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From GitBox <...@apache.org>
Subject [GitHub] [incubator-tvm] junrushao1994 commented on a change in pull request #5740: [Object] Introduce POD-C Compliant tvm::Map
Date Tue, 16 Jun 2020 07:46:15 GMT

junrushao1994 commented on a change in pull request #5740:
URL: https://github.com/apache/incubator-tvm/pull/5740#discussion_r440651195



##########
File path: include/tvm/node/container.h
##########
@@ -43,51 +44,1188 @@ using runtime::Downcast;
 using runtime::IterAdapter;
 using runtime::make_object;
 using runtime::Object;
+using runtime::ObjectEqual;
+using runtime::ObjectHash;
 using runtime::ObjectPtr;
 using runtime::ObjectPtrEqual;
 using runtime::ObjectPtrHash;
 using runtime::ObjectRef;
 using runtime::String;
 using runtime::StringObj;
 
-/*! \brief String-aware ObjectRef hash functor */
-struct ObjectHash {
-  size_t operator()(const ObjectRef& a) const {
-    if (const auto* str = a.as<StringObj>()) {
-      return String::HashBytes(str->data, str->size);
-    }
-    return ObjectPtrHash()(a);
+#if (TVM_USE_STL_MAP != 0)
+
+/*! \brief Shared content of all specializations of hash map */
+class MapNode : public Object {
+ public:
+  /*! \brief Type of the keys in the hash map */
+  using key_type = ObjectRef;
+  /*! \brief Type of the values in the hash map */
+  using mapped_type = ObjectRef;
+  /*! \brief Type of the actual underlying container */
+  using ContainerType = std::unordered_map<ObjectRef, ObjectRef, ObjectHash, ObjectEqual>;
+  /*! \brief Iterator class */
+  using iterator = ContainerType::iterator;
+  /*! \brief Iterator class */
+  using const_iterator = ContainerType::const_iterator;
+  /*! \brief Type of value stored in the hash map */
+  using KVType = ContainerType::value_type;
+
+  static_assert(std::is_standard_layout<KVType>::value, "KVType is not standard layout");
+  static_assert(sizeof(KVType) == 16 || sizeof(KVType) == 8, "sizeof(KVType) incorrect");
+
+  static constexpr const uint32_t _type_index = runtime::TypeIndex::kRuntimeMap;
+  static constexpr const char* _type_key = "Map";
+  TVM_DECLARE_FINAL_OBJECT_INFO(MapNode, Object);
+
+  /*!
+   * \brief Number of elements in the SmallMapNode
+   * \return The result
+   */
+  size_t size() const { return data_.size(); }
+  /*!
+   * \brief Count the number of times a key exists in the hash map
+   * \param key The indexing key
+   * \return The result, 0 or 1
+   */
+  size_t count(const key_type& key) const { return data_.count(key); }
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The const reference to the value
+   */
+  const mapped_type& at(const key_type& key) const { return data_.at(key); }
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The mutable reference to the value
+   */
+  mapped_type& at(const key_type& key) { return data_.at(key); }
+  /*! \return begin iterator */
+  iterator begin() { return data_.begin(); }
+  /*! \return const begin iterator */
+  const_iterator begin() const { return data_.begin(); }
+  /*! \return end iterator */
+  iterator end() { return data_.end(); }
+  /*! \return end iterator */
+  const_iterator end() const { return data_.end(); }
+  /*!
+   * \brief Index value associated with a key
+   * \param key The indexing key
+   * \return The iterator of the entry associated with the key, end iterator if not exists
+   */
+  const_iterator find(const key_type& key) const { return data_.find(key); }
+  /*!
+   * \brief Index value associated with a key
+   * \param key The indexing key
+   * \return The iterator of the entry associated with the key, end iterator if not exists
+   */
+  iterator find(const key_type& key) { return data_.find(key); }
+  /*!
+   * \brief Erase the entry associated with the iterator
+   * \param position The iterator
+   */
+  void erase(const iterator& position) { data_.erase(position); }
+  /*!
+   * \brief Erase the entry associated with the key, do nothing if not exists
+   * \param key The indexing key
+   */
+  void erase(const key_type& key) { data_.erase(key); }
+
+ protected:
+  /*!
+   * \brief Create an empty container
+   * \return The object created
+   */
+  static ObjectPtr<MapNode> Empty() { return make_object<MapNode>(); }
+  /*!
