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

Contents

• Overview
  1. Requirements
     1. Hardware
     2. Software
• Getting Started Quickly (A Summary)
• Getting started with LLVM
  1. Terminology and Notation
  2. Setting Up Your Environment
  3. Unpacking the LLVM Archives
  4. Checkout LLVM from CVS
  5. Install the C Front End
  6. Local LLVM Configuration
  7. Compiling the LLVM Suite Source Code
  8. The Location of LLVM Object Files
• Program layout
  1. CVS directories
  2. llvm/include
  3. llvm/lib
  4. llvm/test
  5. llvm/tools
  6. llvm/utils
• Compiling the LLVM GCC Front End
• An Example Using the LLVM Tool Chain
• Common Problems
• Links

First, LLVM comes in two pieces. The first piece is the LLVM suite. This contains all of the tools, libraries, and header files needed to use the low level virtual machine. It also contains a test suite that can be used to test the LLVM tools and the GCC front end.

The second piece is the GCC front end. This component provides a version of GCC that compiles C and C++ code into LLVM bytecode. Currently, the GCC front end is a modified version of GCC 3.4 (we track the GCC 3.4 development). Once compiled into LLVM bytecode, a program can be manipulated with the LLVM tools from the LLVM suite.

Requirements

Hardware

• Linux on x86
  • Approximately 760 MB of Free Disk Space
    • Source code: 30 MB
    • Object code: 670 MB
    • GCC front end: 60 MB

• Solaris on SparcV9 (Ultrasparc)
  • Approximately 1.24 GB of Free Disk Space
    • Source code: 30 MB
    • Object code: 1000 MB
    • GCC front end: 210 MB

If you want to compile your own version of the GCC front end, you will need additional disk space:

• Linux on x86
  • Approximately 249 MB of Free Disk Space
    • Source code: 146 MB
    • Object code: 82 MB
    • Installed binaries: 21 MB

• Solaris on Sparc
  • Approximately 264 MB of Free Disk Space
    • Source code: 146 MB
    • Object code: 93 MB
    • Installed binaries: 25 MB

LLVM may compile on other platforms. The LLVM utilities should work on other platforms, so it should be possible to generate and produce LLVM bytecode on unsupported platforms (although bytecode generated on one platform may not work on another platform). However, the code generators and Just-In-Time (JIT) compilers only generate SparcV9 or x86 machine code.

Software

Unpacking the distribution requires the following tools:

GNU Zip (gzip)

GNU Tar

These tools are needed to uncompress and unarchive the software. Regular Solaris tar may work for unpacking the TAR archive but is untested.

GCC

The GNU Compiler Collection must be installed with C and C++ language support. GCC 3.2.x works, and GCC 3.x is generally supported.

Note that we currently do not support any other C++ compiler.

GNU Make

The LLVM build system relies upon GNU Make extensions. Therefore, you will need GNU Make (sometimes known as gmake) to build LLVM.

Flex and Bison

The LLVM source code is built using flex and bison. You will not be able to configure and compile LLVM without them.

GNU M4

If you are installing Bison on your machine for the first time, you will need GNU M4 (version 1.4 or higher).

There are some additional tools that you may want to have when working with LLVM:

• GNU Autoconf
• GNU M4

  If you want to make changes to the configure scripts, you will need GNU autoconf (2.53 or higher), and consequently, GNU M4 (version 1.4 or higher).

The remainder 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 next section gives a short summary for those who are already familiar with the system and want to get started as quickly as possible. A more complete description is provided after that.

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.

1. Install the GCC front end:
   1. cd where-you-want-the-C-front-end-to-live
   2. gunzip --stdout cfrontend.platform.tar.gz | tar -xvf -

2. Get the Source Code
   • With the distributed files:
     1. cd where-you-want-llvm-to-live
     2. gunzip --stdout llvm.tar.gz | tar -xvf -
     3. cd llvm

   • With anonymous CVS access:
     1. Find the path to the CVS repository containing LLVM (we'll call this CVSROOTDIR).
     2. cd where-you-want-llvm-to-live
     3. cvs -d CVSROOTDIR checkout llvm
     4. cd llvm

3. Configure the LLVM Build Environment
   1. Change directory to where you want to store the LLVM object files and run configure to configure the Makefiles and header files for the default platform. Useful options include:
      • --with-llvmgccdir=directory
        Specify where the LLVM GCC frontend is installed.

4. Build the LLVM Suite
   1. Set your LLVM_LIB_SEARCH_PATH environment variable.
   2. gmake -k |& tee gnumake.out    # this is csh or tcsh syntax

See Setting Up Your Environment on tips to simplify working with the LLVM front-end and compiled tools. See the next section for other useful details in working with LLVM, or go straight to Program Layout to learn about the layout of the source code tree. For information on building the GCC front end yourself, see Compiling the LLVM GCC Front End for information.

