1. Introduction
2. General Information
   • The C++ Standard Template Library
3. Important and useful LLVM APIs
   • The isa<>, cast<> and dyn_cast<> templates
   • The DEBUG() macro & -debug option
   • The Statistic template & -stats option
4. Helpful Hints for Common Operations
   • Basic Inspection and Traversal Routines
     • Iterating over the BasicBlocks in a Function
     • Iterating over the Instructions in a BasicBlock
     • Iterating over the Instructions in a Function
     • Turning an iterator into a class pointer
     • Finding call sites: a more complex example
     • Iterating over def-use & use-def chains
   • Making simple changes
     • Creating and inserting new Instructions
     • Deleting Instructions
     • Replacing an Instruction with another Value
5. The Core LLVM Class Hierarchy Reference
   • 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 Chris Lattner, Dinakar Dhurjati, and Joel Stanley

This document is meant to highlight some of the important classes and interfaces available in the LLVM source-base. This manual is not intended 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++ In a Nutshell - This is an O'Reilly book in the making. It has a decent Standard Library Reference that rivals Dinkumware's, and is actually free until the book is published.
3. C++ Frequently Asked Questions
4. SGI's STL Programmer's Guide - Contains a useful Introduction to the STL.
5. 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.

Here we highlight some LLVM APIs that are generally useful and good to know about when writing transformations.

The LLVM source-base makes extensive use of a custom form of RTTI. These templates have many similarities to the C++ dynamic_cast<> operator, but they don't have some drawbacks (primarily stemming from the fact that dynamic_cast<> only works on classes that have a v-table). Because they are used so often, you must know what they do and how they work. All of these templates are defined in the Support/Casting.h file (note that you very rarely have to include this file directly).

isa<>:

The isa<> operator works exactly like the Java "instanceof" operator. It returns true or false depending on whether a reference or pointer points to an instance of the specified class. This can be very useful for constraint checking of various sorts (example below).

cast<>:

The cast<> operator is a "checked cast" operation. It converts a pointer or reference from a base class to a derived cast, causing an assertion failure if it is not really an instance of the right type. This should be used in cases where you have some information that makes you believe that something is of the right type. An example of the isa<> and cast<> template is:

static bool isLoopInvariant(const Value *V, const Loop *L) {
  if (isa<Constant>(V) || isa<Argument>(V) || isa<GlobalValue>(V))
    return true;

  // Otherwise, it must be an instruction...
  return !L->contains(cast<Instruction>(V)->getParent());

Note that you should not use an isa<> test followed by a cast<>, for that use the dyn_cast<> operator.

dyn_cast<>:

The dyn_cast<> operator is a "checking cast" operation. It checks to see if the operand is of the specified type, and if so, returns a pointer to it (this operator does not work with references). If the operand is not of the correct type, a null pointer is returned. Thus, this works very much like the dynamic_cast operator in C++, and should be used in the same circumstances. Typically, the dyn_cast<> operator is used in an if statement or some other flow control statement like this:

  if (AllocationInst *AI = dyn_cast<AllocationInst>(Val)) {
    ...
  }

This form of the if statement effectively combines together a call to isa<> and a call to cast<> into one statement, which is very convenient.

Another common example is:

  // Loop over all of the phi nodes in a basic block
  BasicBlock::iterator BBI = BB->begin();
  for (; PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
    cerr << *PN;

Note that the dyn_cast<> operator, like C++'s dynamic_cast or Java's instanceof operator, can be abused. In particular you should not use big chained if/then/else blocks to check for lots of different variants of classes. If you find yourself wanting to do this, it is much cleaner and more efficient to use the InstVisitor class to dispatch over the instruction type directly.

cast_or_null<>:

The cast_or_null<> operator works just like the cast<> operator, except that it allows for a null pointer as an argument (which it then propagates). This can sometimes be useful, allowing you to combine several null checks into one.

dyn_cast_or_null<>:

The dyn_cast_or_null<> operator works just like the dyn_cast<> operator, except that it allows for a null pointer as an argument (which it then propagates). This can sometimes be useful, allowing you to combine several null checks into one.

These five templates can be used with any classes, whether they have a v-table or not. To add support for these templates, you simply need to add classof static methods to the class you are interested casting to. Describing this is currently outside the scope of this document, but there are lots of examples in the LLVM source base.

