1. Introduction
2. General Information
   • The C++ Standard Template Library
   • The isa<>, cast<> and dyn_cast<> templates
3. The Core LLVM Class Heirarchy
   • The Value class
     • The User class
       • The Instruction class
       • The GlobalValue class
         • The BasicBlock class
         • The Function class
         • The GlobalVariable class
       • The Module class
       • The Constant class
     • The Type class
     • The Argument class
   • The SymbolTable class
   • The ilist and iplist classes
     • Creating, inserting, moving and deleting from LLVM lists
   • Important iterator invalidation semantics to be aware of

   Written by Dinakar Dhurjati and Chris Lattner

This document is meant to hi-light some of the important classes and interfaces available in the LLVM source-base. This manual is not indended to explain what LLVM is, how it works, and what LLVM code looks like. It assumes that you know the basics of LLVM and are interested in writing transformations or otherwise analyzing or manipulating the code.

This document should get you oriented so that you can find your way in the continuously growing source code that makes up the LLVM infrastructure. Note that this manual is not intended to serve as a replacement for reading the source code, so if you think there should be a method in one of these classes to do something, but it's not listed, check the source. Links to the doxygen sources are provided to make this as easy as possible.

The first section of this document describes general information that is useful to know when working in the LLVM infrastructure, and the second describes the Core LLVM classes. In the future this manual will be extended with information describing how to use extension libraries, such as dominator information, CFG traversal routines, and useful utilities like the InstVisitor template.

This section contains general information that is useful if you are working in the LLVM source-base, but that isn't specific to any particular API.

LLVM makes heavy use of the C++ Standard Template Library (STL), perhaps much more than you are used to, or have seen before. Because of this, you might want to do a little background reading in the techniques used and capabilities of the library. There are many good pages that discuss the STL, and several books on the subject that you can get, so it will not be discussed in this document.

Here are some useful links:

1. Dinkumware C++ Library reference - an excellent reference for the STL and other parts of the standard C++ library.
   
2. C++ Frequently Asked Questions
3. SGI's STL Programmer's Guide - Contains a useful Introduction to the STL.
4. Bjarne Stroustrup's C++ Page

You are also encouraged to take a look at the LLVM Coding Standards guide which focuses on how to write maintainable code more than where to put your curly braces.

The Core LLVM classes are the primary means of representing the program being inspected or transformed. The core LLVM classes are defined in header files in the include/llvm/ directory, and implemented in the lib/VMCore directory.

#include "llvm/Value.h"
doxygen info: Value Class

The Value class is the most important class in LLVM Source base. It represents a typed value that may be used (among other things) as an operand to an instruction. There are many different types of Values, such as Constants, Arguments, and even Instructions and Functions are Values.

A particular Value may be used many times in the LLVM representation for a program. For example, an incoming argument to a function (represented with an instance of the Argument class) is "used" by every instruction in the function that references the argument. To keep track of this relationship, the Value class keeps a list of all of the Users that is using it (the User class is a base class for all nodes in the LLVM graph that can refer to Values). This use list is how LLVM represents def-use information in the program, and is accessable through the use_* methods, shown below.

Because LLVM is a typed representation, every LLVM Value is typed, and this Type is available through the getType() method. In addition, all LLVM values can be named. The "name" of the Value is symbolic string printed in the LLVM code:

   %foo = add int 1, 2

The name of this instruction is "foo". NOTE that the name of any value may be missing (an empty string), so names should ONLY be used for debugging (making the source code easier to read, debugging printouts), they should not be used to keep track of values or map between them. For this purpose, use a std::map of pointers to the Value itself instead.

One important aspect of LLVM is that there is no distinction between an SSA variable and the operation that produces it. Because of this, any reference to the value produced by an instruction (or the value available as an incoming argument, for example) is represented as a direct pointer to the class that represents this value. Although this may take some getting used to, it simplifies the representation and makes it easier to manipulate.

---

Important Public Members of the Value class

• Value::use_iterator - Typedef for iterator over the use-list
  Value::use_const_iterator - Typedef for const_iterator over the use-list
  unsigned use_size() - Returns the number of users of the value.
  bool use_empty() - Returns true if there are no users.
  use_iterator use_begin() - Get an iterator to the start of the use-list.
  use_iterator use_end() - Get an iterator to the end of the use-list.
  User *use_back() - Returns the last element in the list.

