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 LLVM Programmer's Manual
LLVM Programmer's Manual
1. Introduction
2. General Information
3. Important and useful LLVM APIs
4. Helpful Hints for Common Operations
5. The Core LLVM Class Hierarchy Reference

Written by Chris Lattner, Dinakar Dhurjati, and Joel Stanley

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
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 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.

The DEBUG() macro & -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 "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!
$Using the DEBUG() macro instead of a home-brewed solution allows you to now have to create "yet another" command line option for the debug output for your pass. Note that DEBUG() macros are disabled for optimized builds, so they do not cause a performance impact at all (for the same reason, they should also not contain side-effects!). One additional nice thing about the DEBUG() macro is that you can enable or disable it directly in gdb. Just use "set DebugFlag=0" or "set DebugFlag=1" from the gdb if the program is running. If the program hasn't been started yet, you can always just run it with -debug. Fine grained debug info with DEBUG_TYPE() and the -debug-only option Sometimes you may find yourself in a situation where enabling -debug just turns on too much information (such as when working on the code generator). If you want to enable debug information with more fine-grained control, you define the DEBUG_TYPE macro and the -debug only option as follows: ... DEBUG(std::cerr << "No debug type\n"); #undef DEBUG_TYPE #define DEBUG_TYPE "foo" DEBUG(std::cerr << "'foo' debug type\n"); #undef DEBUG_TYPE #define DEBUG_TYPE "bar" DEBUG(std::cerr << "'bar' debug type\n"); #undef DEBUG_TYPE #define DEBUG_TYPE "" DEBUG(std::cerr << "No debug type (2)\n"); ... Then you can run your pass like this:$ 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 "Support/Debug.h", you don't have to insert the ugly #undef's). Also, you should use names more meaningful that "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 all, say, instruction scheduling, debug information to be enabled with -debug-type=InstrSched, even if the source lives in multiple files.

The Statistic template & -stats option
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 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.

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
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&,...)
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)

Finding call sites: a slightly more complex example

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;
...
}

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 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

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 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

Important Public Members of the Value class

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 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 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 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.

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 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.

The GlobalValue class
#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

The Function class
#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 necessary 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 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, 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.

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 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

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

The Type class and Derived Types

Important Public Methods

The Argument class

By: Dinakar Dhurjati and Chris Lattner
The LLVM Compiler Infrastructure