Often when working on your pass you will put a bunch of debugging printouts and other code into your pass. After you get it working, you want to remove it... but you may need it again in the future (to work out new bugs that you run across).

Naturally, because of this, you don't want to delete the debug printouts, but you don't want them to always be noisy. A standard compromise is to comment them out, allowing you to enable them if you need them in the future.

The "Support/Statistic.h" file provides a macro named DEBUG() that is a much nicer solution to this problem. Basically, you can put arbitrary code into the argument of the DEBUG macro, and it is only executed if 'opt' is run with the '-debug' command line argument:

     ... 
     DEBUG(std::cerr << "I am here!\n");
     ...

Then you can run your pass like this:

  $ opt < a.bc > /dev/null -mypass
    <no output>
  $ opt < a.bc > /dev/null -mypass -debug
    I am here!
  $

Using the DEBUG() macro instead of a home brewed solution allows you to now have to create "yet another" command line option for the debug output for your pass. Note that DEBUG() macros are disabled for optimized builds, so they do not cause a performance impact at all (for the same reason, they should also not contain side-effects!).

One additional nice thing about the DEBUG() macro is that you can enable or disable it directly in gdb. Just use "set DebugFlag=0" or "set DebugFlag=1" from the gdb if the program is running. If the program hasn't been started yet, you can always just run it with -debug.

The "Support/Statistic.h" file provides a template named Statistic that is used as a unified way to keeping track of what the LLVM compiler is doing and how effective various optimizations are. It is useful to see what optimizations are contributing to making a particular program run faster.

Often you may run your pass on some big program, and you're interested to see how many times it makes a certain transformation. Although you can do this with hand inspection, or some ad-hoc method, this is a real pain and not very useful for big programs. Using the Statistic template makes it very easy to keep track of this information, and the calculated information is presented in a uniform manner with the rest of the passes being executed.

There are many examples of Statistic users, but this basics of using it are as follows:

1. Define your statistic like this:

   static Statistic<> NumXForms("mypassname", "The # of times I did stuff");
   

   The Statistic template can emulate just about any data-type, but if you do not specify a template argument, it defaults to acting like an unsigned int counter (this is usually what you want).

2. Whenever you make a transformation, bump the counter:

      ++NumXForms;   // I did stuff
   

That's all you have to do. To get 'opt' to print out the statistics gathered, use the '-stats' option:

   $ opt -stats -mypassname < program.bc > /dev/null
    ... statistic output ...

When running gccas on a C file from the SPEC benchmark suite, it gives a report that looks like this:

   7646 bytecodewriter  - Number of normal instructions
    725 bytecodewriter  - Number of oversized instructions
 129996 bytecodewriter  - Number of bytecode bytes written
   2817 raise           - Number of insts DCEd or constprop'd
   3213 raise           - Number of cast-of-self removed
   5046 raise           - Number of expression trees converted
     75 raise           - Number of other getelementptr's formed
    138 raise           - Number of load/store peepholes
     42 deadtypeelim    - Number of unused typenames removed from symtab
    392 funcresolve     - Number of varargs functions resolved
     27 globaldce       - Number of global variables removed
      2 adce            - Number of basic blocks removed
    134 cee             - Number of branches revectored
     49 cee             - Number of setcc instruction eliminated
    532 gcse            - Number of loads removed
   2919 gcse            - Number of instructions removed
     86 indvars         - Number of cannonical indvars added
     87 indvars         - Number of aux indvars removed
     25 instcombine     - Number of dead inst eliminate
    434 instcombine     - Number of insts combined
    248 licm            - Number of load insts hoisted
   1298 licm            - Number of insts hoisted to a loop pre-header
      3 licm            - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
     75 mem2reg         - Number of alloca's promoted
   1444 cfgsimplify     - Number of blocks simplified

Obviously, with so many optimizations, having a unified framework for this stuff is very nice. Making your pass fit well into the framework makes it more maintainable and useful.

This section describes how to perform some very simple transformations of LLVM code. This is meant to give examples of common idioms used, showing the practical side of LLVM transformations.

Because this is a "how-to" section, you should also read about the main classes that you will be working with. The Core LLVM Class Hierarchy Reference contains details and descriptions of the main classes that you should know about.

The LLVM compiler infrastructure have many different data structures that may be traversed. Following the example of the C++ standard template library, the techniques used to traverse these various data structures are all basically the same. For a enumerable sequence of values, the XXXbegin() function (or method) returns an iterator to the start of the sequence, the XXXend() function returns an iterator pointing to one past the last valid element of the sequence, and there is some XXXiterator data type that is common between the two operations.