  These methods are the interface to access the def-use information in LLVM. As with all other iterators in LLVM, the naming conventions follow the conventions defined by the STL.

• Type *getType() const

  This method returns the Type of the Value.

• bool hasName() const
  std::string getName() const
  void setName(const std::string &Name)

  This family of methods is used to access and assign a name to a Value, be aware of the precaution above.

• void replaceAllUsesWith(Value *V)

  This method traverses the use list of a Value changing all User's of the current value to refer to "V" instead. For example, if you detect that an instruction always produces a constant value (for example through constant folding), you can replace all uses of the instruction with the constant like this:

    Inst->replaceAllUsesWith(ConstVal);
  

#include "llvm/User.h"
doxygen info: User Class
Superclass: Value

The User class is the common base class of all LLVM nodes that may refer to Values. It exposes a list of "Operands" that are all of the Values that the User is referring to. The User class itself is a subclass of Value.

The operands of a User point directly to the LLVM Value that it refers to. Because LLVM uses Static Single Assignment (SSA) form, there can only be one definition referred to, allowing this direct connection. This connection provides the use-def information in LLVM.

---

Important Public Members of the User class

The User class exposes the operand list in two ways: through an index access interface and through an iterator based interface.
• Value *getOperand(unsigned i)
  unsigned getNumOperands()

  These two methods expose the operands of the User in a convenient form for direct access.

• User::op_iterator - Typedef for iterator over the operand list
  User::op_const_iterator use_iterator op_begin() - Get an iterator to the start of the operand list.
  use_iterator op_end() - Get an iterator to the end of the operand list.

  Together, these methods make up the iterator based interface to the operands of a User.

#include "llvm/Instruction.h"
doxygen info: Instruction Class
Superclasses: User, Value

The Instruction class is the common base class for all LLVM instructions. It provides only a few methods, but is a very commonly used class. The primary data tracked by the Instruction class itself is the opcode (instruction type) and the parent BasicBlock the Instruction is embedded into. To represent a specific type of instruction, one of many subclasses of Instruction are used.

Because the Instruction class subclasses the User class, its operands can be accessed in the same way as for other Users (with the getOperand()/getNumOperands() and op_begin()/op_end() methods).

---

Important Public Members of the Instruction class

• BasicBlock *getParent()

  Returns the BasicBlock that this Instruction is embedded into.

• bool hasSideEffects()

  Returns true if the instruction has side effects, i.e. it is a call, free, invoke, or store.

• unsigned getOpcode()

  Returns the opcode for the Instruction.

#include "llvm/BasicBlock.h"
doxygen info: BasicBlock Class
Superclass: Value

This class represents a single entry multiple exit section of the code, commonly known as a basic block by the compiler community. The BasicBlock class maintains a list of Instructions, which form the body of the block. Matching the language definition, the last element of this list of instructions is always a terminator instruction (a subclass of the TerminatorInst class).

In addition to tracking the list of instructions that make up the block, the BasicBlock class also keeps track of the Function that it is embedded into.

Note that BasicBlocks themselves are Values, because they are referenced by instructions like branches and can go in the switch tables. BasicBlocks have type label.

---

Important Public Members of the BasicBlock class

• BasicBlock(const std::string &Name = "", Function *Parent = 0)

  The BasicBlock constructor is used to create new basic blocks for insertion into a function. The constructor simply takes a name for the new block, and optionally a Function to insert it into. If the Parent parameter is specified, the new BasicBlock is automatically inserted at the end of the specified Function, if not specified, the BasicBlock must be manually inserted into the Function.

• BasicBlock::iterator - Typedef for instruction list iterator
  BasicBlock::const_iterator - Typedef for const_iterator.
  begin(), end(), front(), back(), size(), empty(), rbegin(), rend()

  These methods and typedefs are forwarding functions that have the same semantics as the standard library methods of the same names. These methods expose the underlying instruction list of a basic block in a way that is easy to manipulate. To get the full complement of container operations (including operations to update the list), you must use the getInstList() method.

• BasicBlock::InstListType &getInstList()

  This method is used to get access to the underlying container that actually holds the Instructions. This method must be used when there isn't a forwarding function in the BasicBlock class for the operation that you would like to perform. Because there are no forwarding functions for "updating" operations, you need to use this if you want to update the contents of a BasicBlock.

