1 //===- llvm/Use.h - Definition of the Use class -----------------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
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:
21 /// http://www.llvm.org/docs/ProgrammersManual.html#UserLayout
23 //===----------------------------------------------------------------------===//
28 #include "llvm-c/Types.h"
29 #include "llvm/ADT/PointerIntPair.h"
30 #include "llvm/Support/CBindingWrapping.h"
31 #include "llvm/Support/Compiler.h"
35 template <typename> struct simplify_type;
39 /// A Use represents the edge between a Value definition and its users.
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.
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:
50 /// http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm
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
58 Use(const Use &U) = delete;
60 /// Provide a fast substitute to std::swap<Use>
61 /// that also works with less standard-compliant compilers
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; }
69 static inline User *getFromVoidPointer(void *P) {
73 enum { NumLowBitsAvailable = 1 };
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>;
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; }
85 static inline Use **getFromVoidPointer(void *P) {
89 enum { NumLowBitsAvailable = 2 };
93 /// Destructor - Only for zap()
99 enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag };
102 Use(PrevPtrTag tag) { Prev.setInt(tag); }
107 operator Value *() const { return Val; }
108 Value *get() const { return Val; }
110 /// Returns the User that contains this Use.
112 /// For an instruction operand, for example, this will return the
114 User *getUser() const LLVM_READONLY;
116 inline void set(Value *Val);
118 inline Value *operator=(Value *RHS);
119 inline const Use &operator=(const Use &RHS);
121 Value *operator->() { return Val; }
122 const Value *operator->() const { return Val; }
124 Use *getNext() const { return Next; }
126 /// Return the operand # of this use in its User.
127 unsigned getOperandNo() const;
129 /// Initializes the waymarking tags on an array of Uses.
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);
135 /// Destroys Use operands when the number of operands of
137 static void zap(Use *Start, const Use *Stop, bool del = false);
140 const Use *getImpliedUser() const LLVM_READONLY;
142 Value *Val = nullptr;
144 PointerIntPair<Use **, 2, PrevPtrTag, PrevPointerTraits> Prev;
146 void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); }
148 void addToList(Use **List) {
151 Next->setPrev(&Next);
156 void removeFromList() {
157 Use **StrippedPrev = Prev.getPointer();
158 *StrippedPrev = Next;
160 Next->setPrev(StrippedPrev);
164 /// Allow clients to treat uses just like values when using
165 /// casting operators.
166 template <> struct simplify_type<Use> {
167 using SimpleType = Value *;
169 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); }
171 template <> struct simplify_type<const Use> {
172 using SimpleType = /*const*/ Value *;
174 static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); }
177 // Create wrappers for C Binding types (see CBindingWrapping.h).
178 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef)
180 } // end namespace llvm
182 #endif // LLVM_IR_USE_H