Because the pattern for iteration is common across many different aspects of the program representation, the standard template library algorithms may be used on them, and it is easier to remember how to iterate. First we show a few common examples of the data structures that need to be traversed. Other data structures are traversed in very similar ways.

---

Iterating over the BasicBlocks in a Function

It's quite common to have a Function instance that you'd like to transform in some way; in particular, you'd like to manipulate its BasicBlocks. To facilitate this, you'll need to iterate over all of the BasicBlocks that constitute the Function. The following is an example that prints the name of a BasicBlock and the number of Instructions it contains:

  // func is a pointer to a Function instance
  for(Function::iterator i = func->begin(), e = func->end(); i != e; ++i) {

      // print out the name of the basic block if it has one, and then the
      // number of instructions that it contains

      cerr << "Basic block (name=" << i->getName() << ") has " 
           << i->size() << " instructions.\n";
  }

Note that i can be used as if it were a pointer for the purposes of invoking member functions of the Instruction class. This is because the indirection operator is overloaded for the iterator classes. In the above code, the expression i->size() is exactly equivalent to (*i).size() just like you'd expect.

---

Iterating over the Instructions in a BasicBlock

Just like when dealing with BasicBlocks in Functions, it's easy to iterate over the individual instructions that make up BasicBlocks. Here's a code snippet that prints out each instruction in a BasicBlock:

  // blk is a pointer to a BasicBlock instance
  for(BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
     // the next statement works since operator<<(ostream&,...) 
     // is overloaded for Instruction&
     cerr << *i << "\n";

However, this isn't really the best way to print out the contents of a BasicBlock! Since the ostream operators are overloaded for virtually anything you'll care about, you could have just invoked the print routine on the basic block itself: cerr << *blk << "\n";.

Note that currently operator<< is implemented for Value*, so it will print out the contents of the pointer, instead of the pointer value you might expect. This is a deprecated interface that will be removed in the future, so it's best not to depend on it. To print out the pointer value for now, you must cast to void*.

---

Iterating over the Instructions in a Function

If you're finding that you commonly iterate over a Function's BasicBlocks and then that BasicBlock's Instructions, InstIterator should be used instead. You'll need to include llvm/Support/InstIterator.h, and then instantiate InstIterators explicitly in your code. Here's a small example that shows how to dump all instructions in a function to stderr (Note: Dereferencing an InstIterator yields an Instruction*, not an Instruction&!):

#include "llvm/Support/InstIterator.h"
...
// Suppose F is a ptr to a function
for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
  cerr << **i << "\n";

Easy, isn't it? You can also use InstIterators to fill a worklist with its initial contents. For example, if you wanted to initialize a worklist to contain all instructions in a Function F, all you would need to do is something like:

std::set<Instruction*> worklist;
worklist.insert(inst_begin(F), inst_end(F));

The STL set worklist would now contain all instructions in the Function pointed to by F.

---

Turning an iterator into a class pointer (and vice-versa)

Sometimes, it'll be useful to grab a reference (or pointer) to a class instance when all you've got at hand is an iterator. Well, extracting a reference or a pointer from an iterator is very straightforward. Assuming that i is a BasicBlock::iterator and j is a BasicBlock::const_iterator:

    Instruction& inst = *i;   // grab reference to instruction reference
    Instruction* pinst = &*i; // grab pointer to instruction reference
    const Instruction& inst = *j;

However, the iterators you'll be working with in the LLVM framework are special: they will automatically convert to a ptr-to-instance type whenever they need to. Instead of dereferencing the iterator and then taking the address of the result, you can simply assign the iterator to the proper pointer type and you get the dereference and address-of operation as a result of the assignment (behind the scenes, this is a result of overloading casting mechanisms). Thus the last line of the last example,

Instruction* pinst = &*i;

is semantically equivalent to

Instruction* pinst = i;

It's also possible to turn a class pointer into the corresponding iterator. Usually, this conversion is quite inexpensive. The following code snippet illustrates use of the conversion constructors provided by LLVM iterators. By using these, you can explicitly grab the iterator of something without actually obtaining it via iteration over some structure:

void printNextInstruction(Instruction* inst) {
    BasicBlock::iterator it(inst);
    ++it; // after this line, it refers to the instruction after *inst.
    if(it != inst->getParent()->end()) cerr << *it << "\n";
}

Of course, this example is strictly pedagogical, because it'd be much better to explicitly grab the next instruction directly from inst.

