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-c/Types.h" 31 32 namespace llvm { 33 34 class Value; 35 class User; 36 class Use; 37 template <typename> struct simplify_type; 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 // A type for the word following an array of hung-off Uses in memory, which is 65 // a pointer back to their User with the bottom bit set. 66 typedef PointerIntPair<User *, 1, unsigned> UserRef; 67 68 private: 69 /// Destructor - Only for zap() ~Use()70 ~Use() { 71 if (Val) 72 removeFromList(); 73 } 74 75 enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag }; 76 77 /// Constructor Use(PrevPtrTag tag)78 Use(PrevPtrTag tag) : Val(nullptr) { Prev.setInt(tag); } 79 80 public: 81 operator Value *() const { return Val; } get()82 Value *get() const { return Val; } 83 84 /// \brief Returns the User that contains this Use. 85 /// 86 /// For an instruction operand, for example, this will return the 87 /// instruction. 88 User *getUser() const; 89 90 inline void set(Value *Val); 91 92 inline Value *operator=(Value *RHS); 93 inline const Use &operator=(const Use &RHS); 94 95 Value *operator->() { return Val; } 96 const Value *operator->() const { return Val; } 97 getNext()98 Use *getNext() const { return Next; } 99 100 /// \brief Return the operand # of this use in its User. 101 unsigned getOperandNo() const; 102 103 /// \brief Initializes the waymarking tags on an array of Uses. 104 /// 105 /// This sets up the array of Uses such that getUser() can find the User from 106 /// any of those Uses. 107 static Use *initTags(Use *Start, Use *Stop); 108 109 /// \brief Destroys Use operands when the number of operands of 110 /// a User changes. 111 static void zap(Use *Start, const Use *Stop, bool del = false); 112 113 private: 114 const Use *getImpliedUser() const; 115 116 Value *Val; 117 Use *Next; 118 PointerIntPair<Use **, 2, PrevPtrTag> Prev; 119 setPrev(Use ** NewPrev)120 void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); } 121 addToList(Use ** List)122 void addToList(Use **List) { 123 Next = *List; 124 if (Next) 125 Next->setPrev(&Next); 126 setPrev(List); 127 *List = this; 128 } 129 removeFromList()130 void removeFromList() { 131 Use **StrippedPrev = Prev.getPointer(); 132 *StrippedPrev = Next; 133 if (Next) 134 Next->setPrev(StrippedPrev); 135 } 136 137 friend class Value; 138 }; 139 140 /// \brief Allow clients to treat uses just like values when using 141 /// casting operators. 142 template <> struct simplify_type<Use> { 143 typedef Value *SimpleType; 144 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); } 145 }; 146 template <> struct simplify_type<const Use> { 147 typedef /*const*/ Value *SimpleType; 148 static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); } 149 }; 150 151 // Create wrappers for C Binding types (see CBindingWrapping.h). 152 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef) 153 154 } // end namespace llvm 155 156 #endif // LLVM_IR_USE_H 157