LLVM Programmer's Manual

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 and `-debug` option
     - Fine grained debug info with `DEBUG_TYPE` and the `-debug-only` option
   - The `Statistic` template & `-stats` option
   - Viewing graphs while debugging code
4. Helpful Hints for Common Operations
   - Basic Inspection and Traversal Routines
     - Iterating over the `BasicBlock`s in a `Function`
     - Iterating over the `Instruction`s in a `BasicBlock`
     - Iterating over the `Instruction`s in a `Function`
     - Turning an iterator into a class pointer
     - Finding call sites: a more complex example
     - Treating calls and invokes the same way
     - Iterating over def-use & use-def chains
   - Making simple changes
     - Creating and inserting new `Instruction`s
     - Deleting `Instruction`s
     - Replacing an `Instruction` with another `Value`
5. Advanced Topics
   - LLVM Type Resolution
     - Basic Recursive Type Construction
     - The `refineAbstractTypeTo` method
     - The PATypeHolder Class
     - The AbstractTypeUser Class
   - The `SymbolTable` class
6. The Core LLVM Class Hierarchy Reference
   - The `Value` class
     - The `User` class
       - The `Instruction` class
         - The `GetElementPtrInst` class
       - The `Module` class
       - The `Constant` class
         - The `GlobalValue` class
           - The `BasicBlock`class
           - The `Function` class
           - The `GlobalVariable` class
     - The `Type` class
     - The `Argument` class

Written by Chris Lattner, Dinakar Dhurjati, Joel Stanley, and Reid Spencer

Introduction

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.

General Information

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.

The C++ Standard Template Library

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 unfortunately no longer free since the book has been 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
6. Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get the book).

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.

Other useful references

1. CVS Branch and Tag Primer
2. Using static and shared libraries across platforms

Important and useful LLVM APIs

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

The `isa<>`, `cast<>` and `dyn_cast<>` templates

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 `llvm/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.

  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.

The `DEBUG()` macro and `-debug` option

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 "`llvm/Support/Debug.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`' (or any other tool) 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 not 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`.

Fine grained debug info with `DEBUG_TYPE` and the `-debug-only` option

Sometimes you may find yourself in a situation where enabling `-debug` just turns on too much information (such as when working on the code generator). If you want to enable debug information with more fine-grained control, you define the `DEBUG_TYPE` macro and the `-debug` only option as follows:

         ...
         DEBUG(std::cerr << "No debug type\n");
         #undef  DEBUG_TYPE
         #define DEBUG_TYPE "foo"
         DEBUG(std::cerr << "'foo' debug type\n");
         #undef  DEBUG_TYPE
         #define DEBUG_TYPE "bar"
         DEBUG(std::cerr << "'bar' debug type\n");
         #undef  DEBUG_TYPE
         #define DEBUG_TYPE ""
         DEBUG(std::cerr << "No debug type (2)\n");
         ...

Then you can run your pass like this:

      $ opt < a.bc > /dev/null -mypass
        <no output>
      $ opt < a.bc > /dev/null -mypass -debug
        No debug type
        'foo' debug type
        'bar' debug type
        No debug type (2)
      $ opt < a.bc > /dev/null -mypass -debug-only=foo
        'foo' debug type
      $ opt < a.bc > /dev/null -mypass -debug-only=bar
        'bar' debug type
      $

Of course, in practice, you should only set `DEBUG_TYPE` at the top of a file, to specify the debug type for the entire module (if you do this before you `#include "llvm/Support/Debug.h"`, you don't have to insert the ugly `#undef`'s). Also, you should use names more meaningful than "foo" and "bar", because there is no system in place to ensure that names do not conflict. If two different modules use the same string, they will all be turned on when the name is specified. This allows, for example, all debug information for instruction scheduling to be enabled with `-debug-type=InstrSched`, even if the source lives in multiple files.

The `Statistic` template & `-stats` option

The "`llvm/ADT/Statistic.h`" file provides a template named `Statistic` that is used as a unified way to keep 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` uses, but the 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 canonical 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.

Viewing graphs while debugging code

Several of the important data structures in LLVM are graphs: for example CFGs made out of LLVM BasicBlocks, CFGs made out of LLVM MachineBasicBlocks, and Instruction Selection DAGs. In many cases, while debugging various parts of the compiler, it is nice to instantly visualize these graphs.

