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// Copyright 2018 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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#include "absl/container/internal/hashtablez_sampler.h"
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#include <atomic>
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#include <cassert>
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#include <cmath>
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#include <functional>
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#include <limits>
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#include "absl/base/attributes.h"
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#include "absl/base/internal/exponential_biased.h"
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#include "absl/container/internal/have_sse.h"
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#include "absl/debugging/stacktrace.h"
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#include "absl/memory/memory.h"
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#include "absl/synchronization/mutex.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace container_internal {
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constexpr int HashtablezInfo::kMaxStackDepth;
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namespace {
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ABSL_CONST_INIT std::atomic<bool> g_hashtablez_enabled{
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false
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};
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ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_sample_parameter{1 << 10};
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ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_max_samples{1 << 20};
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#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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ABSL_PER_THREAD_TLS_KEYWORD absl::base_internal::ExponentialBiased
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g_exponential_biased_generator;
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#endif
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} // namespace
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#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample = 0;
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#endif // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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HashtablezSampler& HashtablezSampler::Global() {
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static auto* sampler = new HashtablezSampler();
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return *sampler;
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}
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HashtablezSampler::DisposeCallback HashtablezSampler::SetDisposeCallback(
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DisposeCallback f) {
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return dispose_.exchange(f, std::memory_order_relaxed);
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}
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HashtablezInfo::HashtablezInfo() { PrepareForSampling(); }
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HashtablezInfo::~HashtablezInfo() = default;
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void HashtablezInfo::PrepareForSampling() {
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capacity.store(0, std::memory_order_relaxed);
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size.store(0, std::memory_order_relaxed);
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num_erases.store(0, std::memory_order_relaxed);
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max_probe_length.store(0, std::memory_order_relaxed);
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total_probe_length.store(0, std::memory_order_relaxed);
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hashes_bitwise_or.store(0, std::memory_order_relaxed);
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hashes_bitwise_and.store(~size_t{}, std::memory_order_relaxed);
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create_time = absl::Now();
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// The inliner makes hardcoded skip_count difficult (especially when combined
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// with LTO). We use the ability to exclude stacks by regex when encoding
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// instead.
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depth = absl::GetStackTrace(stack, HashtablezInfo::kMaxStackDepth,
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/* skip_count= */ 0);
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dead = nullptr;
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}
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HashtablezSampler::HashtablezSampler()
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: dropped_samples_(0), size_estimate_(0), all_(nullptr), dispose_(nullptr) {
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absl::MutexLock l(&graveyard_.init_mu);
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graveyard_.dead = &graveyard_;
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}
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HashtablezSampler::~HashtablezSampler() {
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HashtablezInfo* s = all_.load(std::memory_order_acquire);
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while (s != nullptr) {
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HashtablezInfo* next = s->next;
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delete s;
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s = next;
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}
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}
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void HashtablezSampler::PushNew(HashtablezInfo* sample) {
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sample->next = all_.load(std::memory_order_relaxed);
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while (!all_.compare_exchange_weak(sample->next, sample,
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std::memory_order_release,
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std::memory_order_relaxed)) {
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}
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}
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void HashtablezSampler::PushDead(HashtablezInfo* sample) {
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if (auto* dispose = dispose_.load(std::memory_order_relaxed)) {
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dispose(*sample);
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}
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absl::MutexLock graveyard_lock(&graveyard_.init_mu);
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absl::MutexLock sample_lock(&sample->init_mu);
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sample->dead = graveyard_.dead;
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graveyard_.dead = sample;
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}
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HashtablezInfo* HashtablezSampler::PopDead() {
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absl::MutexLock graveyard_lock(&graveyard_.init_mu);
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// The list is circular, so eventually it collapses down to
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// graveyard_.dead == &graveyard_
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// when it is empty.
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HashtablezInfo* sample = graveyard_.dead;
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if (sample == &graveyard_) return nullptr;
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absl::MutexLock sample_lock(&sample->init_mu);
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graveyard_.dead = sample->dead;
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sample->PrepareForSampling();
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return sample;
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}
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HashtablezInfo* HashtablezSampler::Register() {
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int64_t size = size_estimate_.fetch_add(1, std::memory_order_relaxed);
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if (size > g_hashtablez_max_samples.load(std::memory_order_relaxed)) {
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size_estimate_.fetch_sub(1, std::memory_order_relaxed);
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dropped_samples_.fetch_add(1, std::memory_order_relaxed);
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return nullptr;
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}
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HashtablezInfo* sample = PopDead();
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if (sample == nullptr) {
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// Resurrection failed. Hire a new warlock.
