fc8dc48020
git-subtree-dir: third_party/abseil_cpp git-subtree-mainline:ffb2ae54begit-subtree-split:768eb2ca28
211 lines
7.5 KiB
C++
211 lines
7.5 KiB
C++
// Copyright 2019 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
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#define ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
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#include <stdint.h>
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#include <atomic>
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#include "absl/base/internal/exponential_biased.h"
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#include "absl/base/optimization.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace base_internal {
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// PeriodicSamplerBase provides the basic period sampler implementation.
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//
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// This is the base class for the templated PeriodicSampler class, which holds
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// a global std::atomic value identified by a user defined tag, such that
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// each specific PeriodSampler implementation holds its own global period.
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//
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// PeriodicSamplerBase is thread-compatible except where stated otherwise.
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class PeriodicSamplerBase {
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public:
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// PeriodicSamplerBase is trivial / copyable / movable / destructible.
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PeriodicSamplerBase() = default;
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PeriodicSamplerBase(PeriodicSamplerBase&&) = default;
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PeriodicSamplerBase(const PeriodicSamplerBase&) = default;
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// Returns true roughly once every `period` calls. This is established by a
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// randomly picked `stride` that is counted down on each call to `Sample`.
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// This stride is picked such that the probability of `Sample()` returning
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// true is 1 in `period`.
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inline bool Sample() noexcept;
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// The below methods are intended for optimized use cases where the
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// size of the inlined fast path code is highly important. Applications
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// should use the `Sample()` method unless they have proof that their
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// specific use case requires the optimizations offered by these methods.
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//
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// An example of such a use case is SwissTable sampling. All sampling checks
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// are in inlined SwissTable methods, and the number of call sites is huge.
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// In this case, the inlined code size added to each translation unit calling
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// SwissTable methods is non-trivial.
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//
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// The `SubtleMaybeSample()` function spuriously returns true even if the
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// function should not be sampled, applications MUST match each call to
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// 'SubtleMaybeSample()' returning true with a `SubtleConfirmSample()` call,
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// and use the result of the latter as the sampling decision.
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// In other words: the code should logically be equivalent to:
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//
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// if (SubtleMaybeSample() && SubtleConfirmSample()) {
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// // Sample this call
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// }
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//
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// In the 'inline-size' optimized case, the `SubtleConfirmSample()` call can
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// be placed out of line, for example, the typical use case looks as follows:
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//
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// // --- frobber.h -----------
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// void FrobberSampled();
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//
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// inline void FrobberImpl() {
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// // ...
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// }
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//
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// inline void Frobber() {
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// if (ABSL_PREDICT_FALSE(sampler.SubtleMaybeSample())) {
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// FrobberSampled();
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// } else {
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// FrobberImpl();
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// }
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// }
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//
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// // --- frobber.cc -----------
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// void FrobberSampled() {
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// if (!sampler.SubtleConfirmSample())) {
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// // Spurious false positive
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// FrobberImpl();
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// return;
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// }
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//
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// // Sampled execution
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// // ...
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// }
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inline bool SubtleMaybeSample() noexcept;
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bool SubtleConfirmSample() noexcept;
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protected:
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// We explicitly don't use a virtual destructor as this class is never
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// virtually destroyed, and it keeps the class trivial, which avoids TLS
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// prologue and epilogue code for our TLS instances.
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~PeriodicSamplerBase() = default;
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// Returns the next stride for our sampler.
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// This function is virtual for testing purposes only.
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virtual int64_t GetExponentialBiased(int period) noexcept;
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private:
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// Returns the current period of this sampler. Thread-safe.
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virtual int period() const noexcept = 0;
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// Keep and decrement stride_ as an unsigned integer, but compare the value
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// to zero casted as a signed int. clang and msvc do not create optimum code
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// if we use signed for the combined decrement and sign comparison.
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//
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// Below 3 alternative options, all compiles generate the best code
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// using the unsigned increment <---> signed int comparison option.
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//
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// Option 1:
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// int64_t stride_;
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// if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... }
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//
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// GCC x64 (OK) : https://gcc.godbolt.org/z/R5MzzA
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// GCC ppc (OK) : https://gcc.godbolt.org/z/z7NZAt
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// Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd
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// ICC x64 (OK) : https://gcc.godbolt.org/z/rE6s8W
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// MSVC x64 (OK) : https://gcc.godbolt.org/z/ARMXqS
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//
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// Option 2:
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// int64_t stride_ = 0;
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// if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... }
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//
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// GCC x64 (OK) : https://gcc.godbolt.org/z/jSQxYK
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// GCC ppc (OK) : https://gcc.godbolt.org/z/VJdYaA
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// Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX
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// ICC x64 (OK) : https://gcc.godbolt.org/z/4snaFd
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// MSVC x64 (BAD): https://gcc.godbolt.org/z/BgnEKE
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//
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// Option 3:
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// uint64_t stride_;
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// if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... }
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//
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// GCC x64 (OK) : https://gcc.godbolt.org/z/bFbfPy
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// GCC ppc (OK) : https://gcc.godbolt.org/z/S9KkUE
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// Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4
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// ICC x64 (OK) : https://gcc.godbolt.org/z/ptTNfD
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// MSVC x64 (OK) : https://gcc.godbolt.org/z/76j4-5
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uint64_t stride_ = 0;
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ExponentialBiased rng_;
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};
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inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept {
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// See comments on `stride_` for the unsigned increment / signed compare.
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if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) {
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return false;
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}
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return true;
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}
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inline bool PeriodicSamplerBase::Sample() noexcept {
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return ABSL_PREDICT_FALSE(SubtleMaybeSample()) ? SubtleConfirmSample()
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: false;
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}
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// PeriodicSampler is a concreted periodic sampler implementation.
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// The user provided Tag identifies the implementation, and is required to
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// isolate the global state of this instance from other instances.
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//
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// Typical use case:
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//
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// struct HashTablezTag {};
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// thread_local PeriodicSampler sampler;
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//
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// void HashTableSamplingLogic(...) {
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// if (sampler.Sample()) {
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// HashTableSlowSamplePath(...);
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// }
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// }
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//
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template <typename Tag, int default_period = 0>
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class PeriodicSampler final : public PeriodicSamplerBase {
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public:
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~PeriodicSampler() = default;
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int period() const noexcept final {
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return period_.load(std::memory_order_relaxed);
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}
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// Sets the global period for this sampler. Thread-safe.
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// Setting a period of 0 disables the sampler, i.e., every call to Sample()
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// will return false. Setting a period of 1 puts the sampler in 'always on'
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// mode, i.e., every call to Sample() returns true.
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static void SetGlobalPeriod(int period) {
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period_.store(period, std::memory_order_relaxed);
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}
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private:
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static std::atomic<int> period_;
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};
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template <typename Tag, int default_period>
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std::atomic<int> PeriodicSampler<Tag, default_period>::period_(default_period);
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} // namespace base_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
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