LLVM provides several callbacks that are available in a debug build to do exactly that. If you call the `Function::viewCFG()` method, for example, the current LLVM tool will pop up a window containing the CFG for the function where each basic block is a node in the graph, and each node contains the instructions in the block. Similarly, there also exists `Function::viewCFGOnly()` (does not include the instructions), the `MachineFunction::viewCFG()` and `MachineFunction::viewCFGOnly()`, and the `SelectionDAG::viewGraph()` methods. Within GDB, for example, you can usually use something like "`call DAG.viewGraph()`" to pop up a window. Alternatively, you can sprinkle calls to these functions in your code in places you want to debug.

Getting this to work requires a small amount of configuration. On Unix systems with X11, install the graphviz toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on Mac OS/X, download and install the Mac OS/X Graphviz program, and add `/Applications/Graphviz.app/Contents/MacOS/` (or whereever you install it) to your path. Once in your system and path are set up, rerun the LLVM configure script and rebuild LLVM to enable this functionality.

Helpful Hints for Common Operations

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.

Basic Inspection and Traversal Routines

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 `BasicBlock`s 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 `BasicBlock`s. To facilitate this, you'll need to iterate over all of the `BasicBlock`s that constitute the `Function`. The following is an example that prints the name of a `BasicBlock` and the number of `Instruction`s 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

          std::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 `Instruction`s in a `BasicBlock`

Just like when dealing with `BasicBlock`s in `Function`s, it's easy to iterate over the individual instructions that make up `BasicBlock`s. 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&
         std::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: `std::cerr << *blk << "\n";`.

Iterating over the `Instruction`s in a `Function`

If you're finding that you commonly iterate over a `Function`'s `BasicBlock`s and then that `BasicBlock`'s `Instruction`s, `InstIterator` should be used instead. You'll need to include `llvm/Support/InstIterator.h`, and then instantiate `InstIterator`s explicitly in your code. Here's a small example that shows how to dump all instructions in a function to the standard error stream:

    #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)
      std::cerr << *i << "\n";

Easy, isn't it? You can also use `InstIterator`s 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 straight-forward. 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, and this is a constant time operation (very efficient). 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()) std::cerr << *it << "\n";
    }

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`*) is already in scope. As you'll learn later, you may want to use an `InstVisitor` to accomplish this in a much more straight-forward 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;
    };

Treating calls and invokes the same way

You may have noticed that the previous example was a bit oversimplified in that it did not deal with call sites generated by 'invoke' instructions. In this, and in other situations, you may find that you want to treat `CallInst`s and `InvokeInst`s the same way, even though their most-specific common base class is `Instruction`, which includes lots of less closely-related things. For these cases, LLVM provides a handy wrapper class called `CallSite`. It is essentially a wrapper around an `Instruction` pointer, with some methods that provide functionality common to `CallInst`s and `InvokeInst`s.

This class has "value semantics": it should be passed by value, not by reference and it should not be dynamically allocated or deallocated using `operator new` or `operator delete`. It is efficiently copyable, assignable and constructable, with costs equivalents to that of a bare pointer. If you look at its definition, it has only a single pointer member.

Iterating over def-use & use-def chains

Frequently, we might have an instance of the Value Class and we want to determine which `User`s use the `Value`. The list of all `User`s 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)) {
            std::cerr << "F is used in instruction:\n";
            std::cerr << *Inst << "\n";
        }
    }

Alternately, it's common to have an instance of the User Class and need to know what `Value`s are used by it. The list of all `Value`s used by a `User` is known as a use-def chain. Instances of class `Instruction` are common `User`s, 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;
        ...
    }

Making simple changes

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 `Instruction`s

Instantiating Instructions

Creation of `Instruction`s is straight-forward: 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

  Appending to the end of a `BasicBlock` is so common that the `Instruction` class and `Instruction`-derived classes provide constructors which take a pointer to a `BasicBlock` to be appended to. For example code that looked like:

        BasicBlock *pb = ...;
        Instruction *newInst = new Instruction(...);
        pb->getInstList().push_back(newInst); // appends newInst to pb

  becomes:

        BasicBlock *pb = ...;
        Instruction *newInst = new Instruction(..., pb);

  which is much cleaner, especially if you are creating long instruction streams.