---

Finding call sites: a slightly more complex example

Say that you're writing a FunctionPass and would like to count all the locations in the entire module (that is, across every Function) where a certain function (i.e. some Function*) already in scope. As you'll learn later, you may want to use an InstVisitor to accomplish this in a much more straightforward manner, but this example will allow us to explore how you'd do it if you didn't have InstVisitor around. In pseudocode, this is what we want to do:

initialize callCounter to zero
for each Function f in the Module
    for each BasicBlock b in f
      for each Instruction i in b
        if(i is a CallInst and calls the given function)
          increment callCounter

And the actual code is (remember, since we're writing a FunctionPass, our FunctionPass-derived class simply has to override the runOnFunction method...):

Function* targetFunc = ...;

class OurFunctionPass : public FunctionPass {
  public:
    OurFunctionPass(): callCounter(0) { }

    virtual runOnFunction(Function& F) {
 	for(Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
 	    for(BasicBlock::iterator i = b->begin(); ie = b->end(); i != ie; ++i) {
 		if (CallInst* callInst = dyn_cast<CallInst>(&*i)) {
 		    // we know we've encountered a call instruction, so we
 		    // need to determine if it's a call to the
	            // function pointed to by m_func or not.
  
 		    if(callInst->getCalledFunction() == targetFunc)
 			++callCounter;
 	    }
 	}
    }
    
  private:
    unsigned  callCounter;
};

---

Iterating over def-use & use-def chains

Frequently, we might have an instance of the Value Class and we want to determine which Users use the Value. The list of all Users of a particular Value is called a def-use chain. For example, let's say we have a Function* named F to a particular function foo. Finding all of the instructions that use foo is as simple as iterating over the def-use chain of F:

Function* F = ...;

for(Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i) {
    if(Instruction* Inst = dyn_cast<Instruction>(*i)) {
        cerr << "F is used in instruction:\n";
        cerr << *Inst << "\n";
    }
}

Alternately, it's common to have an instance of the User Class and need to know what Values are used by it. The list of all Values used by a User is known as a use-def chain. Instances of class Instruction are common Users, so we might want to iterate over all of the values that a particular instruction uses (that is, the operands of the particular Instruction):

Instruction* pi = ...;

for(User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
    Value* v = *i;
    ...
}

There are some primitive transformation operations present in the LLVM infrastructure that are worth knowing about. When performing transformations, it's fairly common to manipulate the contents of basic blocks. This section describes some of the common methods for doing so and gives example code.

---

Creating and inserting new Instructions

Instantiating Instructions

Creation of Instructions is straightforward: simply call the constructor for the kind of instruction to instantiate and provide the necessary parameters. For example, an AllocaInst only requires a (const-ptr-to) Type. Thus:

AllocaInst* ai = new AllocaInst(Type::IntTy);

will create an AllocaInst instance that represents the allocation of one integer in the current stack frame, at runtime. Each Instruction subclass is likely to have varying default parameters which change the semantics of the instruction, so refer to the doxygen documentation for the subclass of Instruction that you're interested in instantiating.

Naming values

It is very useful to name the values of instructions when you're able to, as this facilitates the debugging of your transformations. If you end up looking at generated LLVM machine code, you definitely want to have logical names associated with the results of instructions! By supplying a value for the Name (default) parameter of the Instruction constructor, you associate a logical name with the result of the instruction's execution at runtime. For example, say that I'm writing a transformation that dynamically allocates space for an integer on the stack, and that integer is going to be used as some kind of index by some other code. To accomplish this, I place an AllocaInst at the first point in the first BasicBlock of some Function, and I'm intending to use it within the same Function. I might do:

AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc");

where indexLoc is now the logical name of the instruction's execution value, which is a pointer to an integer on the runtime stack.