+   * \brief Create the map using contents from the given iterators.
+   * \param first Begin of iterator
+   * \param last End of iterator
+   * \tparam IterType The type of iterator
+   * \return ObjectPtr to the map created
+   */
+  template <typename IterType>
+  static ObjectPtr<Object> CreateFromRange(IterType first, IterType last) {
+    ObjectPtr<MapNode> p = make_object<MapNode>();
+    p->data_ = std::unordered_map<ObjectRef, ObjectRef, ObjectHash, ObjectEqual>(first,
last);
+    return p;
+  }
+  /*!
+   * \brief InsertMaybeReHash an entry into the given hash map
+   * \param kv The entry to be inserted
+   * \param map The pointer to the map, can be changed if re-hashing happens
+   */
+  static void InsertMaybeReHash(const KVType& kv, ObjectPtr<Object>* map) {
+    MapNode* m = static_cast<MapNode*>(map->get());
+    m->data_[kv.first] = kv.second;
   }
+  /*!
+   * \brief Create an empty container with elements copying from another MapNode
+   * \param m The source container
+   * \return The object created
+   */
+  static ObjectPtr<MapNode> CopyFrom(MapNode* m) {
+    ObjectPtr<MapNode> p = make_object<MapNode>();
+    p->data_ = std::unordered_map<ObjectRef, ObjectRef, ObjectHash, ObjectEqual>(m->data_.begin(),
+                                                                                 m->data_.end());
+    return p;
+  }
+  /*! \brief The real container storing data */
+  ContainerType data_;
+  template <typename, typename, typename, typename>
+  friend class Map;
 };
 
-/*! \brief String-aware ObjectRef equal functor */
-struct ObjectEqual {
-  bool operator()(const ObjectRef& a, const ObjectRef& b) const {
-    if (a.same_as(b)) {
-      return true;
+#else
+
+/*! \brief Shared content of all specializations of hash map */
+class MapNode : public Object {
+ public:
+  /*! \brief Type of the keys in the hash map */
+  using key_type = ObjectRef;
+  /*! \brief Type of the values in the hash map */
+  using mapped_type = ObjectRef;
+  /*! \brief Type of value stored in the hash map */
+  using KVType = std::pair<ObjectRef, ObjectRef>;
+  /*! \brief Iterator class */
+  class iterator;
+
+  static_assert(std::is_standard_layout<KVType>::value, "KVType is not standard layout");
+  static_assert(sizeof(KVType) == 16 || sizeof(KVType) == 8, "sizeof(KVType) incorrect");
+
+  static constexpr const uint32_t _type_index = runtime::TypeIndex::kRuntimeMap;
+  static constexpr const char* _type_key = "Map";
+  TVM_DECLARE_FINAL_OBJECT_INFO(MapNode, Object);
+
+  /*!
+   * \brief Number of elements in the SmallMapNode
+   * \return The result
+   */
+  size_t size() const { return size_; }
+  /*!
+   * \brief Count the number of times a key exists in the hash map
+   * \param key The indexing key
+   * \return The result, 0 or 1
+   */
+  size_t count(const key_type& key) const;
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The const reference to the value
+   */
+  const mapped_type& at(const key_type& key) const;
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The mutable reference to the value
+   */
+  mapped_type& at(const key_type& key);
+  /*! \return begin iterator */
+  iterator begin() const;
+  /*! \return end iterator */
+  iterator end() const;
+  /*!
+   * \brief Index value associated with a key
+   * \param key The indexing key
+   * \return The iterator of the entry associated with the key, end iterator if not exists
+   */
+  iterator find(const key_type& key) const;
+  /*!
+   * \brief Erase the entry associated with the iterator
+   * \param position The iterator
+   */
+  void erase(const iterator& position);
+  /*!
