LLVM API Documentation

Constant.h
Go to the documentation of this file.
00001 //===-- llvm/Constant.h - Constant class definition -------------*- C++ -*-===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file contains the declaration of the Constant class.
00011 //
00012 //===----------------------------------------------------------------------===//
00013 
00014 #ifndef LLVM_IR_CONSTANT_H
00015 #define LLVM_IR_CONSTANT_H
00016 
00017 #include "llvm/IR/User.h"
00018 
00019 namespace llvm {
00020   class APInt;
00021 
00022   template<typename T> class SmallVectorImpl;
00023 
00024 /// This is an important base class in LLVM. It provides the common facilities
00025 /// of all constant values in an LLVM program. A constant is a value that is
00026 /// immutable at runtime. Functions are constants because their address is
00027 /// immutable. Same with global variables. 
00028 /// 
00029 /// All constants share the capabilities provided in this class. All constants
00030 /// can have a null value. They can have an operand list. Constants can be
00031 /// simple (integer and floating point values), complex (arrays and structures),
00032 /// or expression based (computations yielding a constant value composed of 
00033 /// only certain operators and other constant values).
00034 /// 
00035 /// Note that Constants are immutable (once created they never change) 
00036 /// and are fully shared by structural equivalence.  This means that two 
00037 /// structurally equivalent constants will always have the same address.  
00038 /// Constants are created on demand as needed and never deleted: thus clients 
00039 /// don't have to worry about the lifetime of the objects.
00040 /// @brief LLVM Constant Representation
00041 class Constant : public User {
00042   void operator=(const Constant &) LLVM_DELETED_FUNCTION;
00043   Constant(const Constant &) LLVM_DELETED_FUNCTION;
00044   void anchor() override;
00045 
00046 protected:
00047   Constant(Type *ty, ValueTy vty, Use *Ops, unsigned NumOps)
00048     : User(ty, vty, Ops, NumOps) {}
00049 
00050   void destroyConstantImpl();
00051 public:
00052   /// isNullValue - Return true if this is the value that would be returned by
00053   /// getNullValue.
00054   bool isNullValue() const;
00055 
00056   /// isAllOnesValue - Return true if this is the value that would be returned by
00057   /// getAllOnesValue.
00058   bool isAllOnesValue() const;
00059 
00060   /// isNegativeZeroValue - Return true if the value is what would be returned 
00061   /// by getZeroValueForNegation.
00062   bool isNegativeZeroValue() const;
00063 
00064   /// Return true if the value is negative zero or null value.
00065   bool isZeroValue() const;
00066 
00067   /// canTrap - Return true if evaluation of this constant could trap.  This is
00068   /// true for things like constant expressions that could divide by zero.
00069   bool canTrap() const;
00070 
00071   /// isThreadDependent - Return true if the value can vary between threads.
00072   bool isThreadDependent() const;
00073 
00074   /// isConstantUsed - Return true if the constant has users other than constant
00075   /// exprs and other dangling things.
00076   bool isConstantUsed() const;
00077   
00078   enum PossibleRelocationsTy {
00079     NoRelocation = 0,
00080     LocalRelocation = 1,
00081     GlobalRelocations = 2
00082   };
00083   
00084   /// getRelocationInfo - This method classifies the entry according to
00085   /// whether or not it may generate a relocation entry.  This must be
00086   /// conservative, so if it might codegen to a relocatable entry, it should say
00087   /// so.  The return values are:
00088   /// 
00089   ///  NoRelocation: This constant pool entry is guaranteed to never have a
00090   ///     relocation applied to it (because it holds a simple constant like
00091   ///     '4').
00092   ///  LocalRelocation: This entry has relocations, but the entries are
00093   ///     guaranteed to be resolvable by the static linker, so the dynamic
00094   ///     linker will never see them.
00095   ///  GlobalRelocations: This entry may have arbitrary relocations.
00096   ///
00097   /// FIXME: This really should not be in VMCore.
00098   PossibleRelocationsTy getRelocationInfo() const;
00099   
00100   /// getAggregateElement - For aggregates (struct/array/vector) return the
00101   /// constant that corresponds to the specified element if possible, or null if
00102   /// not.  This can return null if the element index is a ConstantExpr, or if
00103   /// 'this' is a constant expr.
00104   Constant *getAggregateElement(unsigned Elt) const;
00105   Constant *getAggregateElement(Constant *Elt) const;
00106 
00107   /// getSplatValue - If this is a splat vector constant, meaning that all of
00108   /// the elements have the same value, return that value. Otherwise return 0.
00109   Constant *getSplatValue() const;
00110 
00111   /// If C is a constant integer then return its value, otherwise C must be a
00112   /// vector of constant integers, all equal, and the common value is returned.
00113   const APInt &getUniqueInteger() const;
00114 
00115   /// destroyConstant - Called if some element of this constant is no longer
00116   /// valid.  At this point only other constants may be on the use_list for this
00117   /// constant.  Any constants on our Use list must also be destroy'd.  The
00118   /// implementation must be sure to remove the constant from the list of
00119   /// available cached constants.  Implementations should call
00120   /// destroyConstantImpl as the last thing they do, to destroy all users and
00121   /// delete this.
00122   virtual void destroyConstant() { llvm_unreachable("Not reached!"); }
00123 
00124   //// Methods for support type inquiry through isa, cast, and dyn_cast:
00125   static inline bool classof(const Value *V) {
00126     return V->getValueID() >= ConstantFirstVal &&
00127            V->getValueID() <= ConstantLastVal;
00128   }
00129 
00130   /// replaceUsesOfWithOnConstant - This method is a special form of
00131   /// User::replaceUsesOfWith (which does not work on constants) that does work
00132   /// on constants.  Basically this method goes through the trouble of building
00133   /// a new constant that is equivalent to the current one, with all uses of
00134   /// From replaced with uses of To.  After this construction is completed, all
00135   /// of the users of 'this' are replaced to use the new constant, and then
00136   /// 'this' is deleted.  In general, you should not call this method, instead,
00137   /// use Value::replaceAllUsesWith, which automatically dispatches to this
00138   /// method as needed.
00139   ///
00140   virtual void replaceUsesOfWithOnConstant(Value *, Value *, Use *) {
00141     // Provide a default implementation for constants (like integers) that
00142     // cannot use any other values.  This cannot be called at runtime, but needs
00143     // to be here to avoid link errors.
00144     assert(getNumOperands() == 0 && "replaceUsesOfWithOnConstant must be "
00145            "implemented for all constants that have operands!");
00146     llvm_unreachable("Constants that do not have operands cannot be using "
00147                      "'From'!");
00148   }
00149 
00150   static Constant *getNullValue(Type* Ty);
00151 
00152   /// @returns the value for an integer or vector of integer constant of the
00153   /// given type that has all its bits set to true.
00154   /// @brief Get the all ones value
00155   static Constant *getAllOnesValue(Type* Ty);
00156 
00157   /// getIntegerValue - Return the value for an integer or pointer constant,
00158   /// or a vector thereof, with the given scalar value.
00159   static Constant *getIntegerValue(Type* Ty, const APInt &V);
00160   
00161   /// removeDeadConstantUsers - If there are any dead constant users dangling
00162   /// off of this constant, remove them.  This method is useful for clients
00163   /// that want to check to see if a global is unused, but don't want to deal
00164   /// with potentially dead constants hanging off of the globals.
00165   void removeDeadConstantUsers() const;
00166 };
00167 
00168 } // End llvm namespace
00169 
00170 #endif