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:
getAsVoidPointer(Use ** P)45   static inline void *getAsVoidPointer(Use **P) { return P; }
getFromVoidPointer(void * 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) = delete;
81 
82   /// Destructor - Only for zap()
~Use()83   ~Use() {
84     if (Val)
85       removeFromList();
86   }
87 
88   enum PrevPtrTag { zeroDigitTag, oneDigitTag, stopTag, fullStopTag };
89 
90   /// Constructor
Use(PrevPtrTag tag)91   Use(PrevPtrTag tag) : Val(nullptr) { Prev.setInt(tag); }
92 
93 public:
94   operator Value *() const { return Val; }
get()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 
getNext()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 
setPrev(Use ** NewPrev)139   void setPrev(Use **NewPrev) { Prev.setPointer(NewPrev); }
addToList(Use ** List)140   void addToList(Use **List) {
141     Next = *List;
142     if (Next)
143       Next->setPrev(&Next);
144     setPrev(List);
145     *List = this;
146   }
removeFromList()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