Getting Started with the LLVM System
By: Guochun Shi, Chris Lattner and Vikram Adve

Contents

Overview

The next section of this guide is meant to get you up and running with LLVM, and to give you some basic information about the LLVM environment. The first subsection gives a short summary for those who are already familiar with the system and want to get started as quickly as possible.

The later sections of this guide describe the general layout of the the LLVM source-tree, a simple example using the LLVM tool chain, and links to find more information about LLVM or to get help via e-mail.

Getting Started

Getting Started Quickly (A Summary)

Here's the short story for getting up and running quickly with LLVM:

See Setting up your environment on tips to simplify working with the LLVM front-end and compiled tools. See the other sub-sections below for other useful details in working with LLVM, or go straight to Program Layout to learn about the layout of the source code tree.

Terminology and Notation

Through this manual, the following names are used to denote paths specific to the local system and working environment. These are not environment variables you need to set, but just strings used in the rest of this document below.. In any of the examples below, simply replace each of these names with the appropriate pathname on your local system. All these paths are absolute:

Checkout LLVM from CVS

Before checking out the source code, you will need to know the path to CVS repository containing LLVM source code (we'll call this CVSROOTDIR below). Ask the person responsible for your local LLVM installation to give you this path.

To get a fresh copy of the entire source code, all you need to do is check it out from CVS as follows:

This will create an 'llvm' directory in the current directory and fully populate it with the LLVM source code, Makefiles, test directories, and local copies of documentation files.

Local Configuration Options

The file llvm/Makefile.config defines the following path variables, which are specific to a particular installation of LLVM. These should need to be modified only once after checking out a copy of LLVM (if the default values do not already match your system):

The location for LLVM object files

The LLVM make system sends most output files generated during the build into the directory defined by the variable LLVM_OBJ_DIR in llvm/Makefile.config. This can be either just your normal LLVM source tree or some other directory writable by you. You may wish to put object files on a different filesystem either to keep them from being backed up or to speed up local builds.

If you do not wish to use a different location for object files (building into the source tree directly), just set this variable to ".".

Setting up your environment

NOTE: This step is optional but will set up your environment so you can use the compiled LLVM tools with as little hassle as possible.)

Add the following lines to your .cshrc (or the corresponding lines to your .profile if you use a bourne shell derivative). 

       # Make the C front end easy to use...
       alias llvmgcc LLVMGCCDIR/bin/llvm-gcc

       # Make the LLVM tools easy to use...
       setenv PATH LLVM_OBJ_DIR/tools/Debug:${PATH}
The llvmgcc alias is useful because the C compiler is not included in the CVS tree you just checked out.

The other LLVM tools are part of the LLVM source base, and built when compiling LLVM. They will be built into the LLVM_OBJ_DIR/tools/Debug directory.

Compiling the source code

Every directory in the LLVM source tree includes a Makefile to build it, and any subdirectories that it contains. These makefiles require that you use gmake, instead of make to build them, but can otherwise be used freely. To build the entire LLVM system, just enter the top level llvm directory and type gmake. A few minutes later you will hopefully have a freshly compiled toolchain waiting for you in llvm/tools/Debug. If you want to look at the libraries that were compiled, look in llvm/lib/Debug.

If you get an error talking about a /localhome directory, follow the instructions in the section about Setting Up Your Environment.

Program Layout

One useful source of infomation about the LLVM sourcebase is the LLVM doxygen documentation, available at http://llvm.cs.uiuc.edu/doxygen/. The following is a brief introduction to code layout:

CVS directories

Every directory checked out of CVS will contain a CVS directory, for the most part these can just be ignored.

Depend, Debug, & Release directories

If you are building with the "BUILD_ROOT=." option enabled in the Makefile.common file, most source directories will contain two directories, Depend and Debug. The Depend directory contains automatically generated dependance files which are used during compilation to make sure that source files get rebuilt if a header file they use is modified. The Debug directory holds the object files, library files and executables that are used for building a debug enabled build. The Release directory is created to hold the same files when the ENABLE_OPTIMIZED=1 flag is passed to gmake, causing an optimized built to be performed.

llvm/include

This directory contains public header files exported from the LLVM library. The two main subdirectories of this directory are:

  1. llvm/include/llvm - This directory contains all of the LLVM specific header files. This directory also has subdirectories for different portions of LLVM: Analysis, CodeGen, Reoptimizer, Target, Transforms, etc...
  2. llvm/include/Support - This directory contains generic support libraries that are independant of LLVM, but are used by LLVM. For example, some C++ STL utilities and a Command Line option processing library.

llvm/lib

This directory contains most source files of LLVM system. In LLVM almost all code exists in libraries, making it very easy to share code among the different tools.

