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// Copyright 2021 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
//
//
// 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_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
#define ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_
#include <atomic>
#include <cstdint>
#include "absl/base/config.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {
// CordzUpdateTracker tracks counters for Cord update methods.
//
// The purpose of CordzUpdateTracker is to track the number of calls to methods
// updating Cord data for sampled cords. The class internally uses 'lossy'
// atomic operations: Cord is thread-compatible, so there is no need to
// synchronize updates. However, Cordz collection threads may call 'Value()' at
// any point, so the class needs to provide thread safe access.
//
// This class is thread-safe. But as per above comments, all non-const methods
// should be used single-threaded only: updates are thread-safe but lossy.
class CordzUpdateTracker {
public:
// Tracked update methods.
enum MethodIdentifier {
kUnknown,
kAppendCord,
kAppendCordBuffer,
kAppendExternalMemory,
kAppendString,
kAssignCord,
kAssignString,
kClear,
kConstructorCord,
kConstructorString,
kCordReader,
kFlatten,
kGetAppendBuffer,
kGetAppendRegion,
kMakeCordFromExternal,
kMoveAppendCord,
kMoveAssignCord,
kMovePrependCord,
kPrependCord,
kPrependCordBuffer,
kPrependString,
kRemovePrefix,
kRemoveSuffix,
kSetExpectedChecksum,
kSubCord,
// kNumMethods defines the number of entries: must be the last entry.
kNumMethods,
};
// Constructs a new instance. All counters are zero-initialized.
constexpr CordzUpdateTracker() noexcept : values_{} {}
// Copy constructs a new instance.
CordzUpdateTracker(const CordzUpdateTracker& rhs) noexcept { *this = rhs; }
// Assigns the provided value to this instance.
CordzUpdateTracker& operator=(const CordzUpdateTracker& rhs) noexcept {
for (int i = 0; i < kNumMethods; ++i) {
values_[i].store(rhs.values_[i].load(std::memory_order_relaxed),
std::memory_order_relaxed);
}
return *this;
}
// Returns the value for the specified method.
int64_t Value(MethodIdentifier method) const {
return values_[method].load(std::memory_order_relaxed);
}
// Increases the value for the specified method by `n`
void LossyAdd(MethodIdentifier method, int64_t n = 1) {
auto& value = values_[method];
value.store(value.load(std::memory_order_relaxed) + n,
std::memory_order_relaxed);
}
// Adds all the values from `src` to this instance
void LossyAdd(const CordzUpdateTracker& src) {
for (int i = 0; i < kNumMethods; ++i) {
MethodIdentifier method = static_cast<MethodIdentifier>(i);
if (int64_t value = src.Value(method)) {
LossyAdd(method, value);
}
}
}
private:
// Until C++20 std::atomic is not constexpr default-constructible, so we need
// a wrapper for this class to be constexpr constructible.
class Counter : public std::atomic<int64_t> {
public:
constexpr Counter() noexcept : std::atomic<int64_t>(0) {}
};
Counter values_[kNumMethods];
};
} // namespace cord_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_