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sample = new HashtablezInfo();
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PushNew(sample);
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}
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return sample;
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}
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void HashtablezSampler::Unregister(HashtablezInfo* sample) {
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PushDead(sample);
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size_estimate_.fetch_sub(1, std::memory_order_relaxed);
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}
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int64_t HashtablezSampler::Iterate(
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const std::function<void(const HashtablezInfo& stack)>& f) {
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HashtablezInfo* s = all_.load(std::memory_order_acquire);
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while (s != nullptr) {
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absl::MutexLock l(&s->init_mu);
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if (s->dead == nullptr) {
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f(*s);
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}
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s = s->next;
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}
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return dropped_samples_.load(std::memory_order_relaxed);
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}
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static bool ShouldForceSampling() {
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enum ForceState {
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kDontForce,
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kForce,
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kUninitialized
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};
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ABSL_CONST_INIT static std::atomic<ForceState> global_state{
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kUninitialized};
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ForceState state = global_state.load(std::memory_order_relaxed);
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if (ABSL_PREDICT_TRUE(state == kDontForce)) return false;
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if (state == kUninitialized) {
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state = AbslContainerInternalSampleEverything() ? kForce : kDontForce;
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global_state.store(state, std::memory_order_relaxed);
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}
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return state == kForce;
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}
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HashtablezInfo* SampleSlow(int64_t* next_sample) {
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if (ABSL_PREDICT_FALSE(ShouldForceSampling())) {
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*next_sample = 1;
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return HashtablezSampler::Global().Register();
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}
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#if !defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
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*next_sample = std::numeric_limits<int64_t>::max();
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return nullptr;
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#else
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bool first = *next_sample < 0;
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*next_sample = g_exponential_biased_generator.GetStride(
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g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
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// Small values of interval are equivalent to just sampling next time.
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ABSL_ASSERT(*next_sample >= 1);
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// g_hashtablez_enabled can be dynamically flipped, we need to set a threshold
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// low enough that we will start sampling in a reasonable time, so we just use
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// the default sampling rate.
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if (!g_hashtablez_enabled.load(std::memory_order_relaxed)) return nullptr;
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// We will only be negative on our first count, so we should just retry in
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// that case.
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if (first) {
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if (ABSL_PREDICT_TRUE(--*next_sample > 0)) return nullptr;
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return SampleSlow(next_sample);
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}
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return HashtablezSampler::Global().Register();
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#endif
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}
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void UnsampleSlow(HashtablezInfo* info) {
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HashtablezSampler::Global().Unregister(info);
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}
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void RecordInsertSlow(HashtablezInfo* info, size_t hash,
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size_t distance_from_desired) {
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// SwissTables probe in groups of 16, so scale this to count items probes and
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// not offset from desired.
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size_t probe_length = distance_from_desired;
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#if ABSL_INTERNAL_RAW_HASH_SET_HAVE_SSE2
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probe_length /= 16;
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#else
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probe_length /= 8;
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#endif
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info->hashes_bitwise_and.fetch_and(hash, std::memory_order_relaxed);
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info->hashes_bitwise_or.fetch_or(hash, std::memory_order_relaxed);
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info->max_probe_length.store(
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std::max(info->max_probe_length.load(std::memory_order_relaxed),
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probe_length),
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std::memory_order_relaxed);
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info->total_probe_length.fetch_add(probe_length, std::memory_order_relaxed);
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info->size.fetch_add(1, std::memory_order_relaxed);
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}
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void SetHashtablezEnabled(bool enabled) {
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g_hashtablez_enabled.store(enabled, std::memory_order_release);
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}
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void SetHashtablezSampleParameter(int32_t rate) {
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if (rate > 0) {
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g_hashtablez_sample_parameter.store(rate, std::memory_order_release);
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} else {
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ABSL_RAW_LOG(ERROR, "Invalid hashtablez sample rate: %lld",
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static_cast<long long>(rate)); // NOLINT(runtime/int)
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}
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}
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void SetHashtablezMaxSamples(int32_t max) {
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if (max > 0) {
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g_hashtablez_max_samples.store(max, std::memory_order_release);
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} else {
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ABSL_RAW_LOG(ERROR, "Invalid hashtablez max samples: %lld",
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static_cast<long long>(max)); // NOLINT(runtime/int)
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}
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}
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} // namespace container_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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