+   * \brief Erase the entry associated with the key, do nothing if not exists
+   * \param key The indexing key
+   */
+  void erase(const key_type& key) { erase(find(key)); }
+
+  class iterator {
+   public:
+    using iterator_category = std::forward_iterator_tag;
+    using difference_type = int64_t;
+    using value_type = KVType;
+    using pointer = KVType*;
+    using reference = KVType&;
+    /*! \brief Default constructor */
+    iterator() : i(0), self(nullptr) {}
+    /*! \brief Compare iterators */
+    bool operator==(const iterator& other) const { return i == other.i && self
== other.self; }
+    /*! \brief Compare iterators */
+    bool operator!=(const iterator& other) const { return !(*this == other); }
+    /*! \brief De-reference iterators */
+    pointer operator->() const;
+    /*! \brief De-reference iterators */
+    reference operator*() const { return *((*this).operator->()); }
+    /*! \brief Prefix self increment, e.g. ++iter */
+    iterator& operator++();
+    /*! \brief Prefix self decrement, e.g. --iter */
+    iterator& operator--();
+    /*! \brief Suffix self increment */
+    iterator operator++(int) {
+      iterator copy = *this;
+      ++(*this);
+      return copy;
     }
-    if (const auto* str_a = a.as<StringObj>()) {
-      if (const auto* str_b = b.as<StringObj>()) {
-        return String::memncmp(str_a->data, str_b->data, str_a->size, str_b->size)
== 0;
+    /*! \brief Suffix self decrement */
+    iterator operator--(int) {
+      iterator copy = *this;
+      --(*this);
+      return copy;
+    }
+
+   protected:
+    /*! \brief Construct by value */
+    iterator(uint64_t i, const MapNode* self) : i(i), self(self) {}
+    /*! \brief The position on the array */
+    uint64_t i;
+    /*! \brief The container it points to */
+    const MapNode* self;
+
+    friend class DenseMapNode;
+    friend class SmallMapNode;
+  };
+
+ protected:
+  /*!
+   * \brief Create an empty container
+   * \return The object created
+   */
+  static ObjectPtr<MapNode> Empty();
+  /*!
+   * \brief Create the map using contents from the given iterators.
+   * \param first Begin of iterator
+   * \param last End of iterator
+   * \tparam IterType The type of iterator
+   * \return ObjectPtr to the map created
+   */
+  template <typename IterType>
+  static ObjectPtr<Object> CreateFromRange(IterType first, IterType last);
+  /*!
+   * \brief InsertMaybeReHash an entry into the given hash map
+   * \param kv The entry to be inserted
+   * \param map The pointer to the map, can be changed if re-hashing happens
+   */
+  static void InsertMaybeReHash(const KVType& kv, ObjectPtr<Object>* map);
+  /*!
+   * \brief Create an empty container with elements copying from another SmallMapNode
+   * \param m The source container
+   * \return The object created
+   */
+  static ObjectPtr<MapNode> CopyFrom(MapNode* m);
+  /*! \brief number of slots minus 1 */
+  uint64_t slots_;
+  /*! \brief number of entries in the container */
+  uint64_t size_;
+  // Reference class
+  template <typename, typename, typename, typename>
+  friend class Map;
+};
+
+/*! \brief A specialization of small-sized hash map */
+class SmallMapNode : public MapNode,
+                     public runtime::InplaceArrayBase<SmallMapNode, MapNode::KVType>
{
+ private:
+  static constexpr uint64_t kInitSize = 2;
+  static constexpr uint64_t kMaxSize = 4;
+
+ public:
+  using MapNode::iterator;
+  using MapNode::KVType;
+
+  /*! \brief Defaults to the destructor of InplaceArrayBase */
+  ~SmallMapNode() = default;
+  /*!
+   * \brief Count the number of times a key exists in the SmallMapNode
+   * \param key The indexing key
+   * \return The result, 0 or 1
+   */
+  size_t count(const key_type& key) const { return find(key).i < size_; }
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The const reference to the value
+   */
+  const mapped_type& at(const key_type& key) const {
+    iterator itr = find(key);
+    CHECK(itr.i < size_) << "IndexError: key is not in Map";
+    return itr->second;
+  }
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The mutable reference to the value
+   */
+  mapped_type& at(const key_type& key) {
+    iterator itr = find(key);
+    CHECK(itr.i < size_) << "IndexError: key is not in Map";
+    return itr->second;
+  }
+  /*! \return begin iterator */
+  iterator begin() const { return iterator(0, this); }
+  /*! \return end iterator */
+  iterator end() const { return iterator(size_, this); }
+  /*!