- Insertion into an implicit instruction list

  `Instruction` instances that are already in `BasicBlock`s 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 `BasicBlock`s.

Deleting `Instruction`s

Deleting an instruction from an existing sequence of instructions that form a `BasicBlock` is very straight-forward. 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`.

#### Deleting `Instruction`s

- `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 a 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 `User`s and `Value`s

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.

Advanced Topics

This section describes some of the advanced or obscure API's that most clients do not need to be aware of. These API's tend manage the inner workings of the LLVM system, and only need to be accessed in unusual circumstances.

LLVM Type Resolution

The LLVM type system has a very simple goal: allow clients to compare types for structural equality with a simple pointer comparison (aka a shallow compare). This goal makes clients much simpler and faster, and is used throughout the LLVM system.

Unfortunately achieving this goal is not a simple matter. In particular, recursive types and late resolution of opaque types makes the situation very difficult to handle. Fortunately, for the most part, our implementation makes most clients able to be completely unaware of the nasty internal details. The primary case where clients are exposed to the inner workings of it are when building a recursive type. In addition to this case, the LLVM bytecode reader, assembly parser, and linker also have to be aware of the inner workings of this system.

For our purposes below, we need three concepts. First, an "Opaque Type" is exactly as defined in the language reference. Second an "Abstract Type" is any type which includes an opaque type as part of its type graph (for example "`{ opaque, int }`"). Third, a concrete type is a type that is not an abstract type (e.g. "`[ int, float }`").

Basic Recursive Type Construction

Because the most common question is "how do I build a recursive type with LLVM", we answer it now and explain it as we go. Here we include enough to cause this to be emitted to an output .ll file:

       %mylist = type { %mylist*, int }

To build this, use the following LLVM APIs:

      // Create the initial outer struct.
      PATypeHolder StructTy = OpaqueType::get();
      std::vector<const Type*> Elts;
      Elts.push_back(PointerType::get(StructTy));
      Elts.push_back(Type::IntTy);
      StructType *NewSTy = StructType::get(Elts);

      // At this point, NewSTy = "{ opaque*, int }". Tell VMCore that
      // the struct and the opaque type are actually the same.
      cast<OpaqueType>(StructTy.get())->refineAbstractTypeTo(NewSTy);

      // NewSTy is potentially invalidated, but StructTy (a PATypeHolder) is
      // kept up-to-date.
      NewSTy = cast<StructType>(StructTy.get());

      // Add a name for the type to the module symbol table (optional).
      MyModule->addTypeName("mylist", NewSTy);

This code shows the basic approach used to build recursive types: build a non-recursive type using 'opaque', then use type unification to close the cycle. The type unification step is performed by the `refineAbstractTypeTo` method, which is described next. After that, we describe the PATypeHolder class.

The `refineAbstractTypeTo` method

The `refineAbstractTypeTo` method starts the type unification process. While this method is actually a member of the DerivedType class, it is most often used on OpaqueType instances. Type unification is actually a recursive process. After unification, types can become structurally isomorphic to existing types, and all duplicates are deleted (to preserve pointer equality).

In the example above, the OpaqueType object is definitely deleted. Additionally, if there is an "{ \2*, int}" type already created in the system, the pointer and struct type created are also deleted. Obviously whenever a type is deleted, any "Type*" pointers in the program are invalidated. As such, it is safest to avoid having any "Type*" pointers to abstract types live across a call to `refineAbstractTypeTo` (note that non-abstract types can never move or be deleted). To deal with this, the PATypeHolder class is used to maintain a stable reference to a possibly refined type, and the AbstractTypeUser class is used to update more complex datastructures.

The PATypeHolder Class

PATypeHolder is a form of a "smart pointer" for Type objects. When VMCore happily goes about nuking types that become isomorphic to existing types, it automatically updates all PATypeHolder objects to point to the new type. In the example above, this allows the code to maintain a pointer to the resultant resolved recursive type, even though the Type*'s are potentially invalidated.

PATypeHolder is an extremely light-weight object that uses a lazy union-find implementation to update pointers. For example the pointer from a Value to its Type is maintained by PATypeHolder objects.

The AbstractTypeUser Class

Some data structures need more to perform more complex updates when types get resolved. The SymbolTable class, for example, needs move and potentially merge type planes in its representation when a pointer changes.

