1//===-- llvm/Use.h - Definition of the Use class ----------------*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9/// \file
10///
11/// This defines the Use class.  The Use class represents the operand of an
12/// instruction or some other User instance which refers to a Value.  The Use
13/// class keeps the "use list" of the referenced value up to date.
14///
15/// Pointer tagging is used to efficiently find the User corresponding to a Use
16/// without having to store a User pointer in every Use. A User is preceded in
17/// memory by all the Uses corresponding to its operands, and the low bits of
18/// one of the fields (Prev) of the Use class are used to encode offsets to be
19/// able to find that User given a pointer to any Use. For details, see:
20///
21///   http://www.llvm.org/docs/ProgrammersManual.html#UserLayout
22///
23//===----------------------------------------------------------------------===//
24
25#ifndef LLVM_IR_USE_H
26#define LLVM_IR_USE_H
27
28#include "llvm-c/Core.h"
29#include "llvm/ADT/PointerIntPair.h"
30#include "llvm/Support/CBindingWrapping.h"
31#include "llvm/Support/Compiler.h"
32#include <cstddef>
33#include <iterator>
34
35namespace llvm {
36
37class Value;
38class User;
39class Use;
40template <typename> struct simplify_type;
41
42// Use** is only 4-byte aligned.
43template <> class PointerLikeTypeTraits<Use **> {
44public:
45  static inline void *getAsVoidPointer(Use **P) { return P; }
46  static inline Use **getFromVoidPointer(void *P) {
47    return static_cast<Use **>(P);
48  }
49  enum { NumLowBitsAvailable = 2 };
50};
51
52/// \brief A Use represents the edge between a Value definition and its users.
53///
54/// This is notionally a two-dimensional linked list. It supports traversing
55/// all of the uses for a particular value definition. It also supports jumping
56/// directly to the used value when we arrive from the User's operands, and
57/// jumping directly to the User when we arrive from the Value's uses.
58///
59/// The pointer to the used Value is explicit, and the pointer to the User is
60/// implicit. The implicit pointer is found via a waymarking algorithm
61/// described in the programmer's manual:
62///
63///   http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm
64///
65/// This is essentially the single most memory intensive object in LLVM because
66/// of the number of uses in the system. At the same time, the constant time
67/// operations it allows are essential to many optimizations having reasonable
68/// time complexity.
69class Use {
70public:
71  /// \brief Provide a fast substitute to std::swap<Use>
72  /// that also works with less standard-compliant compilers
73  void swap(Use &RHS);
74
75  // A type for the word following an array of hung-off Uses in memory, which is
76  // a pointer back to their User with the bottom bit set.
77  typedef PointerIntPair<User *, 1, unsigned> UserRef;
78
79private:
80  Use(const Use &U) LLVM_DELETED_FUNCTION;
81
82  /// Destructor - Only for zap()
83  ~Use() {
84    if (Val)
85      removeFromList();
86  }
87
88  enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag };
89
90  /// Constructor
91  Use(PrevPtrTag tag) : Val(nullptr) { Prev.setInt(tag); }
92
93public:
94  operator Value *() const { return Val; }
95  Value *get() const { return Val; }
96
97  /// \brief Returns the User that contains this Use.
98  ///
99  /// For an instruction operand, for example, this will return the
100  /// instruction.
101  User *getUser() const;
102
103  inline void set(Value *Val);
104
105  Value *operator=(Value *RHS) {
106    set(RHS);
107    return RHS;
108  }
109  const Use &operator=(const Use &RHS) {
110    set(RHS.Val);
111    return *this;
112  }
113
114  Value *operator->() { return Val; }
115  const Value *operator->() const { return Val; }
116
117  Use *getNext() const { return Next; }
118
119  /// \brief Return the operand # of this use in its User.
120  unsigned getOperandNo() const;
121
122  /// \brief Initializes the waymarking tags on an array of Uses.
123  ///
124  /// This sets up the array of Uses such that getUser() can find the User from
125  /// any of those Uses.
126  static Use *initTags(Use *Start, Use *Stop);
127
128  /// \brief Destroys Use operands when the number of operands of
129  /// a User changes.
130  static void zap(Use *Start, const Use *Stop, bool del = false);
131
132private:
133  const Use *getImpliedUser() const;
134
135  Value *Val;
136  Use *Next;
137  PointerIntPair<Use **, 2, PrevPtrTag> Prev;
138
139  void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); }
140  void addToList(Use **List) {
141    Next = *List;
142    if (Next)
143      Next->setPrev(&Next);
144    setPrev(List);
145    *List = this;
146  }
147  void removeFromList() {
148    Use **StrippedPrev = Prev.getPointer();
149    *StrippedPrev = Next;
150    if (Next)
151      Next->setPrev(StrippedPrev);
152  }
153
154  friend class Value;
155};
156
157/// \brief Allow clients to treat uses just like values when using
158/// casting operators.
159template <> struct simplify_type<Use> {
160  typedef Value *SimpleType;
161  static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); }
162};
163template <> struct simplify_type<const Use> {
164  typedef /*const*/ Value *SimpleType;
165  static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); }
166};
167
168// Create wrappers for C Binding types (see CBindingWrapping.h).
169DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef)
170
171}
172
173#endif
174