+   * \brief Index value associated with a key
+   * \param key The indexing key
+   * \return The iterator of the entry associated with the key, end iterator if not exists
+   */
+  iterator find(const key_type& key) const {
+    KVType* ptr = static_cast<KVType*>(AddressOf(0));
+    for (uint64_t i = 0; i < size_; ++i, ++ptr) {
+      if (ObjectEqual()(ptr->first, key)) {
+        return iterator(i, this);
       }
     }
-    return false;
+    return iterator(size_, this);
+  }
+  /*!
+   * \brief Erase the entry associated with the iterator
+   * \param position The iterator
+   */
+  void erase(const iterator& position) { Erase(position.i); }
+
+ private:
+  /*!
+   * \brief Remove a position in SmallMapNode
+   * \param i The position to be removed
+   */
+  void Erase(const uint64_t i) {
+    if (i >= size_) {
+      return;
+    }
+    KVType* begin = static_cast<KVType*>(AddressOf(0));
+    KVType* last = begin + (size_ - 1);
+    if (i + 1 == size_) {
+      last->first.ObjectRef::~ObjectRef();
+      last->second.ObjectRef::~ObjectRef();
+    } else {
+      *(begin + i) = std::move(*last);
+    }
+    size_ -= 1;
+  }
+  /*!
+   * \brief Create an empty container
+   * \param n Number of empty slots
+   * \return The object created
+   */
+  static ObjectPtr<SmallMapNode> Empty(uint64_t n = kInitSize) {
+    using ::tvm::runtime::make_inplace_array_object;
+    ObjectPtr<SmallMapNode> p = make_inplace_array_object<SmallMapNode, KVType>(n);
+    p->size_ = 0;
+    p->slots_ = n;
+    return p;
+  }
+  /*!
+   * \brief Create an empty container initialized with a given range
+   * \param n Number of empty slots
+   * \param first begin of iterator
+   * \param last end of iterator
+   * \tparam IterType The type of iterator
+   * \return The object created
+   */
+  template <typename IterType>
+  static ObjectPtr<SmallMapNode> CreateFromRange(uint64_t n, IterType first, IterType
last) {
+    ObjectPtr<SmallMapNode> p = Empty(n);
+    KVType* ptr = static_cast<KVType*>(p->AddressOf(0));
+    for (; first != last; ++first, ++p->size_) {
+      new (ptr++) KVType(*first);
+    }
+    return p;
+  }
+  /*!
+   * \brief Create an empty container with elements copying from another SmallMapNode
+   * \param m The source container
+   * \return The object created
+   */
+  static ObjectPtr<SmallMapNode> CopyFrom(SmallMapNode* m) {
+    KVType* first = static_cast<KVType*>(m->AddressOf(0));
+    KVType* last = first + m->size_;
+    return CreateFromRange(m->size_, first, last);
+  }
+  /*!
+   * \brief InsertMaybeReHash an entry into the given hash map
+   * \param kv The entry to be inserted
+   * \param map The pointer to the map, can be changed if re-hashing happens
+   */
+  static void InsertMaybeReHash(const KVType& kv, ObjectPtr<Object>* map) {
+    SmallMapNode* m = static_cast<SmallMapNode*>(map->get());
+    iterator itr = m->find(kv.first);
+    if (itr.i < m->size_) {
+      itr->second = kv.second;
+      return;
+    }
+    if (m->size_ < m->slots_) {
+      KVType* ptr = static_cast<KVType*>(m->AddressOf(m->size_));
+      new (ptr) KVType(kv);
+      ++m->size_;
+      return;
+    }
+    uint64_t next_size = std::max(m->slots_ * 2, uint64_t(kInitSize));
+    next_size = std::min(next_size, uint64_t(kMaxSize));
+    CHECK_GT(next_size, m->slots_);
+    ObjectPtr<Object> n = CreateFromRange(next_size, m->begin(), m->end());
+    InsertMaybeReHash(kv, &n);
+    *map = std::move(n);
   }
+  /*!
+   * \brief Increment the pointer
+   * \param i The pointer to be incremented
+   * \return The increased pointer
+   */
+  uint64_t IncItr(uint64_t i) const { return i + 1 < size_ ? i + 1 : size_; }
+  /*!
+   * \brief Decrement the pointer
+   * \param i The pointer to be decremented
+   * \return The decreased pointer
+   */
+  uint64_t DecItr(uint64_t i) const { return i > 0 ? i - 1 : size_; }
+  /*!
