1//===- llvm/Use.h - Definition of the Use class -----------------*- C++ -*-===// 2// 3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4// See https://llvm.org/LICENSE.txt for license information. 5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6// 7//===----------------------------------------------------------------------===// 8/// \file 9/// 10/// This defines the Use class. The Use class represents the operand of an 11/// instruction or some other User instance which refers to a Value. The Use 12/// class keeps the "use list" of the referenced value up to date. 13/// 14/// Pointer tagging is used to efficiently find the User corresponding to a Use 15/// without having to store a User pointer in every Use. A User is preceded in 16/// memory by all the Uses corresponding to its operands, and the low bits of 17/// one of the fields (Prev) of the Use class are used to encode offsets to be 18/// able to find that User given a pointer to any Use. For details, see: 19/// 20/// http://www.llvm.org/docs/ProgrammersManual.html#UserLayout 21/// 22//===----------------------------------------------------------------------===// 23 24#ifndef LLVM_IR_USE_H 25#define LLVM_IR_USE_H 26 27#include "llvm-c/Types.h" 28#include "llvm/ADT/PointerIntPair.h" 29#include "llvm/Support/CBindingWrapping.h" 30#include "llvm/Support/Compiler.h" 31 32namespace llvm { 33 34template <typename> struct simplify_type; 35class User; 36class Value; 37 38/// A Use represents the edge between a Value definition and its users. 39/// 40/// This is notionally a two-dimensional linked list. It supports traversing 41/// all of the uses for a particular value definition. It also supports jumping 42/// directly to the used value when we arrive from the User's operands, and 43/// jumping directly to the User when we arrive from the Value's uses. 44/// 45/// The pointer to the used Value is explicit, and the pointer to the User is 46/// implicit. The implicit pointer is found via a waymarking algorithm 47/// described in the programmer's manual: 48/// 49/// http://www.llvm.org/docs/ProgrammersManual.html#the-waymarking-algorithm 50/// 51/// This is essentially the single most memory intensive object in LLVM because 52/// of the number of uses in the system. At the same time, the constant time 53/// operations it allows are essential to many optimizations having reasonable 54/// time complexity. 55class Use { 56public: 57 Use(const Use &U) = delete; 58 59 /// Provide a fast substitute to std::swap<Use> 60 /// that also works with less standard-compliant compilers 61 void swap(Use &RHS); 62 63 /// Pointer traits for the UserRef PointerIntPair. This ensures we always 64 /// use the LSB regardless of pointer alignment on different targets. 65 struct UserRefPointerTraits { 66 static inline void *getAsVoidPointer(User *P) { return P; } 67 68 static inline User *getFromVoidPointer(void *P) { 69 return (User *)P; 70 } 71 72 enum { NumLowBitsAvailable = 1 }; 73 }; 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 using UserRef = PointerIntPair<User *, 1, unsigned, UserRefPointerTraits>; 78 79 /// Pointer traits for the Prev PointerIntPair. This ensures we always use 80 /// the two LSBs regardless of pointer alignment on different targets. 81 struct PrevPointerTraits { 82 static inline void *getAsVoidPointer(Use **P) { return P; } 83 84 static inline Use **getFromVoidPointer(void *P) { 85 return (Use **)P; 86 } 87 88 enum { NumLowBitsAvailable = 2 }; 89 }; 90 91private: 92 /// Destructor - Only for zap() 93 ~Use() { 94 if (Val) 95 removeFromList(); 96 } 97 98 enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag }; 99 100 /// Constructor 101 Use(PrevPtrTag tag) { Prev.setInt(tag); } 102 103public: 104 friend class Value; 105 106 operator Value *() const { return Val; } 107 Value *get() const { return Val; } 108 109 /// Returns the User that contains this Use. 110 /// 111 /// For an instruction operand, for example, this will return the 112 /// instruction. 113 User *getUser() const LLVM_READONLY; 114 115 inline void set(Value *Val); 116 117 inline Value *operator=(Value *RHS); 118 inline const Use &operator=(const Use &RHS); 119 120 Value *operator->() { return Val; } 121 const Value *operator->() const { return Val; } 122 123 Use *getNext() const { return Next; } 124 125 /// Return the operand # of this use in its User. 126 unsigned getOperandNo() const; 127 128 /// Initializes the waymarking tags on an array of Uses. 129 /// 130 /// This sets up the array of Uses such that getUser() can find the User from 131 /// any of those Uses. 132 static Use *initTags(Use *Start, Use *Stop); 133 134 /// Destroys Use operands when the number of operands of 135 /// a User changes. 136 static void zap(Use *Start, const Use *Stop, bool del = false); 137 138private: 139 const Use *getImpliedUser() const LLVM_READONLY; 140 141 Value *Val = nullptr; 142 Use *Next = nullptr; 143 PointerIntPair<Use **, 2, PrevPtrTag, PrevPointerTraits> Prev; 144 145 void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); } 146 147 void addToList(Use **List) { 148 Next = *List; 149 if (Next) 150 Next->setPrev(&Next); 151 setPrev(List); 152 *List = this; 153 } 154 155 void removeFromList() { 156 Use **StrippedPrev = Prev.getPointer(); 157 *StrippedPrev = Next; 158 if (Next) 159 Next->setPrev(StrippedPrev); 160 } 161}; 162 163/// Allow clients to treat uses just like values when using 164/// casting operators. 165template <> struct simplify_type<Use> { 166 using SimpleType = Value *; 167 168 static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); } 169}; 170template <> struct simplify_type<const Use> { 171 using SimpleType = /*const*/ Value *; 172 173 static SimpleType getSimplifiedValue(const Use &Val) { return Val.get(); } 174}; 175 176// Create wrappers for C Binding types (see CBindingWrapping.h). 177DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef) 178 179} // end namespace llvm 180 181#endif // LLVM_IR_USE_H 182