• Function *getParent()

  Returns a pointer to Function the block is embedded into, or a null pointer if it is homeless.

• TerminatorInst *getTerminator()

  Returns a pointer to the terminator instruction that appears at the end of the BasicBlock. If there is no terminator instruction, or if the last instruction in the block is not a terminator, then a null pointer is returned.

#include "llvm/GlobalValue.h"
doxygen info: GlobalValue Class
Superclasses: User, Value

Global values (GlobalVariables or Functions) are the only LLVM values that are visible in the bodies of all Functions. Because they are visible at global scope, they are also subject to linking with other globals defined in different translation units. To control the linking process, GlobalValues know their linkage rules. Specifically, GlobalValues know whether they have internal or external linkage.

If a GlobalValue has internal linkage (equivalent to being static in C), it is not visible to code outside the current translation unit, and does not participate in linking. If it has external linkage, it is visible to external code, and does participate in linking. In addition to linkage information, GlobalValues keep track of which Module they are currently part of.

Because GlobalValues are memory objects, they are always referred to by their address. As such, the Type of a global is always a pointer to its contents. This is explained in the LLVM Language Reference Manual.

---

Important Public Members of the GlobalValue class

• bool hasInternalLinkage() const
  bool hasExternalLinkage() const
  void setInternalLinkage(bool HasInternalLinkage)

  These methods manipulate the linkage characteristics of the GlobalValue.

• Module *getParent()

  This returns the Module that the GlobalValue is currently embedded into.

#include "llvm/Function.h"
doxygen info: Function Class
Superclasses: GlobalValue, User, Value

The Function class represents a single procedure in LLVM. It is actually one of the more complex classes in the LLVM heirarchy because it must keep track of a large amount of data. The Function class keeps track of a list of BasicBlocks, a list of formal Arguments, and a SymbolTable.

The list of BasicBlocks is the most commonly used part of Function objects. The list imposes an implicit ordering of the blocks in the function, which indicate how the code will be layed out by the backend. Additionally, the first BasicBlock is the implicit entry node for the Function. It is not legal in LLVM explicitly branch to this initial block. There are no implicit exit nodes, and in fact there may be multiple exit nodes from a single Function. If the BasicBlock list is empty, this indicates that the Function is actually a function declaration: the actual body of the function hasn't been linked in yet.

In addition to a list of BasicBlocks, the Function class also keeps track of the list of formal Arguments that the function receives. This container manages the lifetime of the Argument nodes, just like the BasicBlock list does for the BasicBlocks.

The SymbolTable is a very rarely used LLVM feature that is only used when you have to look up a value by name. Aside from that, the SymbolTable is used internally to make sure that there are not conflicts between the names of Instructions, BasicBlocks, or Arguments in the function body.

---

Important Public Members of the Function class

• Function(const FunctionType *Ty, bool isInternal, const std::string &N = "")

  Constructor used when you need to create new Functions to add the the program. The constructor must specify the type of the function to create and whether or not it should start out with internal or external linkage.

• bool isExternal()

  Return whether or not the Function has a body defined. If the function is "external", it does not have a body, and thus must be resolved by linking with a function defined in a different translation unit.

• Function::iterator - Typedef for basic block list iterator
  Function::const_iterator - Typedef for const_iterator.
  begin(), end(), front(), back(), size(), empty(), rbegin(), rend()

  These are forwarding methods that make it easy to access the contents of a Function object's BasicBlock list.

• Function::BasicBlockListType &getBasicBlockList()

  Returns the list of BasicBlocks. This is neccesary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

• Function::aiterator - Typedef for the argument list iterator
  Function::const_aiterator - Typedef for const_iterator.
  abegin(), aend(), afront(), aback(), asize(), aempty(), arbegin(), arend()

  These are forwarding methods that make it easy to access the contents of a Function object's Argument list.

• Function::ArgumentListType &getArgumentList()

  Returns the list of Arguments. This is neccesary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

• BasicBlock &getEntryNode()

  Returns the entry BasicBlock for the function. Because the entry block for the function is always the first block, this returns the first block of the Function.

• Type *getReturnType()
  FunctionType *getFunctionType()

  This traverses the Type of the Function and returns the return type of the function, or the FunctionType of the actual function.