+   * \brief De-reference the pointer
+   * \param i The pointer to be dereferenced
+   * \return The result
+   */
+  KVType* DeRefItr(uint64_t i) const { return static_cast<KVType*>(AddressOf(i)); }
+  /*! \brief A size function used by InplaceArrayBase */
+  uint64_t GetSize() const { return size_; }
+
+ protected:
+  friend class MapNode;
+  friend class DenseMapNode;
+  friend ObjectPtr<MapNode> runtime::make_object<>();
+  friend class runtime::InplaceArrayBase<SmallMapNode, MapNode::KVType>;
 };
 
-/*! \brief map node content */
-class MapNode : public Object {
+/*! \brief A specialization of hash map that implements the idea of array-based hash map.
+ * Another reference implementation can be found [1].
+ *
+ * A. Overview
+ *
+ * DenseMapNode did several improvements over traditional separate chaining hash,
+ * in terms of cache locality, memory footprints and data organization.
+ *
+ * A1. Implicit linked list. For better cache locality, instead of using linked list
+ * explicitly for each bucket, we store list data into a single array that spans contiguously
+ * in memory, and then carefully design access patterns to make sure most of them fall into
+ * a single cache line.
+ *
+ * A2. 1-byte metadata. There is only 1 byte overhead for each slot in the array to indexing
and
+ * traversal. This can be divided in 3 parts.
+ * 1) Reserved code: (0b11111111)_2 indicates a slot is empty; (0b11111110)_2 indicates protected,
+ * which means the slot is empty but not allowed to be written.
+ * 2) If not empty or protected, the highest bit is used to indicate whether data in the
slot is
+ * head of a linked list.
+ * 3) The rest 7 bits are used as the "next pointer" (i.e. pointer to the next element).
On 64-bit
+ * architecture, an ordinary pointer can take up to 8 bytes, which is not acceptable overhead
when
+ * dealing with 16-byte ObjectRef pairs. Based on a commonly noticed fact that the lists
are
+ * relatively short (length <= 3) in hash maps, we follow [1]'s idea that only allows
the pointer to
+ * be one of the 126 possible values, i.e. if the next element of i-th slot is (i + x)-th
element,
+ * then x must be one of the 126 pre-defined values.
+ *
+ * A3. Data blocking. We organize the array in the way that every 16 elements forms a data
block.
+ * The 16-byte metadata of those 16 elements are stored together, followed by the real data,
i.e.
+ * 16 key-value pairs.
+ *
+ * B. Implementation details
+ *
+ * B1. Power-of-2 table size and Fibonacci Hashing. We use power-of-two as table size to
avoid
+ * modulo for more efficient arithmetics. To make the hash-to-slot mapping distribute more
evenly,
+ * we use the Fibonacci Hashing [2] trick.
+ *
+ * B2. Traverse a linked list in the array.
+ * 1) List head. Assume Fibonacci Hashing maps a given key to slot i, if metadata at slot
i
+ * indicates that it is list head, then we found the head; otherwise the list is empty. No
probing
+ * is done in this procedure. 2) Next element. To find the next element of a non-empty slot
i, we
+ * look at the last 7 bits of the metadata at slot i. If they are all zeros, then it is the
end of
+ * list; otherwise, we know that the next element is (i + candidates[the-last-7-bits]).
+ *
+ * B3. InsertMaybeReHash an element. Following B2, we first traverse the linked list to see
if this
+ * element is in the linked list, and if not, we put it at the end by probing the next empty
+ * position in one of the 126 candidate positions. If the linked list does not even exist,
but the
+ * slot for list head has been occupied by another linked list, we should find this intruder
another
+ * place.
+ *
+ * B4. Quadratic probing with triangle numbers. In open address hashing, it is provable that
probing
+ * with triangle numbers can traverse power-of-2-sized table [3]. In our algorithm, we follow
the
+ * suggestion in [1] that also use triangle numbers for "next pointer" as well as sparing
for list
+ * head.
