include/llvm/IR/Use.h

   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
  32 namespace llvm {
  33
  34 template <typename> struct simplify_type;
  35 class User;
  36 class 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.
  55 class Use {
  56 public:
  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
  91 private:
  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
 103 public:
 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
 138 private:
 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.
 165 template <> struct simplify_type<Use> {
 166   using SimpleType = Value *;
 167
 168   static SimpleType getSimplifiedValue(Use &Val) { return Val.get(); }
 169 };
 170 template <> 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).
 177 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(Use, LLVMUseRef)
 178
 179 } // end namespace llvm
 180
 181 #endif // LLVM_IR_USE_H