EVOLUTION-MANAGER

Edit File: refcounting_hash_map.h

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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http://www.apache.org/licenses/LICENSE-2.0

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distributed under the License is distributed on an "AS IS" BASIS,
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==============================================================================*/

#ifndef TENSORFLOW_COMPILER_XLA_REFCOUNTING_HASH_MAP_H_
#define TENSORFLOW_COMPILER_XLA_REFCOUNTING_HASH_MAP_H_

#include <functional>
#include <memory>

#include "absl/base/thread_annotations.h"
#include "absl/container/node_hash_map.h"
#include "absl/memory/memory.h"
#include "absl/synchronization/mutex.h"

namespace xla {

// RefcountingHashMap is an "eager, thread-safe cache".
//
// Given a key k you can retrieve a shared_ptr to a value v.  If k is not
// already in the map, we construct a new V; if it is already in the map, we'll
// return the existing v.  Once all shared_ptrs are destroyed, the entry is
// removed from the map.
//
// This class is thread-safe.
//
// Word to the wise: You might want an erase() function here that removes a
// value from the map but leaves existing shared_ptrs intact.  My experience is,
// this is extremely complicated to implement correctly.
template <typename K, typename V>
class RefcountingHashMap {
 public:
  // Default-constructs new values.
  RefcountingHashMap() = default;

// Not copyable or movable because this contains internal pointers (namely,
  // instances of Deleter contain pointers to `this` and into `map_`).
  RefcountingHashMap(const RefcountingHashMap&) = delete;
  RefcountingHashMap(RefcountingHashMap&&) = delete;
  RefcountingHashMap& operator=(const RefcountingHashMap&) = delete;
  RefcountingHashMap& operator=(RefcountingHashMap&&) = delete;

// Gets the value for the given key.
  //
  // If the map doesn't contain a live value for the key, constructs one
  // using `value_factory`.
  std::shared_ptr<V> GetOrCreateIfAbsent(
      const K& key,
      const std::function<std::unique_ptr<V>(const K&)>& value_factory) {
    absl::MutexLock lock(&mu_);
    auto it = map_.find(key);
    // We ensure that the entry has not expired in case deleter was running when
    // we have entered this block.
    if (it != map_.end()) {
      if (std::shared_ptr<V> value = it->second.lock()) {
        return value;
      }
      map_.erase(it);
    }

// Create entry in the map and then set its value, so the value can
    // contain a pointer back into the map.
    it = map_.emplace(key, std::weak_ptr<V>()).first;
    std::shared_ptr<V> value(value_factory(key).release(),
                             Deleter{&it->first, this});
    it->second = value;  // Set the weak ptr to the shared ptr.
    return value;
  }

// Runs a function over every key/value in the map.
  //
  // Touching the map from within this function may deadlock; don't do it.
  //
  // Function signature must be compatible with
  //   void fn(const K&, std::shared_ptr<V>)
  //
  template <typename Fn>
  void ForEach(Fn&& fn) {
    absl::MutexLock lock(&mu_);
    for (const auto& kv : map_) {
      fn(kv.first, kv.second.lock());
    }
  }

private:
  struct Deleter {
    const K* key;  // Points into parent->map_.
    RefcountingHashMap* parent;

void operator()(V* v) {
      delete v;
      absl::MutexLock lock(&parent->mu_);
      auto it = parent->map_.find(*key);
      if (it != parent->map_.end() && it->second.expired()) {
        parent->map_.erase(it);
      }
    }
  };

absl::Mutex mu_;
  absl::node_hash_map<K, std::weak_ptr<V>> map_ ABSL_GUARDED_BY(mu_);
};

}  // namespace xla

#endif  // TENSORFLOW_COMPILER_XLA_REFCOUNTING_HASH_MAP_H_