From c2e754829628d1e9b7a16b3389cfdace76950fdf Mon Sep 17 00:00:00 2001 From: misterg Date: Tue, 19 Sep 2017 16:54:40 -0400 Subject: Initial Commit --- absl/base/call_once.h | 210 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 210 insertions(+) create mode 100644 absl/base/call_once.h (limited to 'absl/base/call_once.h') diff --git a/absl/base/call_once.h b/absl/base/call_once.h new file mode 100644 index 000000000000..478a41c1fafb --- /dev/null +++ b/absl/base/call_once.h @@ -0,0 +1,210 @@ +// 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. +// +// ----------------------------------------------------------------------------- +// 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 +#include +#include + +#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/spinlock_wait.h" + +namespace absl { + +class once_flag; + +namespace base_internal { +// Implementation detail. +std::atomic* 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 +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* base_internal::ControlWord(once_flag* flag); + std::atomic 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 +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, + kOnceDone = 0x3F2D8AB0, +}; + +template +void CallOnceImpl(std::atomic* control, + base_internal::SchedulingMode scheduling_mode, Callable&& fn, + Args&&... args) { +#ifndef NDEBUG + { + uint32_t old_control = control->load(std::memory_order_acquire); + if (old_control != kOnceInit && + old_control != kOnceRunning && + old_control != kOnceWaiter && + old_control != kOnceDone) { + ABSL_RAW_LOG( + FATAL, + "Unexpected value for control word: %d. Either the control word " + "has non-static storage duration (where GoogleOnceDynamic might " + "be appropriate), or there's been a memory corruption.", + old_control); + } + } +#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. + uint32_t old_control = kOnceInit; + if (control->compare_exchange_strong(old_control, kOnceRunning, + std::memory_order_acquire, + std::memory_order_relaxed) || + base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans, + scheduling_mode) == kOnceInit) { + base_internal::Invoke(std::forward(fn), + std::forward(args)...); + 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* ControlWord(once_flag* flag) { + return &flag->control_; +} + +template +void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args) { + std::atomic* 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(fn), + std::forward(args)...); + } +} + +} // namespace base_internal + +template +void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args) { + std::atomic* 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(fn), std::forward(args)...); + } +} + +} // namespace absl + +#endif // ABSL_BASE_CALL_ONCE_H_ -- cgit 1.4.1