//===-- llvm/Use.h - Definition of the Use class ----------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This defines the Use class.  The Use class represents the operand of an
// instruction or some other User instance which refers to a Value.  The Use
// class keeps the "use list" of the referenced value up to date.
//
// Pointer tagging is used to efficiently find the User corresponding
// to a Use without having to store a User pointer in every Use. A
// User is preceded in memory by all the Uses corresponding to its
// operands, and the low bits of one of the fields (Prev) of the Use
// class are used to encode offsets to be able to find that User given
// a pointer to any Use. For details, see:
//
//   http://www.llvm.org/docs/ProgrammersManual.html#UserLayout
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_USE_H
#define LLVM_USE_H

#include "llvm/ADT/PointerIntPair.h"
#include <cstddef>
#include <iterator>

namespace llvm {

class Value;
class User;
class Use;
template<typename>
struct simplify_type;

// Use** is only 4-byte aligned.
template<>
class PointerLikeTypeTraits<Use**> {
public:
  static inline void *getAsVoidPointer(Use** P) { return P; }
  static inline Use **getFromVoidPointer(void *P) {
    return static_cast<Use**>(P);
  }
  enum { NumLowBitsAvailable = 2 };
};

//===----------------------------------------------------------------------===//
//                                  Use Class
//===----------------------------------------------------------------------===//

/// Use is here to make keeping the "use" list of a Value up-to-date really
/// easy.
class Use {
public:
  /// swap - provide a fast substitute to std::swap<Use>
  /// that also works with less standard-compliant compilers
  void swap(Use &RHS);

  // A type for the word following an array of hung-off Uses in memory, which is
  // a pointer back to their User with the bottom bit set.
  typedef PointerIntPair<User*, 1, unsigned> UserRef;

private:
  /// Copy ctor - do not implement
  Use(const Use &U);

  /// Destructor - Only for zap()
  ~Use() {
    if (Val) removeFromList();
  }

  enum PrevPtrTag { zeroDigitTag
                  , oneDigitTag
                  , stopTag
                  , fullStopTag };

  /// Constructor
  Use(PrevPtrTag tag) : Val(0) {
    Prev.setInt(tag);
  }

public:
  /// Normally Use will just implicitly convert to a Value* that it holds.
  operator Value*() const { return Val; }
  
  /// If implicit conversion to Value* doesn't work, the get() method returns
  /// the Value*.
  Value *get() const { return Val; }
  
  /// getUser - This returns the User that contains this Use.  For an
  /// instruction operand, for example, this will return the instruction.
  User *getUser() const;

  inline void set(Value *Val);

  Value *operator=(Value *RHS) {
    set(RHS);
    return RHS;
  }
  const Use &operator=(const Use &RHS) {
    set(RHS.Val);
    return *this;
  }

        Value *operator->()       { return Val; }
  const Value *operator->() const { return Val; }

  Use *getNext() const { return Next; }

  
  /// initTags - initialize the waymarking tags on an array of Uses, so that
  /// getUser() can find the User from any of those Uses.
  static Use *initTags(Use *Start, Use *Stop);

  /// zap - This is used to destroy Use operands when the number of operands of
  /// a User changes.
  static void zap(Use *Start, const Use *Stop, bool del = false);

private:
  const Use* getImpliedUser() const;
  
  Value *Val;
  Use *Next;
  PointerIntPair<Use**, 2, PrevPtrTag> Prev;

  void setPrev(Use **NewPrev) {
    Prev.setPointer(NewPrev);
  }
  void addToList(Use **List) {
    Next = *List;
    if (Next) Next->setPrev(&Next);
    setPrev(List);
    *List = this;
  }
  void removeFromList() {
    Use **StrippedPrev = Prev.getPointer();
    *StrippedPrev = Next;
    if (Next) Next->setPrev(StrippedPrev);
  }

  friend class Value;
};

// simplify_type - Allow clients to treat uses just like values when using
// casting operators.
template<> struct simplify_type<Use> {
  typedef Value* SimpleType;
  static SimpleType getSimplifiedValue(const Use &Val) {
    return static_cast<SimpleType>(Val.get());
  }
};
template<> struct simplify_type<const Use> {
  typedef Value* SimpleType;
  static SimpleType getSimplifiedValue(const Use &Val) {
    return static_cast<SimpleType>(Val.get());
  }
};


template<typename UserTy>  // UserTy == 'User' or 'const User'
class value_use_iterator : public std::iterator<std::forward_iterator_tag,
                                                UserTy*, ptrdiff_t> {
  typedef std::iterator<std::forward_iterator_tag, UserTy*, ptrdiff_t> super;
  typedef value_use_iterator<UserTy> _Self;

  Use *U;
  explicit value_use_iterator(Use *u) : U(u) {}
  friend class Value;
public:
  typedef typename super::reference reference;
  typedef typename super::pointer pointer;

  value_use_iterator(const _Self &I) : U(I.U) {}
  value_use_iterator() {}

  bool operator==(const _Self &x) const {
    return U == x.U;
  }
  bool operator!=(const _Self &x) const {
    return !operator==(x);
  }

  /// atEnd - return true if this iterator is equal to use_end() on the value.
  bool atEnd() const { return U == 0; }

  // Iterator traversal: forward iteration only
  _Self &operator++() {          // Preincrement
    assert(U && "Cannot increment end iterator!");
    U = U->getNext();
    return *this;
  }
  _Self operator++(int) {        // Postincrement
    _Self tmp = *this; ++*this; return tmp;
  }

  // Retrieve a pointer to the current User.
  UserTy *operator*() const {
    assert(U && "Cannot dereference end iterator!");
    return U->getUser();
  }

  UserTy *operator->() const { return operator*(); }

  Use &getUse() const { return *U; }
  
  /// getOperandNo - Return the operand # of this use in its User.  Defined in
  /// User.h
  ///
  unsigned getOperandNo() const;
};

} // End llvm namespace

#endif