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/Types.h"
29#include "llvm/ADT/PointerIntPair.h"
30#include "llvm/Support/CBindingWrapping.h"
31#include "llvm/Support/Compiler.h"
32
33namespace llvm {
34
35template <typename> struct simplify_type;
36class User;
37class Value;
38
39/// A Use represents the edge between a Value definition and its users.
40///
41/// This is notionally a two-dimensional linked list. It supports traversing
42/// all of the uses for a particular value definition. It also supports jumping
43/// directly to the used value when we arrive from the User's operands, and
44/// jumping directly to the User when we arrive from the Value's uses.
45///
46/// The pointer to the used Value is explicit, and the pointer to the User is
47/// implicit. The implicit pointer is found via a waymarking algorithm
48/// described in the programmer's manual:
49///
50/// http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm
51///
52/// This is essentially the single most memory intensive object in LLVM because
53/// of the number of uses in the system. At the same time, the constant time
54/// operations it allows are essential to many optimizations having reasonable
55/// time complexity.
56class Use {
57public:
58 Use(const Use &U) = delete;
59
60 /// Provide a fast substitute to std::swap<Use>
61 /// that also works with less standard-compliant compilers
62 void swap(Use &RHS);
63
64 /// Pointer traits for the UserRef PointerIntPair. This ensures we always
65 /// use the LSB regardless of pointer alignment on different targets.
66 struct UserRefPointerTraits {
67 static inline void *getAsVoidPointer(User *P) { return P; }
68
69 static inline User *getFromVoidPointer(void *P) {
70 return (User *)P;
71 }
72
73 enum { NumLowBitsAvailable = 1 };
74 };
75
76 // A type for the word following an array of hung-off Uses in memory, which is
77 // a pointer back to their User with the bottom bit set.
78 using UserRef = PointerIntPair<User *, 1, unsigned, UserRefPointerTraits>;
79
80 /// Pointer traits for the Prev PointerIntPair. This ensures we always use
81 /// the two LSBs regardless of pointer alignment on different targets.
82 struct PrevPointerTraits {
83 static inline void *getAsVoidPointer(Use **P) { return P; }
84
85 static inline Use **getFromVoidPointer(void *P) {
86 return (Use **)P;
87 }
88
89 enum { NumLowBitsAvailable = 2 };
90 };
91
92private:
93 /// Destructor - Only for zap()
94 ~Use() {
95 if (Val)
96 removeFromList();
97 }
98
99 enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag };
100
101 /// Constructor
102 Use(PrevPtrTag tag) { Prev.setInt(tag); }
103
104public:
105 friend class Value;
106
107 operator Value *() const { return Val; }
108 Value *get() const { return Val; }
109
110 /// Returns the User that contains this Use.
111 ///
112 /// For an instruction operand, for example, this will return the
113 /// instruction.
114 User *getUser() const LLVM_READONLY;
115
116 inline void set(Value *Val);
117
118 inline Value *operator=(Value *RHS);
119 inline const Use &operator=(const Use &RHS);
120
121 Value *operator->() { return Val; }
122 const Value *operator->() const { return Val; }
123
124 Use *getNext() const { return Next; }
125
126 /// Return the operand # of this use in its User.
127 unsigned getOperandNo() const;
128
129 /// Initializes the waymarking tags on an array of Uses.
130 ///
131 /// This sets up the array of Uses such that getUser() can find the User from
132 /// any of those Uses.
133 static Use *initTags(Use *Start, Use *Stop);
134
135 /// Destroys Use operands when the number of operands of
136 /// a User changes.
137 static void zap(Use *Start, const Use *Stop, bool del = false);
138
139private:
140 const Use *getImpliedUser() const LLVM_READONLY;
141
142 Value *Val = nullptr;
143 Use *Next;
144 PointerIntPair<Use **, 2, PrevPtrTag, PrevPointerTraits> Prev;
145
146 void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); }
147
148 void addToList(Use **List) {
149 Next = *List;
150 if (Next)
151 Next->setPrev(&Next);
152 setPrev(List);
153 *List = this;
154 }
155
156 void removeFromList() {
157 Use **StrippedPrev = Prev.getPointer();
158 *StrippedPrev = Next;
159 if (Next)
160 Next->setPrev(StrippedPrev);
161 }
162};
163
164/// Allow clients to treat uses just like values when using
165/// casting operators.
166template <> struct simplify_type<Use> {
167 using SimpleType = Value *;
168
169 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); }
170};
171template <> struct simplify_type<const Use> {
172 using SimpleType = /*const*/ Value *;
173
174 static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); }
175};
176
177// Create wrappers for C Binding types (see CBindingWrapping.h).
178DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef)
179
180} // end namespace llvm
181
182#endif // LLVM_IR_USE_H
183