Terminology and Notation

Throughout 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:

CVSROOTDIR

This is the path for the CVS repository containing the LLVM source code. Ask the person responsible for your local LLVM installation to give you this path.

SRC_ROOT

This is the top level directory of the LLVM source tree.

OBJ_ROOT

This is the top level directory of the LLVM object tree (i.e. the tree where object files and compiled programs will be placed. It can be the same as SRC_ROOT).

LLVMGCCDIR

This is the where the LLVM GCC Front End is installed.

For the pre-built GCC front end binaries, the LLVMGCCDIR is the cfrontend/platform/llvm-gcc.

GCCSRC

This is the location of the LLVM GCC front end source code (used only if the LLVM GCC front end is being compiled).

GCCOBJ

This is the location of the LLVM GCC front end object code (used only if the LLVM GCC front end is being compiled). It can be safely removed once the LLVM GCC front end is built and installed.

Setting Up Your Environment

In order to compile and use LLVM, you will need to set some environment variables. There are also some shell aliases which you may find useful. You can set these on the command line, or better yet, set them in your .cshrc or .profile.

LLVM_LIB_SEARCH_PATH=LLVMGCCDIR/llvm-gcc/bytecode-libs

This environment variable helps the LLVM GCC front end find bytecode libraries that it will need for compilation.

alias llvmgcc LLVMGCCDIR/bin/llvm-gcc

alias llvmg++ LLVMGCCDIR/bin/llvm-g++

This alias allows you to use the LLVM C and C++ front ends without putting them in your PATH or typing in their complete pathnames.

Unpacking the LLVM Archives

If you have the LLVM distribution, you will need to unpack it before you can begin to compile it. LLVM is distributed as a set of four files. Each file is a TAR archive that is compressed with the gzip program.

The four files are as follows:

llvm.tar.gz

This is the source code to the LLVM suite.

cfrontend.sparc.tar.gz

This is the binary release of the GCC front end for Solaris/Sparc.

cfrontend.x86.tar.gz

This is the binary release of the GCC front end for Linux/x86.

cfrontend-src.tar.gz

This is the source code release of the GCC front end.

Checkout LLVM from CVS

If you have access to our CVS repository, you can get a fresh copy of the entire source code. All you need to do is check it out from CVS as follows:

• cd where-you-want-llvm-to-live
• cvs -d CVSROOTDIR checkout llvm

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.

Note that the GCC front end is not included in the CVS repository. You should have either downloaded the source, or better yet, downloaded the binary distribution for your platform.

Install the GCC Front End

Before configuring and compiling the LLVM suite, you need to extract the LLVM GCC front end from the binary distribution. It is used for building the bytecode libraries later used by the GCC front end for linking programs, and its location must be specified when the LLVM suite is configured.

To install the GCC front end, do the following:

1. cd where-you-want-the-front-end-to-live
2. gunzip --stdout cfrontend.platform.tar.gz | tar -xvf -

Local LLVM Configuration

Once checked out from the CVS repository, the LLVM suite source code must be configured via the configure script. This script sets variables in llvm/Makefile.config and llvm/include/Config/config.h. It also populates OBJ_ROOT with the Makefiles needed to build LLVM.

The following environment variables are used by the configure script to configure the build system:

The following options can be used to set or enable LLVM specific options:

--with-llvmgccdir=LLVMGCCDIR

Path to the location where the LLVM C front end binaries and associated libraries will be installed.

--enable-optimized

Enables optimized compilation by defaulat (debugging symbols are removed and GCC optimization flags are enabled). The default is to use an unoptimized build (also known as a debug build).

--enable-jit

Compile the Just In Time (JIT) functionality. This is not available on all platforms. The default is dependent on platform, so it is best to explicitly enable it if you want it.

--enable-spec2000

--enable-spec2000=<directory>

Enable the use of SPEC2000 when testing LLVM. This is disabled by default (unless configure find SPEC2000 installed). By specifying directory, you can tell configure where to find the SPEC2000 benchmarks. If directory is left unspecified, it configure uses a default value for our internal installation of SPEC2000.

To configure LLVM, follow these steps:

1. Change directory into the object root directory:
   cd OBJ_ROOT

2. Run the script located in the LLVM source tree:
   SRC_ROOT/configure

Compiling the LLVM Suite Source Code

Debug Builds

These builds are the default when one types gmake (unless the --enable-optimized option was used during configuration). They compile the tools and libraries with debugging information.