+ *
+ * [1] https://github.com/skarupke/flat_hash_map
+ * [2] https://programmingpraxis.com/2018/06/19/fibonacci-hash/
+ * [3] https://fgiesen.wordpress.com/2015/02/22/triangular-numbers-mod-2n/
+ */
+class DenseMapNode : public MapNode {
+ private:
+  /*! \brief The number of elements in a memory block */
+  static constexpr int kBlockCap = 16;
+  /*! \brief Maximum load factor of the hash map */
+  static constexpr double kMaxLoadFactor = 0.99;
+  /*! \brief Binary representation of the metadata of an empty slot */
+  static constexpr uint8_t kEmptySlot = uint8_t(0b11111111);
+  /*! \brief Binary representation of the metadata of a protected slot */
+  static constexpr uint8_t kProtectedSlot = uint8_t(0b11111110);
+  /*! \brief Number of probing choices available */
+  static constexpr int kNumJumpDists = 126;
+  /*! \brief Head of the implicit linked list */
+  struct ListNode;
+  /*! \brief POD type of a block of memory */
+  struct Block {
+    uint8_t b[kBlockCap + kBlockCap * sizeof(KVType)];
+  };
+  static_assert(sizeof(Block) == kBlockCap * (sizeof(KVType) + 1), "sizeof(Block) incorrect");
+  static_assert(std::is_standard_layout<Block>::value, "Block is not standard layout");
+
  public:
-  /*! \brief The corresponding conatiner type */
-  using ContainerType = std::unordered_map<ObjectRef, ObjectRef, ObjectHash, ObjectEqual>;
+  using MapNode::iterator;
 
-  /*! \brief the data content */
-  ContainerType data;
+  /*!
+   * \brief Destroy the DenseMapNode
+   */
+  ~DenseMapNode() { this->Reset(); }
+  /*! \return The number of elements of the key */
+  size_t count(const key_type& key) const { return !Search(key).IsNone(); }
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The const reference to the value
+   */
+  const mapped_type& at(const key_type& key) const { return At(key); }
+  /*!
+   * \brief Index value associated with a key, throw exception if the key does not exist
+   * \param key The indexing key
+   * \return The mutable reference to the value
+   */
+  mapped_type& at(const key_type& key) { return At(key); }
+  /*!
+   * \brief Index value associated with a key
+   * \param key The indexing key
+   * \return The iterator of the entry associated with the key, end iterator if not exists
+   */
+  iterator find(const key_type& key) const {
+    ListNode n = Search(key);
+    return n.IsNone() ? end() : iterator(n.index, this);
+  }
+  /*!
+   * \brief Erase the entry associated with the iterator
+   * \param position The iterator
+   */
+  void erase(const iterator& position) {
+    uint64_t i = position.i;
+    if (position.self != nullptr && i <= this->slots_) {
+      Erase(ListNode(i, this));
+    }
+  }
+  /*! \return begin iterator */
+  iterator begin() const {
+    if (slots_ == 0) {
+      return iterator(0, this);
+    }
+    for (uint64_t i = 0; i <= slots_; ++i) {
+      if (!ListNode(i, this).IsEmpty()) {
+        return iterator(i, this);
+      }
+    }
+    return iterator(slots_ + 1, this);
+  }
+  /*! \return end iterator */
+  iterator end() const { return slots_ == 0 ? iterator(0, this) : iterator(slots_ + 1, this);
}
 
-  static constexpr const char* _type_key = "Map";
-  TVM_DECLARE_FINAL_OBJECT_INFO(MapNode, Object);
+ private:
+  /*!
+   * \brief Search for the given key
+   * \param key The key
+   * \return ListNode that associated with the key
+   */
+  ListNode Search(const key_type& key) const {
+    if (this->size_ == 0) {
+      return ListNode();
+    }
+    for (ListNode n = GetListHead(ObjectHash()(key)); !n.IsNone(); n.MoveToNext(this)) {
+      if (ObjectEqual()(key, n.Key())) {
+        return n;
+      }
+    }
+    return ListNode();
+  }
+  /*!
+   * \brief Search for the given key, throw exception if not exists
+   * \param key The key
+   * \return ListNode that associated with the key
+   */
+  mapped_type& At(const key_type& key) const {
+    ListNode n = Search(key);
+    CHECK(!n.IsNone()) << "IndexError: key is not in Map";
+    return n.Val();
+  }
+  /*!