To support this, a class can derive from the AbstractTypeUser class. This class allows it to get callbacks when certain types are resolved. To register to get callbacks for a particular type, the DerivedType::{add/remove}AbstractTypeUser methods can be called on a type. Note that these methods only work for abstract types. Concrete types (those that do not include an opaque objects somewhere) can never be refined.

The `SymbolTable` class

This class provides a symbol table that the `Function` and `Module` classes use for naming definitions. The symbol table can provide a name for any `Value` or `Type`. `SymbolTable` is an abstract data type. It hides the data it contains and provides access to it through a controlled interface.

Note that the symbol table class is should not be directly accessed by most clients. It should only be used when iteration over the symbol table names themselves are required, which is very special purpose. Note that not all LLVM Values have names, and those without names (i.e. they have an empty name) do not exist in the symbol table.

To use the `SymbolTable` well, you need to understand the structure of the information it holds. The class contains two `std::map` objects. The first, `pmap`, is a map of `Type*` to maps of name (`std::string`) to `Value*`. The second, `tmap`, is a map of names to `Type*`. Thus, Values are stored in two-dimensions and accessed by `Type` and name. Types, however, are stored in a single dimension and accessed only by name.

The interface of this class provides three basic types of operations:

1. Accessors. Accessors provide read-only access to information such as finding a value for a name with the lookup method.
2. Mutators. Mutators allow the user to add information to the `SymbolTable` with methods like `insert`.
3. Iterators. Iterators allow the user to traverse the content of the symbol table in well defined ways, such as the method `type_begin`.

### Accessors

`Value* lookup(const Type* Ty, const std::string& name) const`:
  The `lookup` method searches the type plane given by the `Ty` parameter for a `Value` with the provided `name`. If a suitable `Value` is not found, null is returned.
`Type* lookupType( const std::string& name) const`:
  The `lookupType` method searches through the types for a `Type` with the provided `name`. If a suitable `Type` is not found, null is returned.
`bool hasTypes() const`:
  This function returns true if an entry has been made into the type map.
`bool isEmpty() const`:
  This function returns true if both the value and types maps are empty

### Mutators

`void insert(Value *Val)`:
  This method adds the provided value to the symbol table. The Value must have both a name and a type which are extracted and used to place the value in the correct type plane under the value's name.
`void insert(const std::string& Name, Value *Val)`:
  Inserts a constant or type into the symbol table with the specified name. There can be a many to one mapping between names and constants or types.
`void insert(const std::string& Name, Type *Typ)`:
  Inserts a type into the symbol table with the specified name. There can be a many-to-one mapping between names and types. This method allows a type with an existing entry in the symbol table to get a new name.
`void remove(Value* Val)`:
  This method removes a named value from the symbol table. The type and name of the Value are extracted from \p N and used to lookup the Value in the correct type plane. If the Value is not in the symbol table, this method silently ignores the request.
`void remove(Type* Typ)`:
  This method removes a named type from the symbol table. The name of the type is extracted from \P T and used to look up the Type in the type map. If the Type is not in the symbol table, this method silently ignores the request.
`Value* remove(const std::string& Name, Value *Val)`:
  Remove a constant or type with the specified name from the symbol table.
`Type* remove(const std::string& Name, Type* T)`:
  Remove a type with the specified name from the symbol table. Returns the removed Type.
`Value *value_remove(const value_iterator& It)`:
  Removes a specific value from the symbol table. Returns the removed value.
`bool strip()`:
  This method will strip the symbol table of its names leaving the type and values.
`void clear()`:
  Empty the symbol table completely.

### Iteration

The following functions describe three types of iterators you can obtain the beginning or end of the sequence for both const and non-const. It is important to keep track of the different kinds of iterators. There are three idioms worth pointing out:

Units | Iterator | Idiom
--- | --- | ---
Planes Of name/Value maps | PI |
    for (SymbolTable::plane_const_iterator PI = ST.plane_begin(),
         PE = ST.plane_end(); PI != PE; ++PI ) {
      PI->first // This is the Type* of the plane
      PI->second // This is the SymbolTable::ValueMap of name/Value pairs
All name/Type Pairs | TI |
    for (SymbolTable::type_const_iterator TI = ST.type_begin(),
         TE = ST.type_end(); TI != TE; ++TI )
      TI->first  // This is the name of the type
      TI->second // This is the Type* value associated with the name
name/Value pairs in a plane | VI |
    for (SymbolTable::value_const_iterator VI = ST.value_begin(SomeType),
         VE = ST.value_end(SomeType); VI != VE; ++VI )
      VI->first  // This is the name of the Value
      VI->second // This is the Value* value associated with the name

Using the recommended iterator names and idioms will help you avoid making mistakes. Of particular note, make sure that whenever you use value_begin(SomeType) that you always compare the resulting iterator with value_end(SomeType) not value_end(SomeOtherType) or else you will loop infinitely.

