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