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      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/PointerUnion.h"
     24 #include "llvm/ADT/Twine.h"
     25 #include "llvm/ADT/ilist_node.h"
     26 #include "llvm/IR/Attributes.h"
     27 #include "llvm/IR/GlobalObject.h"
     28 #include "llvm/IR/OperandTraits.h"
     29 #include "llvm/IR/Value.h"
     30 #include <cassert>
     31 #include <cstddef>
     32 
     33 namespace llvm {
     34 
     35 class Constant;
     36 class Module;
     37 
     38 template <typename ValueSubClass> class SymbolTableListTraits;
     39 class DIGlobalVariable;
     40 class DIGlobalVariableExpression;
     41 
     42 class GlobalVariable : public GlobalObject, public ilist_node<GlobalVariable> {
     43   friend class SymbolTableListTraits<GlobalVariable>;
     44 
     45   AttributeSet Attrs;
     46   bool isConstantGlobal : 1;                   // Is this a global constant?
     47   bool isExternallyInitializedConstant : 1;    // Is this a global whose value
     48                                                // can change from its initial
     49                                                // value before global
     50                                                // initializers are run?
     51 
     52 public:
     53   /// GlobalVariable ctor - If a parent module is specified, the global is
     54   /// automatically inserted into the end of the specified modules global list.
     55   GlobalVariable(Type *Ty, bool isConstant, LinkageTypes Linkage,
     56                  Constant *Initializer = nullptr, const Twine &Name = "",
     57                  ThreadLocalMode = NotThreadLocal, unsigned AddressSpace = 0,
     58                  bool isExternallyInitialized = false);
     59   /// GlobalVariable ctor - This creates a global and inserts it before the
     60   /// specified other global.
     61   GlobalVariable(Module &M, Type *Ty, bool isConstant,
     62                  LinkageTypes Linkage, Constant *Initializer,
     63                  const Twine &Name = "", GlobalVariable *InsertBefore = nullptr,
     64                  ThreadLocalMode = NotThreadLocal, unsigned AddressSpace = 0,
     65                  bool isExternallyInitialized = false);
     66   GlobalVariable(const GlobalVariable &) = delete;
     67   GlobalVariable &operator=(const GlobalVariable &) = delete;
     68 
     69   ~GlobalVariable() {
     70     dropAllReferences();
     71 
     72     // FIXME: needed by operator delete
     73     setGlobalVariableNumOperands(1);
     74   }
     75 
     76   // allocate space for exactly one operand
     77   void *operator new(size_t s) {
     78     return User::operator new(s, 1);
     79   }
     80 
     81   /// Provide fast operand accessors
     82   DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
     83 
     84   /// Definitions have initializers, declarations don't.
     85   ///
     86   inline bool hasInitializer() const { return !isDeclaration(); }
     87 
     88   /// hasDefinitiveInitializer - Whether the global variable has an initializer,
     89   /// and any other instances of the global (this can happen due to weak
     90   /// linkage) are guaranteed to have the same initializer.
     91   ///
     92   /// Note that if you want to transform a global, you must use
     93   /// hasUniqueInitializer() instead, because of the *_odr linkage type.
     94   ///
     95   /// Example:
     96   ///
     97   /// @a = global SomeType* null - Initializer is both definitive and unique.
     98   ///
     99   /// @b = global weak SomeType* null - Initializer is neither definitive nor
    100   /// unique.
    101   ///
    102   /// @c = global weak_odr SomeType* null - Initializer is definitive, but not
    103   /// unique.
    104   inline bool hasDefinitiveInitializer() const {
    105     return hasInitializer() &&
    106       // The initializer of a global variable may change to something arbitrary
    107       // at link time.
    108       !isInterposable() &&
    109       // The initializer of a global variable with the externally_initialized
    110       // marker may change at runtime before C++ initializers are evaluated.
    111       !isExternallyInitialized();
    112   }
    113 
    114   /// hasUniqueInitializer - Whether the global variable has an initializer, and
    115   /// any changes made to the initializer will turn up in the final executable.
    116   inline bool hasUniqueInitializer() const {
    117     return
    118         // We need to be sure this is the definition that will actually be used
    119         isStrongDefinitionForLinker() &&
    120         // It is not safe to modify initializers of global variables with the
    121         // external_initializer marker since the value may be changed at runtime
    122         // before C++ initializers are evaluated.
    123         !isExternallyInitialized();
    124   }
    125 
    126   /// getInitializer - Return the initializer for this global variable.  It is
    127   /// illegal to call this method if the global is external, because we cannot
    128   /// tell what the value is initialized to!
    129   ///
    130   inline const Constant *getInitializer() const {
    131     assert(hasInitializer() && "GV doesn't have initializer!");
    132     return static_cast<Constant*>(Op<0>().get());
    133   }
    134   inline Constant *getInitializer() {
    135     assert(hasInitializer() && "GV doesn't have initializer!");
    136     return static_cast<Constant*>(Op<0>().get());
    137   }
    138   /// setInitializer - Sets the initializer for this global variable, removing
    139   /// any existing initializer if InitVal==NULL.  If this GV has type T*, the
    140   /// initializer must have type T.
