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/Core.h" 29 #include "llvm/ADT/PointerIntPair.h" 30 #include "llvm/Support/CBindingWrapping.h" 31 #include "llvm/Support/Compiler.h" 32 #include <cstddef> 33 #include <iterator> 34 35 namespace llvm { 36 37 class Value; 38 class User; 39 class Use; 40 template <typename> struct simplify_type; 41 42 // Use** is only 4-byte aligned. 43 template <> class PointerLikeTypeTraits<Use **> { 44 public: 45 static inline void *getAsVoidPointer(Use **P) { return P; } 46 static inline Use **getFromVoidPointer(void *P) { 47 return static_cast<Use **>(P); 48 } 49 enum { NumLowBitsAvailable = 2 }; 50 }; 51 52 /// \brief A Use represents the edge between a Value definition and its users. 53 /// 54 /// This is notionally a two-dimensional linked list. It supports traversing 55 /// all of the uses for a particular value definition. It also supports jumping 56 /// directly to the used value when we arrive from the User's operands, and 57 /// jumping directly to the User when we arrive from the Value's uses. 58 /// 59 /// The pointer to the used Value is explicit, and the pointer to the User is 60 /// implicit. The implicit pointer is found via a waymarking algorithm 61 /// described in the programmer's manual: 62 /// 63 /// http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm 64 /// 65 /// This is essentially the single most memory intensive object in LLVM because 66 /// of the number of uses in the system. At the same time, the constant time 67 /// operations it allows are essential to many optimizations having reasonable 68 /// time complexity. 69 class Use { 70 public: 71 /// \brief Provide a fast substitute to std::swap<Use> 72 /// that also works with less standard-compliant compilers 73 void swap(Use &RHS); 74 75 // A type for the word following an array of hung-off Uses in memory, which is 76 // a pointer back to their User with the bottom bit set. 77 typedef PointerIntPair<User *, 1, unsigned> UserRef; 78 79 private: 80 Use(const Use &U) LLVM_DELETED_FUNCTION; 81 82 /// Destructor - Only for zap() 83 ~Use() { 84 if (Val) 85 removeFromList(); 86 } 87 88 enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag }; 89 90 /// Constructor 91 Use(PrevPtrTag tag) : Val(nullptr) { Prev.setInt(tag); } 92 93 public: 94 operator Value *() const { return Val; } 95 Value *get() const { return Val; } 96 97 /// \brief Returns the User that contains this Use. 98 /// 99 /// For an instruction operand, for example, this will return the 100 /// instruction. 101 User *getUser() const; 102 103 inline void set(Value *Val); 104 105 Value *operator=(Value *RHS) { 106 set(RHS); 107 return RHS; 108 } 109 const Use &operator=(const Use &RHS) { 110 set(RHS.Val); 111 return *this; 112 } 113 114 Value *operator->() { return Val; } 115 const Value *operator->() const { return Val; } 116 117 Use *getNext() const { return Next; } 118 119 /// \brief Return the operand # of this use in its User. 120 unsigned getOperandNo() const; 121 122 /// \brief Initializes the waymarking tags on an array of Uses. 123 /// 124 /// This sets up the array of Uses such that getUser() can find the User from 125 /// any of those Uses. 126 static Use *initTags(Use *Start, Use *Stop); 127 128 /// \brief Destroys Use operands when the number of operands of 129 /// a User changes. 130 static void zap(Use *Start, const Use *Stop, bool del = false); 131 132 private: 133 const Use *getImpliedUser() const; 134 135 Value *Val; 136 Use *Next; 137 PointerIntPair<Use **, 2, PrevPtrTag> Prev; 138 139 void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); } 140 void addToList(Use **List) { 141 Next = *List; 142 if (Next) 143 Next->setPrev(&Next); 144 setPrev(List); 145 *List = this; 146 } 147 void removeFromList() { 148 Use **StrippedPrev = Prev.getPointer(); 149 *StrippedPrev = Next; 150 if (Next) 151 Next->setPrev(StrippedPrev); 152 } 153 154 friend class Value; 155 }; 156 157 /// \brief Allow clients to treat uses just like values when using 158 /// casting operators. 159 template <> struct simplify_type<Use> { 160 typedef Value *SimpleType; 161 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); } 162 }; 163 template <> struct simplify_type<const Use> { 164 typedef /*const*/ Value *SimpleType; 165 static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); } 166 }; 167 168 // Create wrappers for C Binding types (see CBindingWrapping.h). 169 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef) 170 171 } 172 173 #endif 174
