How does SDUse work?

I’m reading souce code in llc’s SDUse.
The creation process of instruction‘s node
IR Instruction->SDNode->operands->SDUse->add operands’s SDNode’s UseList to SDUse’next.
But I cannot understand
1.How operand’s SDNode’s UseList is used.
2.UseList head’s **Pre is what?

/// Represents a use of a SDNode. This class holds an SDValue,
/// which records the SDNode being used and the result number, a
/// pointer to the SDNode using the value, and Next and Prev pointers,
/// which link together all the uses of an SDNode.
class SDUse {
  /// Val - The value being used.
  SDValue Val;
  /// User - The user of this value.
  SDNode *User = nullptr;
  /// Prev, Next - Pointers to the uses list of the SDNode referred by
  /// this operand.
  SDUse **Prev = nullptr;
  SDUse *Next = nullptr;

  SDUse() = default;
  SDUse(const SDUse &U) = delete;
  SDUse &operator=(const SDUse &) = delete;

  /// Normally SDUse will just implicitly convert to an SDValue that it holds.
  operator const SDValue&() const { return Val; }

  /// If implicit conversion to SDValue doesn't work, the get() method returns
  /// the SDValue.
  const SDValue &get() const { return Val; }

  /// This returns the SDNode that contains this Use.
  SDNode *getUser() { return User; }

  /// Get the next SDUse in the use list.
  SDUse *getNext() const { return Next; }

  /// Convenience function for get().getNode().
  SDNode *getNode() const { return Val.getNode(); }
  /// Convenience function for get().getResNo().
  unsigned getResNo() const { return Val.getResNo(); }
  /// Convenience function for get().getValueType().
  EVT getValueType() const { return Val.getValueType(); }

  /// Convenience function for get().operator==
  bool operator==(const SDValue &V) const {
    return Val == V;

  /// Convenience function for get().operator!=
  bool operator!=(const SDValue &V) const {
    return Val != V;

  /// Convenience function for get().operator<
  bool operator<(const SDValue &V) const {
    return Val < V;

  friend class SelectionDAG;
  friend class SDNode;
  // TODO: unfriend HandleSDNode once we fix its operand handling.
  friend class HandleSDNode;

  void setUser(SDNode *p) { User = p; }

  /// Remove this use from its existing use list, assign it the
  /// given value, and add it to the new value's node's use list.
  inline void set(const SDValue &V);
  /// Like set, but only supports initializing a newly-allocated
  /// SDUse with a non-null value.
  inline void setInitial(const SDValue &V);
  /// Like set, but only sets the Node portion of the value,
  /// leaving the ResNo portion unmodified.
  inline void setNode(SDNode *N);

  void addToList(SDUse **List) {
    Next = *List;
    if (Next) Next->Prev = &Next;
    Prev = List;
    *List = this;

  void removeFromList() {
    *Prev = Next;
    if (Next) Next->Prev = Prev;