• bool hasSymbolTable() const

  Return true if the Function has a symbol table allocated to it and if there is at least one entry in it.

• SymbolTable *getSymbolTable()

  Return a pointer to the SymbolTable for this Function or a null pointer if one has not been allocated (because there are no named values in the function).

• SymbolTable *getSymbolTableSure()

  Return a pointer to the SymbolTable for this Function or allocate a new SymbolTable if one is not already around. This should only be used when adding elements to the SymbolTable, so that empty symbol tables are not left laying around.

#include "llvm/GlobalVariable.h"
doxygen info: GlobalVariable Class
Superclasses: GlobalValue, User, Value

Global variables are represented with the (suprise suprise) GlobalVariable class. Like functions, GlobalVariables are also subclasses of GlobalValue, and as such are always referenced by their address (global values must live in memory, so their "name" refers to their address). Global variables may have an initial value (which must be a Constant), and if they have an initializer, they may be marked as "constant" themselves (indicating that their contents never change at runtime).

---

Important Public Members of the GlobalVariable class

• GlobalVariable(const Type *Ty, bool isConstant, bool isInternal, Constant *Initializer = 0, const std::string &Name = "")

  Create a new global variable of the specified type. If isConstant is true then the global variable will be marked as unchanging for the program, and if isInternal is true the resultant global variable will have internal linkage. Optionally an initializer and name may be specified for the global variable as well.

• bool isConstant() const

  Returns true if this is a global variable is known not to be modified at runtime.

• bool hasInitializer()

  Returns true if this GlobalVariable has an intializer.

• Constant *getInitializer()

  Returns the intial value for a GlobalVariable. It is not legal to call this method if there is no initializer.

#include "llvm/Module.h"
doxygen info: Module Class

The Module class represents the top level structure present in LLVM programs. An LLVM module is effectively either a translation unit of the original program or a combination of several translation units merged by the linker. The Module class keeps track of a list of Functions, a list of GlobalVariables, and a SymbolTable. Additionally, it contains a few helpful member functions that try to make common operations easy.

---

Important Public Members of the Module class

• Module::iterator - Typedef for function list iterator
  Module::const_iterator - Typedef for const_iterator.
  begin(), end(), front(), back(), size(), empty(), rbegin(), rend()

  These are forwarding methods that make it easy to access the contents of a Module object's Function list.

• Module::FunctionListType &getFunctionList()

  Returns the list of Functions. This is neccesary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

  ---

• Module::giterator - Typedef for global variable list iterator
  Module::const_giterator - Typedef for const_iterator.
  gbegin(), gend(), gfront(), gback(), gsize(), gempty(), grbegin(), grend()

  These are forwarding methods that make it easy to access the contents of a Module object's GlobalVariable list.

• Module::GlobalListType &getGlobalList()

  Returns the list of GlobalVariables. This is neccesary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

  ---

• bool hasSymbolTable() const

  Return true if the Module has a symbol table allocated to it and if there is at least one entry in it.

• SymbolTable *getSymbolTable()

  Return a pointer to the SymbolTable for this Module or a null pointer if one has not been allocated (because there are no named values in the function).

• SymbolTable *getSymbolTableSure()

  Return a pointer to the SymbolTable for this Module or allocate a new SymbolTable if one is not already around. This should only be used when adding elements to the SymbolTable, so that empty symbol tables are not left laying around.

  ---

• Function *getFunction(const std::string &Name, const FunctionType *Ty)

  Look up the specified function in the Module SymbolTable. If it does not exist, return null.

• Function *getOrInsertFunction(const std::string &Name, const FunctionType *T)

  Look up the specified function in the Module SymbolTable. If it does not exist, add an external declaration for the function and return it.

• std::string getTypeName(const Type *Ty)

  If there is at least one entry in the SymbolTable for the specified Type, return it. Otherwise return the empty string.

• bool addTypeName(const std::string &Name, const Type *Ty)

  Insert an entry in the SymbolTable mapping Name to Ty. If there is already an entry for this name, true is returned and the SymbolTable is not modified.

Constant represents a base class for different types of constants. It is subclassed by ConstantBool, ConstantInt, ConstantSInt, ConstantUInt, ConstantArray etc for representing the various types of Constants.

