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 33 namespace llvm { 34 35 template <typename> struct simplify_type; 36 class User; 37 class Value; 38 39 /// \brief 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. 56 class Use { 57 public: 58 Use(const Use &U) = delete; 59 60 /// \brief 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 92 private: 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); } 103 104 public: 105 friend class Value; 106 107 operator Value *() const { return Val; } 108 Value *get() const { return Val; } 109 110 /// \brief 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 /// \brief Return the operand # of this use in its User. 127 unsigned getOperandNo() const; 128 129 /// \brief 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 /// \brief 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); 138 139 private: 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 /// \brief Allow clients to treat uses just like values when using 165 /// casting operators. 166 template <> struct simplify_type<Use> { 167 using SimpleType = Value *; 168 169 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); } 170 }; 171 template <> 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). 178 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef) 179 180 } // end namespace llvm 181 182 #endif // LLVM_IR_USE_H 183
