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Diffstat (limited to 'third_party/abseil_cpp/absl/base/internal/periodic_sampler.h')
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1 files changed, 0 insertions, 211 deletions
diff --git a/third_party/abseil_cpp/absl/base/internal/periodic_sampler.h b/third_party/abseil_cpp/absl/base/internal/periodic_sampler.h deleted file mode 100644 index f8a86796b1..0000000000 --- a/third_party/abseil_cpp/absl/base/internal/periodic_sampler.h +++ /dev/null @@ -1,211 +0,0 @@ -// Copyright 2019 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. - -#ifndef ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ -#define ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ - -#include <stdint.h> - -#include <atomic> - -#include "absl/base/internal/exponential_biased.h" -#include "absl/base/optimization.h" - -namespace absl { -ABSL_NAMESPACE_BEGIN -namespace base_internal { - -// PeriodicSamplerBase provides the basic period sampler implementation. -// -// This is the base class for the templated PeriodicSampler class, which holds -// a global std::atomic value identified by a user defined tag, such that -// each specific PeriodSampler implementation holds its own global period. -// -// PeriodicSamplerBase is thread-compatible except where stated otherwise. -class PeriodicSamplerBase { - public: - // PeriodicSamplerBase is trivial / copyable / movable / destructible. - PeriodicSamplerBase() = default; - PeriodicSamplerBase(PeriodicSamplerBase&&) = default; - PeriodicSamplerBase(const PeriodicSamplerBase&) = default; - - // Returns true roughly once every `period` calls. This is established by a - // randomly picked `stride` that is counted down on each call to `Sample`. - // This stride is picked such that the probability of `Sample()` returning - // true is 1 in `period`. - inline bool Sample() noexcept; - - // The below methods are intended for optimized use cases where the - // size of the inlined fast path code is highly important. Applications - // should use the `Sample()` method unless they have proof that their - // specific use case requires the optimizations offered by these methods. - // - // An example of such a use case is SwissTable sampling. All sampling checks - // are in inlined SwissTable methods, and the number of call sites is huge. - // In this case, the inlined code size added to each translation unit calling - // SwissTable methods is non-trivial. - // - // The `SubtleMaybeSample()` function spuriously returns true even if the - // function should not be sampled, applications MUST match each call to - // 'SubtleMaybeSample()' returning true with a `SubtleConfirmSample()` call, - // and use the result of the latter as the sampling decision. - // In other words: the code should logically be equivalent to: - // - // if (SubtleMaybeSample() && SubtleConfirmSample()) { - // // Sample this call - // } - // - // In the 'inline-size' optimized case, the `SubtleConfirmSample()` call can - // be placed out of line, for example, the typical use case looks as follows: - // - // // --- frobber.h ----------- - // void FrobberSampled(); - // - // inline void FrobberImpl() { - // // ... - // } - // - // inline void Frobber() { - // if (ABSL_PREDICT_FALSE(sampler.SubtleMaybeSample())) { - // FrobberSampled(); - // } else { - // FrobberImpl(); - // } - // } - // - // // --- frobber.cc ----------- - // void FrobberSampled() { - // if (!sampler.SubtleConfirmSample())) { - // // Spurious false positive - // FrobberImpl(); - // return; - // } - // - // // Sampled execution - // // ... - // } - inline bool SubtleMaybeSample() noexcept; - bool SubtleConfirmSample() noexcept; - - protected: - // We explicitly don't use a virtual destructor as this class is never - // virtually destroyed, and it keeps the class trivial, which avoids TLS - // prologue and epilogue code for our TLS instances. - ~PeriodicSamplerBase() = default; - - // Returns the next stride for our sampler. - // This function is virtual for testing purposes only. - virtual int64_t GetExponentialBiased(int period) noexcept; - - private: - // Returns the current period of this sampler. Thread-safe. - virtual int period() const noexcept = 0; - - // Keep and decrement stride_ as an unsigned integer, but compare the value - // to zero casted as a signed int. clang and msvc do not create optimum code - // if we use signed for the combined decrement and sign comparison. - // - // Below 3 alternative options, all compiles generate the best code - // using the unsigned increment <---> signed int comparison option. - // - // Option 1: - // int64_t stride_; - // if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... } - // - // GCC x64 (OK) : https://gcc.godbolt.org/z/R5MzzA - // GCC ppc (OK) : https://gcc.godbolt.org/z/z7NZAt - // Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd - // ICC x64 (OK) : https://gcc.godbolt.org/z/rE6s8W - // MSVC x64 (OK) : https://gcc.godbolt.org/z/ARMXqS - // - // Option 2: - // int64_t stride_ = 0; - // if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... } - // - // GCC x64 (OK) : https://gcc.godbolt.org/z/jSQxYK - // GCC ppc (OK) : https://gcc.godbolt.org/z/VJdYaA - // Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX - // ICC x64 (OK) : https://gcc.godbolt.org/z/4snaFd - // MSVC x64 (BAD): https://gcc.godbolt.org/z/BgnEKE - // - // Option 3: - // uint64_t stride_; - // if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... } - // - // GCC x64 (OK) : https://gcc.godbolt.org/z/bFbfPy - // GCC ppc (OK) : https://gcc.godbolt.org/z/S9KkUE - // Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4 - // ICC x64 (OK) : https://gcc.godbolt.org/z/ptTNfD - // MSVC x64 (OK) : https://gcc.godbolt.org/z/76j4-5 - uint64_t stride_ = 0; - ExponentialBiased rng_; -}; - -inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept { - // See comments on `stride_` for the unsigned increment / signed compare. - if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { - return false; - } - return true; -} - -inline bool PeriodicSamplerBase::Sample() noexcept { - return ABSL_PREDICT_FALSE(SubtleMaybeSample()) ? SubtleConfirmSample() - : false; -} - -// PeriodicSampler is a concreted periodic sampler implementation. -// The user provided Tag identifies the implementation, and is required to -// isolate the global state of this instance from other instances. -// -// Typical use case: -// -// struct HashTablezTag {}; -// thread_local PeriodicSampler sampler; -// -// void HashTableSamplingLogic(...) { -// if (sampler.Sample()) { -// HashTableSlowSamplePath(...); -// } -// } -// -template <typename Tag, int default_period = 0> -class PeriodicSampler final : public PeriodicSamplerBase { - public: - ~PeriodicSampler() = default; - - int period() const noexcept final { - return period_.load(std::memory_order_relaxed); - } - - // Sets the global period for this sampler. Thread-safe. - // Setting a period of 0 disables the sampler, i.e., every call to Sample() - // will return false. Setting a period of 1 puts the sampler in 'always on' - // mode, i.e., every call to Sample() returns true. - static void SetGlobalPeriod(int period) { - period_.store(period, std::memory_order_relaxed); - } - - private: - static std::atomic<int> period_; -}; - -template <typename Tag, int default_period> -std::atomic<int> PeriodicSampler<Tag, default_period>::period_(default_period); - -} // namespace base_internal -ABSL_NAMESPACE_END -} // namespace absl - -#endif // ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_ |