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/Support/Compiler.h" 31 #include <cstddef> 32 #include <iterator> 33 34 namespace llvm { 35 36 class Value; 37 class User; 38 class Use; 39 template <typename> struct simplify_type; 40 41 // Use** is only 4-byte aligned. 42 template <> class PointerLikeTypeTraits<Use **> { 43 public: 44 static inline void *getAsVoidPointer(Use **P) { return P; } 45 static inline Use **getFromVoidPointer(void *P) { 46 return static_cast<Use **>(P); 47 } 48 enum { NumLowBitsAvailable = 2 }; 49 }; 50 51 /// \brief A Use represents the edge between a Value definition and its users. 52 /// 53 /// This is notionally a two-dimensional linked list. It supports traversing 54 /// all of the uses for a particular value definition. It also supports jumping 55 /// directly to the used value when we arrive from the User's operands, and 56 /// jumping directly to the User when we arrive from the Value's uses. 57 /// 58 /// The pointer to the used Value is explicit, and the pointer to the User is 59 /// implicit. The implicit pointer is found via a waymarking algorithm 60 /// described in the programmer's manual: 61 /// 62 /// http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm 63 /// 64 /// This is essentially the single most memory intensive object in LLVM because 65 /// of the number of uses in the system. At the same time, the constant time 66 /// operations it allows are essential to many optimizations having reasonable 67 /// time complexity. 68 class Use { 69 public: 70 /// \brief Provide a fast substitute to std::swap<Use> 71 /// that also works with less standard-compliant compilers 72 void swap(Use &RHS); 73 74 // A type for the word following an array of hung-off Uses in memory, which is 75 // a pointer back to their User with the bottom bit set. 76 typedef PointerIntPair<User *, 1, unsigned> UserRef; 77 78 private: 79 Use(const Use &U) = delete; 80 81 /// Destructor - Only for zap() 82 ~Use() { 83 if (Val) 84 removeFromList(); 85 } 86 87 enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag }; 88 89 /// Constructor 90 Use(PrevPtrTag tag) : Val(nullptr) { Prev.setInt(tag); } 91 92 public: 93 operator Value *() const { return Val; } 94 Value *get() const { return Val; } 95 96 /// \brief Returns the User that contains this Use. 97 /// 98 /// For an instruction operand, for example, this will return the 99 /// instruction. 100 User *getUser() const; 101 102 inline void set(Value *Val); 103 104 Value *operator=(Value *RHS) { 105 set(RHS); 106 return RHS; 107 } 108 const Use &operator=(const Use &RHS) { 109 set(RHS.Val); 110 return *this; 111 } 112 113 Value *operator->() { return Val; } 114 const Value *operator->() const { return Val; } 115 116 Use *getNext() const { return Next; } 117 118 /// \brief Return the operand # of this use in its User. 119 unsigned getOperandNo() const; 120 121 /// \brief Initializes the waymarking tags on an array of Uses. 122 /// 123 /// This sets up the array of Uses such that getUser() can find the User from 124 /// any of those Uses. 125 static Use *initTags(Use *Start, Use *Stop); 126 127 /// \brief Destroys Use operands when the number of operands of 128 /// a User changes. 129 static void zap(Use *Start, const Use *Stop, bool del = false); 130 131 private: 132 const Use *getImpliedUser() const; 133 134 Value *Val; 135 Use *Next; 136 PointerIntPair<Use **, 2, PrevPtrTag> Prev; 137 138 void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); } 139 void addToList(Use **List) { 140 Next = *List; 141 if (Next) 142 Next->setPrev(&Next); 143 setPrev(List); 144 *List = this; 145 } 146 void removeFromList() { 147 Use **StrippedPrev = Prev.getPointer(); 148 *StrippedPrev = Next; 149 if (Next) 150 Next->setPrev(StrippedPrev); 151 } 152 153 friend class Value; 154 }; 155 156 /// \brief Allow clients to treat uses just like values when using 157 /// casting operators. 158 template <> struct simplify_type<Use> { 159 typedef Value *SimpleType; 160 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); } 161 }; 162 template <> struct simplify_type<const Use> { 163 typedef /*const*/ Value *SimpleType; 164 static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); } 165 }; 166 167 // Create wrappers for C Binding types (see CBindingWrapping.h). 168 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef) 169 170 } 171 172 #endif 173