Inserting instructions

There are essentially two ways to insert an Instruction into an existing sequence of instructions that form a BasicBlock:

• Insertion into an explicit instruction list

  Given a BasicBlock* pb, an Instruction* pi within that BasicBlock, and a newly-created instruction we wish to insert before *pi, we do the following:

  BasicBlock* pb = ...;
  Instruction* pi = ...;
  Instruction* newInst = new Instruction(...);
  pb->getInstList().insert(pi, newInst); // inserts newInst before pi in pb
  

• Insertion into an implicit instruction list

  Instruction instances that are already in BasicBlocks are implicitly associated with an existing instruction list: the instruction list of the enclosing basic block. Thus, we could have accomplished the same thing as the above code without being given a BasicBlock by doing:

  Instruction* pi = ...;
  Instruction* newInst = new Instruction(...);
  pi->getParent()->getInstList().insert(pi, newInst);
  

  In fact, this sequence of steps occurs so frequently that the Instruction class and Instruction-derived classes provide constructors which take (as a default parameter) a pointer to an Instruction which the newly-created Instruction should precede. That is, Instruction constructors are capable of inserting the newly-created instance into the BasicBlock of a provided instruction, immediately before that instruction. Using an Instruction constructor with a insertBefore (default) parameter, the above code becomes:

  Instruction* pi = ...;
  Instruction* newInst = new Instruction(..., pi);
  

  which is much cleaner, especially if you're creating a lot of instructions and adding them to BasicBlocks.

---

Deleting Instructions

Deleting an instruction from an existing sequence of instructions that form a BasicBlock is very straightforward. First, you must have a pointer to the instruction that you wish to delete. Second, you need to obtain the pointer to that instruction's basic block. You use the pointer to the basic block to get its list of instructions and then use the erase function to remove your instruction.

For example:

  Instruction *I = .. ;
  BasicBlock *BB = I->getParent();
  BB->getInstList().erase(I);

---

Replacing an Instruction with another Value

Replacing individual instructions

Including "llvm/Transforms/Utils/BasicBlockUtils.h " permits use of two very useful replace functions: ReplaceInstWithValue and ReplaceInstWithInst.

• ReplaceInstWithValue

  This function replaces all uses (within a basic block) of a given instruction with a value, and then removes the original instruction. The following example illustrates the replacement of the result of a particular AllocaInst that allocates memory for a single integer with an null pointer to an integer.

  AllocaInst* instToReplace = ...;
  BasicBlock::iterator ii(instToReplace);
  ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
                       Constant::getNullValue(PointerType::get(Type::IntTy)));
  

• ReplaceInstWithInst

  This function replaces a particular instruction with another instruction. The following example illustrates the replacement of one AllocaInst with another.

  AllocaInst* instToReplace = ...;
  BasicBlock::iterator ii(instToReplace);
  ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
                      new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt");
  

Replacing multiple uses of Users and Values

You can use Value::replaceAllUsesWith and User::replaceUsesOfWith to change more than one use at a time. See the doxygen documentation for the Value Class and User Class, respectively, for more information.

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 accessible 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 Users 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).

An important file for the Instruction class is the llvm/Instruction.def file. This file contains some meta-data about the various different types of instructions in LLVM. It describes the enum values that are used as opcodes (for example Instruction::Add and Instruction::SetLE), as well as the concrete sub-classes of Instruction that implement the instruction (for example BinaryOperator and SetCondInst). Unfortunately, the use of macros in this file confused doxygen, so these enum values don't show up correctly in the doxygen output.

---

Important Public Members of the Instruction class

• BasicBlock *getParent()

  Returns the BasicBlock that this Instruction is embedded into.

• bool mayWriteToMemory()

  Returns true if the instruction writes to memory, i.e. it is a call, free, invoke, or store.

• unsigned getOpcode()

  Returns the opcode for the Instruction.

• Instruction *clone() const

  Returns another instance of the specified instruction, identical in all ways to the original except that the instruction has no parent (ie it's not embedded into a BasicBlock), and it has no name.

#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.

• SymbolTable *getSymbolTable()

  Return a pointer to the SymbolTable for this Function.

#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.

  ---

• SymbolTable *getSymbolTable()

  Return a reference to the SymbolTable for this Module.

  ---

• 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

  ---

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

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.

  ---

  Derived Types
  • SequentialType : This is subclassed by ArrayType and PointerType
    • const Type * getElementType() const: Returns the type of each of the elements in the sequential type.
  • ArrayType : This is a subclass of SequentialType and defines interface for array types.
    • unsigned getNumElements() const: Returns the number of elements in the array.
  • PointerType : Subclass of SequentialType for pointer types.
  • StructType : subclass of DerivedTypes for struct types
  • FunctionType : subclass of DerivedTypes for function types.
    • 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.

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.