+   * \brief Try to insert a key, or do nothing if already exists
+   * \param key The indexing key
+   * \param result The linked-list entry found or just constructed
+   * \return A boolean, indicating if actual insertion happens
+   */
+  bool TryInsert(const key_type& key, ListNode* result) {
+    if (slots_ == 0) {
+      return false;
+    }
+    // required that `m` to be the head of a linked list through which we can iterator
+    ListNode m = IndexFromhash(ObjectHash()(key));
+    // `m` can be: 1) empty; 2) body of an irrelevant list; 3) head of the relevant list
+    // Case 1: empty
+    if (m.IsEmpty()) {
+      m.NewHead(KVType(key, ObjectRef(nullptr)));
+      this->size_ += 1;
+      *result = m;
+      return true;
+    }
+    // Case 2: body of an irrelevant list
+    if (!m.IsHead()) {
+      // we move the elements around and construct the single-element linked list
+      return IsFull() ? false : TrySpareListHead(m, key, result);
+    }
+    // Case 3: head of the relevant list
+    // we iterate through the linked list until the end
+    ListNode n = m;
+    do {
+      // find equal item, do not insert
+      if (ObjectEqual()(key, n.Key())) {
+        *result = n;
+        return true;
+      }
+      // make sure `m` is the previous element of `n`
+      m = n;
+    } while (n.MoveToNext(this));
+    // `m` is the tail of the linked list
+    // always check capacity before insertion
+    if (IsFull()) {
+      return false;
+    }
+    // find the next empty slot
+    uint8_t jump;
+    if (!m.GetNextEmpty(this, &jump, result)) {
+      return false;
+    }
+    result->NewTail(KVType(key, ObjectRef(nullptr)));
+    // link `n` to `empty`, and move forward
+    m.SetJump(jump);
+    this->size_ += 1;
+    return true;
+  }
+  /*!
+   * \brief Spare an entry to be the head of a linked list.
+   * As described in B3, during insertion, it is possible that the entire linked list does
not
+   * exist, but the slot of its head has been occupied by other linked lists. In this case,
we need
+   * to spare the slot by moving away the elements to another valid empty one to make insertion
+   * possible. \param n The given entry to be spared \param key The indexing key \param result
The
+   * linked-list entry constructed as the head \return A boolean, if actual insertion happens
+   */
+  bool TrySpareListHead(ListNode n, const key_type& key, ListNode* result) {
+    // `n` is not the head of the linked list
+    // move the original item of `n` (if any)
+    // and construct new item on the position `n`
+    // To make `n` empty, we
+    // 1) find `w` the previous element of `n` in the linked list
+    // 2) copy the linked list starting from `r = n`
+    // 3) paste them after `w`
+    // read from the linked list after `r`
+    ListNode r = n;
+    // write to the tail of `w`
+    ListNode w = n.GetPrev(this);
+    // after `n` is moved, we disallow writing to the slot
+    bool is_first = true;
+    uint8_t r_meta, jump;
+    ListNode empty;
+    do {
+      // `jump` describes how `w` is jumped to `empty`
+      // rehash if there is no empty space after `w`
+      if (!w.GetNextEmpty(this, &jump, &empty)) {
+        return false;
+      }
+      // move `r` to `empty`
+      empty.NewTail(std::move(r.Data()));
+      // clear the metadata of `r`
+      r_meta = r.Meta();
+      if (is_first) {
+        is_first = false;
+        r.SetProtected();
+      } else {
+        r.SetEmpty();
+      }
+      // link `w` to `empty`, and move forward
+      w.SetJump(jump);
+      w = empty;
+      // move `r` forward as well
+    } while (r.MoveToNext(this, r_meta));
+    // finally we have done moving the linked list
+    // fill data_ into `n`
+    n.NewHead(KVType(key, ObjectRef(nullptr)));
+    this->size_ += 1;
+    *result = n;
+    return true;
+  }
+  /*!