---

Important Public Methods

• bool isConstantExpr(): Returns true if it is a ConstantExpr \subsection{Important Subclasses of Constant} \begin{itemize}
• ConstantSInt : This subclass of Constant represents a signed integer constant. \begin{itemize}
• int64_t getValue () const: Returns the underlying value of this constant. \end{itemize}
• ConstantUInt : This class represents an unsigned integer. \begin{itemize}
• uint64_t getValue () const: Returns the underlying value of this constant. \end{itemize}
• ConstantFP : This class represents a floating point constant. \begin{itemize}
• double getValue () const: Returns the underlying value of this constant. \end{itemize}
• ConstantBool : This represents a boolean constant. \begin{itemize}
• bool getValue () const: Returns the underlying value of this constant. \end{itemize}
• ConstantArray : This represents a constant array. \begin{itemize}
• const std::vector &getValues() const: Returns a Vecotr of component constants that makeup this array. \end{itemize}
• ConstantStruct : This represents a constant struct. \begin{itemize}
• const std::vector &getValues() const: Returns a Vecotr of component constants that makeup this array. \end{itemize}
• ConstantPointerRef : This represents a constant pointer value that is initialized to point to a global value, which lies at a constant fixed address. \begin{itemize}
• GlobalValue *getValue(): Returns the global value to which this pointer is pointing to. \end{itemize} \end{itemize}

Type as noted earlier is also a subclass of a Value class. Any primitive type (like int, short etc) in LLVM is an instance of Type Class. All other types are instances of subclasses of type like FunctionType, ArrayType etc. DerivedType is the interface for all such dervied types including FunctionType, ArrayType, PointerType, StructType. Types can have names. They can be recursive (StructType). There exists exactly one instance of any type structure at a time. This allows using pointer equality of Type *s for comparing types.

---

Important Public Methods

• PrimitiveID getPrimitiveID () const: Returns the base type of the type.
• bool isSigned () const: Returns whether an integral numeric type is signed. This is true for SByteTy, ShortTy, IntTy, LongTy. Note that this is not true for Float and Double.
• bool isUnsigned () const: Returns whether a numeric type is unsigned. This is not quite the complement of isSigned... nonnumeric types return false as they do with isSigned. This returns true for UByteTy, UShortTy, UIntTy, and ULongTy.
• bool isInteger () const: Equilivent to isSigned() || isUnsigned(), but with only a single virtual function invocation.
• bool isIntegral () const: Returns true if this is an integral type, which is either Bool type or one of the Integer types.
• bool isFloatingPoint (): Return true if this is one of the two floating point types.
• bool isRecursive () const: Returns rue if the type graph contains a cycle.
• isLosslesslyConvertableTo (const Type *Ty) const: Return true if this type can be converted to 'Ty' without any reinterpretation of bits. For example, uint to int.
• bool isPrimitiveType () const: Returns true if it is a primitive type.
• bool isDerivedType () const: Returns true if it is a derived type.
• const Type * getContainedType (unsigned i) const: This method is used to implement the type iterator. For derived types, this returns the types 'contained' in the derived type, returning 0 when 'i' becomes invalid. This allows the user to iterate over the types in a struct, for example, really easily.
• unsigned getNumContainedTypes () const: Return the number of types in the derived type. \subsection{Derived Types} \begin{itemize}
• SequentialType : This is subclassed by ArrayType and PointerType \begin{itemize}
• const Type * getElementType () const: Returns the type of each of the elements in the sequential type. \end{itemize}
• ArrayType : This is a subclass of SequentialType and defines interface for array types. \begin{itemize}
• unsigned getNumElements () const: Returns the number of elements in the array. \end{itemize}
• PointerType : Subclass of SequentialType for pointer types.
• StructType : subclass of DerivedTypes for struct types
• FunctionType : subclass of DerivedTypes for function types. \begin{itemize}
• bool isVarArg () const: Returns true if its a vararg function
• const Type * getReturnType () const: Returns the return type of the function.
• const ParamTypes &getParamTypes () const: Returns a vector of parameter types.
• const Type * getParamType (unsigned i): Returns the type of the ith parameter.
• const unsigned getNumParams () const: Returns the number of formal parameters. \end{itemize} \end{itemize}

This subclass of Value defines the interface for incoming formal arguments to a function. A Function maitanis a list of its formal arguments. An argument has a pointer to the parent Function.