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/ADT/PointerIntPair.h"
29 #include "llvm/Support/CBindingWrapping.h"
30 #include "llvm/Support/Compiler.h"
31 #include <cstddef>
32 #include <iterator>
33 
34 namespace llvm {
35 
36 class Value;
37 class User;
38 class Use;
39 template <typename> struct simplify_type;
40 
41 // Use** is only 4-byte aligned.
42 template <> class PointerLikeTypeTraits<Use **> {
43 public:
getAsVoidPointer(Use ** P)44   static inline void *getAsVoidPointer(Use **P) { return P; }
getFromVoidPointer(void * P)45   static inline Use **getFromVoidPointer(void *P) {
46     return static_cast<Use **>(P);
47   }
48   enum { NumLowBitsAvailable = 2 };
49 };
50 
51 /// \brief A Use represents the edge between a Value definition and its users.
52 ///
53 /// This is notionally a two-dimensional linked list. It supports traversing
54 /// all of the uses for a particular value definition. It also supports jumping
55 /// directly to the used value when we arrive from the User's operands, and
56 /// jumping directly to the User when we arrive from the Value's uses.
57 ///
58 /// The pointer to the used Value is explicit, and the pointer to the User is
59 /// implicit. The implicit pointer is found via a waymarking algorithm
60 /// described in the programmer's manual:
61 ///
62 ///   http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm
63 ///
64 /// This is essentially the single most memory intensive object in LLVM because
65 /// of the number of uses in the system. At the same time, the constant time
66 /// operations it allows are essential to many optimizations having reasonable
67 /// time complexity.
68 class Use {
69 public:
70   /// \brief Provide a fast substitute to std::swap<Use>
71   /// that also works with less standard-compliant compilers
72   void swap(Use &RHS);
73 
74   // A type for the word following an array of hung-off Uses in memory, which is
75   // a pointer back to their User with the bottom bit set.
76   typedef PointerIntPair<User *, 1, unsigned> UserRef;
77 
78 private:
79   Use(const Use &U) = delete;
80 
81   /// Destructor - Only for zap()
~Use()82   ~Use() {
83     if (Val)
84       removeFromList();
85   }
86 
87   enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag };
88 
89   /// Constructor
Use(PrevPtrTag tag)90   Use(PrevPtrTag tag) : Val(nullptr) { Prev.setInt(tag); }
91 
92 public:
93   operator Value *() const { return Val; }
get()94   Value *get() const { return Val; }
95 
96   /// \brief Returns the User that contains this Use.
97   ///
98   /// For an instruction operand, for example, this will return the
99   /// instruction.
100   User *getUser() const;
101 
102   inline void set(Value *Val);
103 
104   Value *operator=(Value *RHS) {
105     set(RHS);
106     return RHS;
107   }
108   const Use &operator=(const Use &RHS) {
109     set(RHS.Val);
110     return *this;
111   }
112 
113   Value *operator->() { return Val; }
114   const Value *operator->() const { return Val; }
115 
getNext()116   Use *getNext() const { return Next; }
117 
118   /// \brief Return the operand # of this use in its User.
119   unsigned getOperandNo() const;
120 
121   /// \brief Initializes the waymarking tags on an array of Uses.
122   ///
123   /// This sets up the array of Uses such that getUser() can find the User from
124   /// any of those Uses.
125   static Use *initTags(Use *Start, Use *Stop);
126 
127   /// \brief Destroys Use operands when the number of operands of
128   /// a User changes.
129   static void zap(Use *Start, const Use *Stop, bool del = false);
130 
131 private:
132   const Use *getImpliedUser() const;
133 
134   Value *Val;
135   Use *Next;
136   PointerIntPair<Use **, 2, PrevPtrTag> Prev;
137 
setPrev(Use ** NewPrev)138   void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); }
addToList(Use ** List)139   void addToList(Use **List) {
140     Next = *List;
141     if (Next)
142       Next->setPrev(&Next);
143     setPrev(List);
144     *List = this;
145   }
removeFromList()146   void removeFromList() {
147     Use **StrippedPrev = Prev.getPointer();
148     *StrippedPrev = Next;
149     if (Next)
150       Next->setPrev(StrippedPrev);
151   }
152 
153   friend class Value;
154 };
155 
156 /// \brief Allow clients to treat uses just like values when using
157 /// casting operators.
158 template <> struct simplify_type<Use> {
159   typedef Value *SimpleType;
160   static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); }
161 };
162 template <> struct simplify_type<const Use> {
163   typedef /*const*/ Value *SimpleType;
164   static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); }
165 };
166 
167 // Create wrappers for C Binding types (see CBindingWrapping.h).
168 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef)
169 
170 }
171 
172 #endif
173