`plane_iterator plane_begin()`:
  Get an iterator that starts at the beginning of the type planes. The iterator will iterate over the Type/ValueMap pairs in the type planes.
`plane_const_iterator plane_begin() const`:
  Get a const_iterator that starts at the beginning of the type planes. The iterator will iterate over the Type/ValueMap pairs in the type planes.
`plane_iterator plane_end()`:
  Get an iterator at the end of the type planes. This serves as the marker for end of iteration over the type planes.
`plane_const_iterator plane_end() const`:
  Get a const_iterator at the end of the type planes. This serves as the marker for end of iteration over the type planes.
`value_iterator value_begin(const Type *Typ)`:
  Get an iterator that starts at the beginning of a type plane. The iterator will iterate over the name/value pairs in the type plane. Note: The type plane must already exist before using this.
`value_const_iterator value_begin(const Type *Typ) const`:
  Get a const_iterator that starts at the beginning of a type plane. The iterator will iterate over the name/value pairs in the type plane. Note: The type plane must already exist before using this.
`value_iterator value_end(const Type *Typ)`:
  Get an iterator to the end of a type plane. This serves as the marker for end of iteration of the type plane. Note: The type plane must already exist before using this.
`value_const_iterator value_end(const Type *Typ) const`:
  Get a const_iterator to the end of a type plane. This serves as the marker for end of iteration of the type plane. Note: the type plane must already exist before using this.
`type_iterator type_begin()`:
  Get an iterator to the start of the name/Type map.
`type_const_iterator type_begin() cons`:
  Get a const_iterator to the start of the name/Type map.
`type_iterator type_end()`:
  Get an iterator to the end of the name/Type map. This serves as the marker for end of iteration of the types.
`type_const_iterator type_end() const`:
  Get a const-iterator to the end of the name/Type map. This serves as the marker for end of iteration of the types.
`plane_const_iterator find(const Type* Typ ) const`:
  This method returns a plane_const_iterator for iteration over the type planes starting at a specific plane, given by \p Ty.
`plane_iterator find( const Type* Typ` :
  This method returns a plane_iterator for iteration over the type planes starting at a specific plane, given by \p Ty.

The Core LLVM Class Hierarchy Reference

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.

The `Value` class

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

The `Value` class is the most important class in the 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 `Value`s, such as `Constant`s,`Argument`s. Even `Instruction`s and `Function`s are `Value`s.

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 `User`s that is using it (the `User` class is a base class for all nodes in the LLVM graph that can refer to `Value`s). 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 a 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 instance of 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);

The `User` class

`#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 `Value`s. It exposes a list of "Operands" that are all of the `Value`s 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
  `op_iterator op_begin()` - Get an iterator to the start of the operand list.
  `op_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`.

The `Instruction` class

`#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 `User`s (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 confuses 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

The `BasicBlock` class

`#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 `Instruction`s, 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 `BasicBlock`s themselves are `Value`s, because they are referenced by instructions like branches and can go in the switch tables. `BasicBlock`s 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 optionally takes a name for the new block, and 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()` STL-style functions for accessing the instruction list.

  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.

The `GlobalValue` class

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

Global values (`GlobalVariable`s or `Function`s) are the only LLVM values that are visible in the bodies of all `Function`s. 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, `GlobalValue`s know their linkage rules. Specifically, `GlobalValue`s know whether they have internal or external linkage, as defined by the `LinkageTypes` enumeration.

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, `GlobalValue`s keep track of which `Module` they are currently part of.

