic<uint32_t>* ControlWord(absl::once_flag* flag); -} // namespace base_internal - -// call_once() -// -// For all invocations using a given `once_flag`, invokes a given `fn` exactly -// once across all threads. The first call to `call_once()` with a particular -// `once_flag` argument (that does not throw an exception) will run the -// specified function with the provided `args`; other calls with the same -// `once_flag` argument will not run the function, but will wait -// for the provided function to finish running (if it is still running). -// -// This mechanism provides a safe, simple, and fast mechanism for one-time -// initialization in a multi-threaded process. -// -// Example: -// -// class MyInitClass { -// public: -// ... -// mutable absl::once_flag once_; -// -// MyInitClass* init() const { -// absl::call_once(once_, &MyInitClass::Init, this); -// return ptr_; -// } -// -template <typename Callable, typename... Args> -void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args); - -// once_flag -// -// Objects of this type are used to distinguish calls to `call_once()` and -// ensure the provided function is only invoked once across all threads. This -// type is not copyable or movable. However, it has a `constexpr` -// constructor, and is safe to use as a namespace-scoped global variable. -class once_flag { - public: - constexpr once_flag() : control_(0) {} - once_flag(const once_flag&) = delete; - once_flag& operator=(const once_flag&) = delete; - - private: - friend std::atomic<uint32_t>* base_internal::ControlWord(once_flag* flag); - std::atomic<uint32_t> control_; -}; - -//------------------------------------------------------------------------------ -// End of public interfaces. -// Implementation details follow. -//------------------------------------------------------------------------------ - -namespace base_internal { - -// Like call_once, but uses KERNEL_ONLY scheduling. Intended to be used to -// initialize entities used by the scheduler implementation. -template <typename Callable, typename... Args> -void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args); - -// Disables scheduling while on stack when scheduling mode is non-cooperative. -// No effect for cooperative scheduling modes. -class SchedulingHelper { - public: - explicit SchedulingHelper(base_internal::SchedulingMode mode) : mode_(mode) { - if (mode_ == base_internal::SCHEDULE_KERNEL_ONLY) { - guard_result_ = base_internal::SchedulingGuard::DisableRescheduling(); - } - } - - ~SchedulingHelper() { - if (mode_ == base_internal::SCHEDULE_KERNEL_ONLY) { - base_internal::SchedulingGuard::EnableRescheduling(guard_result_); - } - } - - private: - base_internal::SchedulingMode mode_; - bool guard_result_; -}; - -// Bit patterns for call_once state machine values. Internal implementation -// detail, not for use by clients. -// -// The bit patterns are arbitrarily chosen from unlikely values, to aid in -// debugging. However, kOnceInit must be 0, so that a zero-initialized -// once_flag will be valid for immediate use. -enum { - kOnceInit = 0, - kOnceRunning = 0x65C2937B, - kOnceWaiter = 0x05A308D2, - // A very small constant is chosen for kOnceDone so that it fit in a single - // compare with immediate instruction for most common ISAs. This is verified - // for x86, POWER and ARM. - kOnceDone = 221, // Random Number -}; - -template <typename Callable, typename... Args> -ABSL_ATTRIBUTE_NOINLINE -void CallOnceImpl(std::atomic<uint32_t>* control, - base_internal::SchedulingMode scheduling_mode, Callable&& fn, - Args&&... args) { -#ifndef NDEBUG - { - uint32_t old_control = control->load(std::memory_order_relaxed); - if (old_control != kOnceInit && - old_control != kOnceRunning && - old_control != kOnceWaiter && - old_control != kOnceDone) { - ABSL_RAW_LOG(FATAL, "Unexpected value for control word: 0x%lx", - static_cast<unsigned long>(old_control)); // NOLINT - } - } -#endif // NDEBUG - static const base_internal::SpinLockWaitTransition trans[] = { - {kOnceInit, kOnceRunning, true}, - {kOnceRunning, kOnceWaiter, false}, - {kOnceDone, kOnceDone, true}}; - - // Must do this before potentially modifying control word's state. - base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode); - // Short circuit the simplest case to avoid procedure call overhead. - // The base_internal::SpinLockWait() call returns either kOnceInit or - // kOnceDone. If it returns kOnceDone, it must have loaded the control word - // with std::memory_order_acquire and seen a value of kOnceDone. - uint32_t old_control = kOnceInit; - if (control->compare_exchange_strong(old_control, kOnceRunning, - std::memory_order_relaxed) || - base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans, - scheduling_mode) == kOnceInit) { - base_internal::Invoke(std::forward<Callable>(fn), - std::forward<Args>(args)...); - // The call to SpinLockWake below is an optimization, because the waiter - // in SpinLockWait is waiting with a short timeout. The atomic load/store - // sequence is slightly faster than an atomic exchange: - // old_control = control->exchange(base_internal::kOnceDone, - // std::memory_order_release); - // We opt for a slightly faster case when there are no waiters, in spite - // of longer tail latency when there are waiters. - old_control = control->load(std::memory_order_relaxed); - control->store(base_internal::kOnceDone, std::memory_order_release); - if (old_control == base_internal::kOnceWaiter) { - base_internal::SpinLockWake(control, true); - } - } // else *control is already kOnceDone -} - -inline std::atomic<uint32_t>* ControlWord(once_flag* flag) { - return &flag->control_; -} - -template <typename Callable, typename... Args> -void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args) { - std::atomic<uint32_t>* once = base_internal::ControlWord(flag); - uint32_t s = once->load(std::memory_order_acquire); - if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) { - base_internal::CallOnceImpl(once, base_internal::SCHEDULE_KERNEL_ONLY, - std::forward<Callable>(fn), - std::forward<Args>(args)...); - } -} - -} // namespace base_internal - -template <typename Callable, typename... Args> -void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args) { - std::atomic<uint32_t>* once = base_internal::ControlWord(&flag); - uint32_t s = once->load(std::memory_order_acquire); - if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) { - base_internal::CallOnceImpl( - once, base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL, - std::forward<Callable>(fn), std::forward<Args>(args)...); - } -} - -ABSL_NAMESPACE_END -} // namespace absl - -#endif // ABSL_BASE_CALL_ONCE_H_ |