    141   void setInitializer(Constant *InitVal);
    142 
    143   /// If the value is a global constant, its value is immutable throughout the
    144   /// runtime execution of the program.  Assigning a value into the constant
    145   /// leads to undefined behavior.
    146   ///
    147   bool isConstant() const { return isConstantGlobal; }
    148   void setConstant(bool Val) { isConstantGlobal = Val; }
    149 
    150   bool isExternallyInitialized() const {
    151     return isExternallyInitializedConstant;
    152   }
    153   void setExternallyInitialized(bool Val) {
    154     isExternallyInitializedConstant = Val;
    155   }
    156 
    157   /// copyAttributesFrom - copy all additional attributes (those not needed to
    158   /// create a GlobalVariable) from the GlobalVariable Src to this one.
    159   void copyAttributesFrom(const GlobalVariable *Src);
    160 
    161   /// removeFromParent - This method unlinks 'this' from the containing module,
    162   /// but does not delete it.
    163   ///
    164   void removeFromParent();
    165 
    166   /// eraseFromParent - This method unlinks 'this' from the containing module
    167   /// and deletes it.
    168   ///
    169   void eraseFromParent();
    170 
    171   /// Drop all references in preparation to destroy the GlobalVariable. This
    172   /// drops not only the reference to the initializer but also to any metadata.
    173   void dropAllReferences();
    174 
    175   /// Attach a DIGlobalVariableExpression.
    176   void addDebugInfo(DIGlobalVariableExpression *GV);
    177 
    178   /// Fill the vector with all debug info attachements.
    179   void getDebugInfo(SmallVectorImpl<DIGlobalVariableExpression *> &GVs) const;
    180 
    181   /// Add attribute to this global.
    182   void addAttribute(Attribute::AttrKind Kind) {
    183     Attrs = Attrs.addAttribute(getContext(), Kind);
    184   }
    185 
    186   /// Add attribute to this global.
    187   void addAttribute(StringRef Kind, StringRef Val = StringRef()) {
    188     Attrs = Attrs.addAttribute(getContext(), Kind, Val);
    189   }
    190 
    191   /// Return true if the attribute exists.
    192   bool hasAttribute(Attribute::AttrKind Kind) const {
    193     return Attrs.hasAttribute(Kind);
    194   }
    195 
    196   /// Return true if the attribute exists.
    197   bool hasAttribute(StringRef Kind) const {
    198     return Attrs.hasAttribute(Kind);
    199   }
    200 
    201   /// Return true if any attributes exist.
    202   bool hasAttributes() const {
    203     return Attrs.hasAttributes();
    204   }
    205 
    206   /// Return the attribute object.
    207   Attribute getAttribute(Attribute::AttrKind Kind) const {
    208     return Attrs.getAttribute(Kind);
    209   }
    210 
    211   /// Return the attribute object.
    212   Attribute getAttribute(StringRef Kind) const {
    213     return Attrs.getAttribute(Kind);
    214   }
    215 
    216   /// Return the attribute set for this global
    217   AttributeSet getAttributes() const {
    218     return Attrs;
    219   }
    220 
    221   /// Return attribute set as list with index.
    222   /// FIXME: This may not be required once ValueEnumerators
    223   /// in bitcode-writer can enumerate attribute-set.
    224   AttributeList getAttributesAsList(unsigned index) const {
    225     if (!hasAttributes())
    226       return AttributeList();
    227     std::pair<unsigned, AttributeSet> AS[1] = {{index, Attrs}};
    228     return AttributeList::get(getContext(), AS);
    229   }
    230 
    231   /// Set attribute list for this global
    232   void setAttributes(AttributeSet A) {
    233     Attrs = A;
    234   }
    235 
    236   /// Check if section name is present
    237   bool hasImplicitSection() const {
    238     return getAttributes().hasAttribute("bss-section") ||
    239            getAttributes().hasAttribute("data-section") ||
    240            getAttributes().hasAttribute("rodata-section");
    241   }
    242 
    243   // Methods for support type inquiry through isa, cast, and dyn_cast:
    244   static inline bool classof(const Value *V) {
    245     return V->getValueID() == Value::GlobalVariableVal;
    246   }
    247 };
    248 
    249 template <>
    250 struct OperandTraits<GlobalVariable> :
    251   public OptionalOperandTraits<GlobalVariable> {
    252 };
    253 
    254 DEFINE_TRANSPARENT_OPERAND_ACCESSORS(GlobalVariable, Value)
    255 
    256 } // end namespace llvm
    257 
    258 #endif // LLVM_IR_GLOBALVARIABLE_H
    259