llvm/lib/VMCore/
This directory holds the core LLVM source files that implement core classes like Instruction and BasicBlock.
llvm/lib/AsmParser/
This directory holds the source code for the LLVM assembly language parser library.
llvm/lib/ByteCode/
This directory holds code for reading and write LLVM bytecode.
llvm/lib/CWriter/
This directory implements the LLVM to C converter.
llvm/lib/Analysis/
This directory contains a variety of different program analyses, such as Dominator Information, Call Graphs, Induction Variables, Interval Identification, Natural Loop Identification, etc...
llvm/lib/Transforms/
This directory contains the source code for the LLVM to LLVM program transformations, such as Aggressive Dead Code Elimination, Sparse Conditional Constant Propogation, Inlining, Loop Invarient Code Motion, Dead Global Elimination, Pool Allocation, and many others...
llvm/lib/Target/
This directory contains files that describe various target architectures for code generation. For example, the llvm/lib/Target/Sparc directory holds the Sparc machine description.
llvm/lib/CodeGen/
This directory contains the major parts of the code generator: Instruction Selector, Instruction Scheduling, and Register Allocation.
llvm/lib/Reoptimizer/
This directory holds code related to the runtime reoptimizer framework that is currently under development.
llvm/lib/Support/
This directory contains the source code that corresponds to the header files located in llvm/include/Support/.

llvm/test

This directory contains regression tests and source code that is used to test the LLVM infrastructure...

llvm/tools

The tools directory contains the executables built out of the libraries above, which form the main part of the user interface. You can always get help for a tool by typing tool_name --help. The following is a brief introduction to the most important tools.

as
The assembler transforms the human readable LLVM assembly to LLVM bytecode.

dis
The disassembler transforms the LLVM bytecode to human readable LLVM assembly. Additionally it can convert LLVM bytecode to C, which is enabled with the -c option.

lli
lli is the LLVM interpreter, which can directly execute LLVM bytecode (although very slowly...). In addition to a simple intepreter, lli is also has debugger and tracing modes (entered by specifying -debug or -trace on the command line, respectively).

llc
llc is the LLVM backend compiler, which translates LLVM bytecode to a SPARC assembly file.

llvmgcc
llvmgcc is a GCC based C frontend that has been retargeted to emit LLVM code as the machine code output. It works just like any other GCC compiler, taking the typical -c, -S, -E, -o options that are typically used. The source code for the llvmgcc tool is currently not included in the LLVM cvs tree because it is quite large and not very interesting.

    gccas
    This tool is invoked by the llvmgcc frontend as the "assembler" part of the compiler. This tool actually assembles LLVM assembly to LLVM bytecode, performs a variety of optimizations, and outputs LLVM bytecode. Thus when you invoke llvmgcc -c x.c -o x.o, you are causing gccas to be run, which writes the x.o file (which is an LLVM bytecode file that can be disassembled or manipulated just like any other bytecode file). The command line interface to gccas is designed to be as close as possible to the system as utility so that the gcc frontend itself did not have to be modified to interface to a "wierd" assembler.

    gccld
    gccld links together several LLVM bytecode files into one bytecode file and does some optimization. It is the linker invoked by the gcc frontend when multiple .o files need to be linked together. Like gccas the command line interface of gccld is designed to match the system linker, to aid interfacing with the GCC frontend.

opt
opt reads LLVM bytecode, applies a series of LLVM to LLVM transformations (which are specified on the command line), and then outputs the resultant bytecode. The 'opt --help' command is a good way to get a list of the program transformations available in LLVM.

analyze
analyze is used to run a specific analysis on an input LLVM bytecode file and print out the results. It is primarily useful for debugging analyses, or familiarizing yourself with what an analysis does.

An example using the LLVM tool chain

  1. First, create a simple C file, name it 'hello.c': 
       #include <stdio.h>
   int main() {
     printf("hello world\n");
     return 0;
   }
  2. Next, compile the C file into a LLVM bytecode file:

    % llvmgcc hello.c -o hello

    This will create two result files: hello and hello.bc. The hello.bc is the LLVM bytecode that corresponds the the compiled program and the library facilities that it required. hello is a simple shell script that runs the bytecode file with lli, making the result directly executable.

  3. Run the program. To make sure the program ran, execute one of the following commands:

    % ./hello

    or

    % lli hello.bc

  4. Use the dis utility to take a look at the LLVM assembly code:

    % dis < hello.bc | less

  5. Compile the program to native Sparc assembly using the code generator:

    % llc hello.bc -o hello.s

  6. Assemble the native sparc assemble file into a program:

    % /opt/SUNWspro/bin/cc -xarch=v9 hello.s -o hello.sparc

  7. Execute the native sparc program:

    % ./hello.sparc

Links

This document is just an introduction to how to use LLVM to do some simple things... there are many more interesting and complicated things that you can do that aren't documented here (but we'll gladly accept a patch if you want to write something up!). For more information about LLVM, check out:

If you have any questions or run into any snags (or you have any additions...), please send an email to Nicholas Hildenbrandt or Chris Lattner.

Last modified: Thu Sep 19 14:55:19 CDT 2002