1//===-- llvm/GlobalVariable.h - GlobalVariable 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//
10// This file contains the declaration of the GlobalVariable class, which
11// represents a single global variable (or constant) in the VM.
12//
13// Global variables are constant pointers that refer to hunks of space that are
14// allocated by either the VM, or by the linker in a static compiler.  A global
15// variable may have an initial value, which is copied into the executables .data
16// area.  Global Constants are required to have initializers.
17//
18//===----------------------------------------------------------------------===//
19
20#ifndef LLVM_IR_GLOBALVARIABLE_H
21#define LLVM_IR_GLOBALVARIABLE_H
22
23#include "llvm/ADT/Twine.h"
24#include "llvm/ADT/ilist_node.h"
25#include "llvm/IR/GlobalObject.h"
26#include "llvm/IR/OperandTraits.h"
27
28namespace llvm {
29
30class Module;
31class Constant;
32template<typename ValueSubClass, typename ItemParentClass>
33  class SymbolTableListTraits;
34
35class GlobalVariable : public GlobalObject, public ilist_node<GlobalVariable> {
36  friend class SymbolTableListTraits<GlobalVariable, Module>;
37  void *operator new(size_t, unsigned) LLVM_DELETED_FUNCTION;
38  void operator=(const GlobalVariable &) LLVM_DELETED_FUNCTION;
39  GlobalVariable(const GlobalVariable &) LLVM_DELETED_FUNCTION;
40
41  void setParent(Module *parent);
42
43  bool isConstantGlobal : 1;                   // Is this a global constant?
44  bool isExternallyInitializedConstant : 1;    // Is this a global whose value
45                                               // can change from its initial
46                                               // value before global
47                                               // initializers are run?
48
49public:
50  // allocate space for exactly one operand
51  void *operator new(size_t s) {
52    return User::operator new(s, 1);
53  }
54
55  /// GlobalVariable ctor - If a parent module is specified, the global is
56  /// automatically inserted into the end of the specified modules global list.
57  GlobalVariable(Type *Ty, bool isConstant, LinkageTypes Linkage,
58                 Constant *Initializer = nullptr, const Twine &Name = "",
59                 ThreadLocalMode = NotThreadLocal, unsigned AddressSpace = 0,
60                 bool isExternallyInitialized = false);
61  /// GlobalVariable ctor - This creates a global and inserts it before the
62  /// specified other global.
63  GlobalVariable(Module &M, Type *Ty, bool isConstant,
64                 LinkageTypes Linkage, Constant *Initializer,
65                 const Twine &Name = "", GlobalVariable *InsertBefore = nullptr,
66                 ThreadLocalMode = NotThreadLocal, unsigned AddressSpace = 0,
67                 bool isExternallyInitialized = false);
68
69  ~GlobalVariable() {
70    NumOperands = 1; // FIXME: needed by operator delete
71  }
72
73  /// Provide fast operand accessors
74  DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
75
76  /// Definitions have initializers, declarations don't.
77  ///
78  inline bool hasInitializer() const { return !isDeclaration(); }
79
80  /// hasDefinitiveInitializer - Whether the global variable has an initializer,
81  /// and any other instances of the global (this can happen due to weak
82  /// linkage) are guaranteed to have the same initializer.
83  ///
84  /// Note that if you want to transform a global, you must use
85  /// hasUniqueInitializer() instead, because of the *_odr linkage type.
86  ///
87  /// Example:
88  ///
89  /// @a = global SomeType* null - Initializer is both definitive and unique.
90  ///
91  /// @b = global weak SomeType* null - Initializer is neither definitive nor
92  /// unique.
93  ///
94  /// @c = global weak_odr SomeType* null - Initializer is definitive, but not
95  /// unique.
96  inline bool hasDefinitiveInitializer() const {
97    return hasInitializer() &&
98      // The initializer of a global variable with weak linkage may change at
99      // link time.
100      !mayBeOverridden() &&
101      // The initializer of a global variable with the externally_initialized
102      // marker may change at runtime before C++ initializers are evaluated.
103      !isExternallyInitialized();
104  }
105
106  /// hasUniqueInitializer - Whether the global variable has an initializer, and
107  /// any changes made to the initializer will turn up in the final executable.
108  inline bool hasUniqueInitializer() const {
109    return hasInitializer() &&
110      // It's not safe to modify initializers of global variables with weak
111      // linkage, because the linker might choose to discard the initializer and
112      // use the initializer from another instance of the global variable
113      // instead. It is wrong to modify the initializer of a global variable
114      // with *_odr linkage because then different instances of the global may
115      // have different initializers, breaking the One Definition Rule.
116      !isWeakForLinker() &&
117      // It is not safe to modify initializers of global variables with the
118      // external_initializer marker since the value may be changed at runtime
119      // before C++ initializers are evaluated.
120      !isExternallyInitialized();
121  }
122
123  /// getInitializer - Return the initializer for this global variable.  It is
124  /// illegal to call this method if the global is external, because we cannot
125  /// tell what the value is initialized to!
126  ///
127  inline const Constant *getInitializer() const {
128    assert(hasInitializer() && "GV doesn't have initializer!");
129    return static_cast<Constant*>(Op<0>().get());
130  }
131  inline Constant *getInitializer() {
132    assert(hasInitializer() && "GV doesn't have initializer!");
133    return static_cast<Constant*>(Op<0>().get());
134  }
135  /// setInitializer - Sets the initializer for this global variable, removing
136  /// any existing initializer if InitVal==NULL.  If this GV has type T*, the
137  /// initializer must have type T.
138  void setInitializer(Constant *InitVal);
139
140  /// If the value is a global constant, its value is immutable throughout the
141  /// runtime execution of the program.  Assigning a value into the constant
142  /// leads to undefined behavior.
143  ///
144  bool isConstant() const { return isConstantGlobal; }
145  void setConstant(bool Val) { isConstantGlobal = Val; }
146
147  bool isExternallyInitialized() const {
148    return isExternallyInitializedConstant;
149  }
150  void setExternallyInitialized(bool Val) {
151    isExternallyInitializedConstant = Val;
152  }
153
154  /// copyAttributesFrom - copy all additional attributes (those not needed to
155  /// create a GlobalVariable) from the GlobalVariable Src to this one.
156  void copyAttributesFrom(const GlobalValue *Src) override;
157
158  /// removeFromParent - This method unlinks 'this' from the containing module,
159  /// but does not delete it.
160  ///
161  void removeFromParent() override;
162
163  /// eraseFromParent - This method unlinks 'this' from the containing module
164  /// and deletes it.
165  ///
166  void eraseFromParent() override;
167
168  /// Override Constant's implementation of this method so we can
169  /// replace constant initializers.
170  void replaceUsesOfWithOnConstant(Value *From, Value *To, Use *U) override;
171
172  // Methods for support type inquiry through isa, cast, and dyn_cast:
173  static inline bool classof(const Value *V) {
174    return V->getValueID() == Value::GlobalVariableVal;
175  }
176};
177
178template <>
179struct OperandTraits<GlobalVariable> :
180  public OptionalOperandTraits<GlobalVariable> {
181};
182
183DEFINE_TRANSPARENT_OPERAND_ACCESSORS(GlobalVariable, Value)
184
185} // End llvm namespace
186
187#endif
188