Because `GlobalValue`s 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. It is important to remember this when using the `GetElementPtrInst` instruction because this pointer must be dereferenced first. For example, if you have a `GlobalVariable` (a subclass of `GlobalValue)` that is an array of 24 ints, type `[24 x int]`, then the `GlobalVariable` is a pointer to that array. Although the address of the first element of this array and the value of the `GlobalVariable` are the same, they have different types. The `GlobalVariable`'s type is `[24 x int]`. The first element's type is `int.` Because of this, accessing a global value requires you to dereference the pointer with `GetElementPtrInst` first, then its elements can be accessed. 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.

The `Function` class

`#include "llvm/Function.h"`
doxygen info: Function Class
Superclasses: `GlobalValue`, `Constant`, `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 `BasicBlock`s, a list of formal `Argument`s, and a `SymbolTable`.

The list of `BasicBlock`s 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 to 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 `BasicBlock`s, the `Function` class also keeps track of the list of formal `Argument`s that the function receives. This container manages the lifetime of the `Argument` nodes, just like the `BasicBlock` list does for the `BasicBlock`s.

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 `Instruction`s, `BasicBlock`s, or `Argument`s in the function body.

Note that `Function` is a GlobalValue and therefore also a Constant. The value of the function is its address (after linking) which is guaranteed to be constant.

Important Public Members of the `Function` class

- `Function(const` `FunctionType *Ty, LinkageTypes Linkage, const std::string &N = "", Module* Parent = 0)`

  Constructor used when you need to create new `Function`s to add the the program. The constructor must specify the type of the function to create and what type of linkage the function should have. The `FunctionType` argument specifies the formal arguments and return value for the function. The same `FunctionType` value can be used to create multiple functions. The `Parent` argument specifies the Module in which the function is defined. If this argument is provided, the function will automatically be inserted into that module's list of functions.

- `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()` `size()`, `empty()`

  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 `BasicBlock`s. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

- `Function::arg_iterator` - Typedef for the argument list iterator
  `Function::const_arg_iterator` - Typedef for const_iterator.
  `arg_begin()`, `arg_end()` `arg_size()`, `arg_empty()`

  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 `Argument`s. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

- `BasicBlock &getEntryBlock()`

  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`.

The `GlobalVariable` class

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

Global variables are represented with the (suprise suprise) `GlobalVariable` class. Like functions, `GlobalVariable`s 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 constant address). See `GlobalValue` for more on this. 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, LinkageTypes& Linkage, Constant *Initializer = 0, const std::string &Name = "", Module* Parent = 0)`

  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. The Linkage parameter specifies the type of linkage (internal, external, weak, linkonce, appending) for the variable. If the linkage is InternalLinkage, WeakLinkage, or LinkOnceLinkage, then the resultant global variable will have internal linkage. AppendingLinkage concatenates together all instances (in different translation units) of the variable into a single variable but is only applicable to arrays. See the LLVM Language Reference for further details on linkage types. Optionally an initializer, a name, and the module to put the variable into may be specified for the global variable as well.

- `bool isConstant() const`

  Returns true if this is a global variable that 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.

The `Module` class

`#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 `Function`s, a list of `GlobalVariable`s, 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::Module(std::string name = "")`

Constructing a Module is easy. You can optionally provide a name for it (probably based on the name of the translation unit).

- `Module::iterator` - Typedef for function list iterator
  `Module::const_iterator` - Typedef for const_iterator.
  `begin()`, `end()` `size()`, `empty()`

  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 `Function`s. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

- `Module::global_iterator` - Typedef for global variable list iterator
  `Module::const_global_iterator` - Typedef for const_iterator.
  `global_begin()`, `global_end()` `global_size()`, `global_empty()`

  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 `GlobalVariable`s. This is necessary 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.

The `Constant` class and subclasses

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
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<Use> &getValues() const`: Returns a vector of component constants that makeup this array.
- ConstantStruct : This represents a constant struct.
  - `const std::vector<Use> &getValues() const`: Returns a vector of component constants that makeup this array.
- GlobalValue : This represents either a global variable or a function. In either case, the value is a constant fixed address (after linking).

The `Type` class and Derived Types

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

- `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`: Equivalent to isSigned() || isUnsigned().
- `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.
- `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 or one pointer type to another.

Important 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 Type * getParamType (unsigned i)`: Returns the type of the ith parameter.
  - `const unsigned getNumParams() const`: Returns the number of formal parameters.

The `Argument` class

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