+   * \brief Remove a ListNode
+   * \param n The node to be removed
+   */
+  void Erase(const ListNode& n) {
+    this->size_ -= 1;
+    if (!n.HasNext()) {
+      // `n` is the last
+      if (!n.IsHead()) {
+        // cut the link if there is any
+        n.GetPrev(this).SetJump(0);
+      }
+      n.Data().KVType::~KVType();
+      n.SetEmpty();
+    } else {
+      ListNode last = n, prev = n;
+      for (last.MoveToNext(this); last.HasNext(); prev = last, last.MoveToNext(this)) {
+      }
+      n.Data() = std::move(last.Data());
+      last.SetEmpty();
+      prev.SetJump(0);
+    }
+  }
+  /*! \brief Clear the container to empty, release all entries and memory acquired */
+  void Reset() {
+    uint64_t n_blocks = CalcNumBlocks(this->slots_);
+    DenseMapNode* m = this;
+    for (uint64_t bi = 0; bi < n_blocks; ++bi) {
+      uint8_t* m_m = m->data_[bi].b;
+      KVType* m_d = reinterpret_cast<KVType*>(m->data_[bi].b + kBlockCap);
+      for (int j = 0; j < kBlockCap; ++j, ++m_m, ++m_d) {
+        uint8_t& meta = *m_m;
+        if (meta != uint8_t(kProtectedSlot) && meta != uint8_t(kEmptySlot)) {
+          meta = uint8_t(kEmptySlot);
+          m_d->KVType::~KVType();
+        }
+      }
+    }
+    delete[] data_;
+    data_ = nullptr;
+    slots_ = 0;
+    size_ = 0;
+    fib_shift_ = 63;
+  }
+  /*!
+   * \brief Create an empty container
+   * \param fib_shift The fib shift provided
+   * \param n_slots Number of slots required, should be power-of-two
+   * \return The object created
+   */
+  static ObjectPtr<DenseMapNode> Empty(uint32_t fib_shift, uint64_t n_slots) {
+    CHECK_GT(n_slots, uint64_t(SmallMapNode::kMaxSize));
+    CHECK_EQ((n_slots & -n_slots), n_slots);
+    ObjectPtr<DenseMapNode> p = make_object<DenseMapNode>();
+    uint64_t n_blocks = CalcNumBlocks(n_slots - 1);
+    Block* block = p->data_ = new Block[n_blocks];
+    p->slots_ = n_slots - 1;
+    p->size_ = 0;
+    p->fib_shift_ = fib_shift;
+    for (uint64_t i = 0; i < n_blocks; ++i, ++block) {
+      std::fill(block->b, block->b + kBlockCap, uint8_t(kEmptySlot));
+    }
+    return p;
+  }
+  /*!
+   * \brief Create an empty container with elements copying from another DenseMapNode
+   * \param m The source container
+   * \return The object created
+   */
+  static ObjectPtr<DenseMapNode> CopyFrom(DenseMapNode* m) {
+    ObjectPtr<DenseMapNode> p = make_object<DenseMapNode>();
+    uint64_t n_blocks = CalcNumBlocks(m->slots_);
+    p->data_ = new Block[n_blocks];
+    p->slots_ = m->slots_;
+    p->size_ = m->size_;
+    p->fib_shift_ = m->fib_shift_;
+    for (uint64_t bi = 0; bi < n_blocks; ++bi) {
+      uint8_t* m_m = m->data_[bi].b;
+      uint8_t* p_m = p->data_[bi].b;
+      KVType* m_d = reinterpret_cast<KVType*>(m->data_[bi].b + kBlockCap);
+      KVType* p_d = reinterpret_cast<KVType*>(p->data_[bi].b + kBlockCap);
+      for (int j = 0; j < kBlockCap; ++j, ++m_m, ++m_d, ++p_m, ++p_d) {
+        uint8_t& meta = *p_m = *m_m;
+        CHECK(meta != kProtectedSlot);
+        if (meta != uint8_t(kEmptySlot)) {
+          new (p_d) KVType(*m_d);
+        }
+      }
+    }
+    return p;
+  }
+  /*!
+   * \brief InsertMaybeReHash an entry into the given hash map
+   * \param kv The entry to be inserted
+   * \param map The pointer to the map, can be changed if re-hashing happens
+   */
+  static void InsertMaybeReHash(const KVType& kv, ObjectPtr<Object>* map) {
+    DenseMapNode* m = static_cast<DenseMapNode*>(map->get());
+    ListNode n;
+    if (m->TryInsert(kv.first, &n)) {
+      n.Val() = kv.second;
+      return;
+    }
+    CHECK_GT(m->slots_, uint64_t(SmallMapNode::kMaxSize));
+    ObjectPtr<Object> p = Empty(m->fib_shift_ - 1, m->slots_ * 2 + 2);
+    InsertMaybeReHash(kv, &p);
+    uint64_t n_blocks = CalcNumBlocks(m->slots_);
+    for (uint64_t bi = 0; bi < n_blocks; ++bi) {

Review comment:
       It is an infrequent but performance critical path (10%-20% time), using iterators may
introduce some code that cannot be optimized out by the compiler.




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