Release (Optimized) Builds

These builds are enabled with the --enable-optimized option to configure or by specifying ENABLE_OPTIMIZED=1 on the gmake command line. They compile the tools and libraries with GCC optimizer flags on and strip debugging information from the libraries and executables it generates.

Profile Builds

These builds are for use with profiling. They compile profiling information into the code for use with programs like gprof. Profile builds must be started by specifying ENABLE_PROFILING=1 on the gmake command line.

gmake

If you have multiple processors in your machine, you may wish to use some of the parallel build options provided by GNU Make. For example, you could use the command:

gmake -j2

There are several special targets which are useful when working with the LLVM source code:

gmake clean

Removes all files generated by the build. This includes object files, generated C/C++ files, libraries, and executables.

gmake distclean

Removes everything that gmake clean does, but also removes files generated by configure. It attempts to return the source tree to the original state in which it was shipped.

gmake install

Installs LLVM files into the proper location. For the most part, this does nothing, but it does install bytecode libraries into the GCC front end's bytecode library directory. If you need to update your bytecode libraries, this is the target to use once you've built them.

gmake ENABLE_OPTIMIZED=1

Perform a Release (Optimized) build.

gmake ENABLE_PROFILING=1

Perform a Profiling build.

gmake VERBOSE=1

Print what gmake is doing on standard output.

The Location of LLVM Object Files

The LLVM build system is capable of sharing a single LLVM source tree among several LLVM builds. Hence, it is possible to build LLVM for several different platforms or configurations using the same source tree.

This is accomplished in the typical autoconf manner:

• Change directory to where the LLVM object files should live:

  cd OBJ_ROOT

• Run the configure script found in the LLVM source directory:

  SRC_ROOT/configure

The LLVM build will place files underneath OBJ_ROOT in directories named after the build type:

Debug Builds

Tools

OBJ_ROOT/tools/Debug

Libraries

OBJ_ROOT/lib/Debug

Release Builds

Tools

OBJ_ROOT/tools/Release

Libraries

OBJ_ROOT/lib/Release

Profile Builds

Tools

OBJ_ROOT/tools/Profile

Libraries

OBJ_ROOT/lib/Profile

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

CVS directories

llvm/include

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, Target, Transforms, etc...
2. llvm/include/Support - This directory contains generic support libraries that are independent of LLVM, but are used by LLVM. For example, some C++ STL utilities and a Command Line option processing library.
3. llvm/include/Config - This directory contains header files configured by the configure script. They wrap "standard" UNIX and C header files. Source code can include these header files which automatically take care of the conditional #includes that the configure script generates.

llvm/lib

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 Propagation, Inlining, Loop Invarient Code Motion, Dead Global Elimination, 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/Support/

This directory contains the source code that corresponds to the header files located in llvm/include/Support/.

llvm/runtime

This directory contains libraries which are compiled into LLVM bytecode and used when linking programs with the GCC front end. Most of these libraries are skeleton versions of real libraries; for example, libc is a stripped down version of glibc.

Unlike the rest of the LLVM suite, this directory needs the LLVM GCC front end to compile.

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.

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.

bugpoint

bugpoint is used to debug optimization passes or code generation backends by narrowing down the given test case to the minimum number of passes and/or instructions that still cause a problem, whether it is a crash or miscompilation. See HowToSubmitABug.html for more information on using bugpoint.

llvm-ar

The archiver produces an archive containing the given LLVM bytecode files, optionally with an index for faster lookup.

llvm-as

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

llvm-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.

llvm-link

llvm-link, not surprisingly, links multiple LLVM modules into a single program.

lli

lli is the LLVM interpreter, which can directly execute LLVM bytecode (although very slowly...). In addition to a simple interpreter, lli also has a tracing mode (entered by specifying -trace on the command line). Finally, for architectures that support it (currently only x86 and Sparc), by default, lli will function as a Just-In-Time compiler (if the functionality was compiled in), and will execute the code much faster than the interpreter.

llc

llc is the LLVM backend compiler, which translates LLVM bytecode to a SPARC or x86 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 "weird" 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.

llvm/utils

Burg/

Burg is an instruction selector generator -- it builds trees on which it then performs pattern-matching to select instructions according to the patterns the user has specified. Burg is currently used in the Sparc V9 backend.

codegen-diff

codegen-diff is a script that finds differences between code that LLC generates and code that LLI generates. This is a useful tool if you are debugging one of them, assuming that the other generates correct output. For the full user manual, run `perldoc codegen-diff'.

cvsupdate

cvsupdate is a script that will update your CVS tree, but produce a much cleaner and more organized output than simply running `cvs up -dP' will. For example, it will group together all the new and updated files and modified files in separate sections, so you can see at a glance what has changed. If you are at the top of your LLVM CVS tree, running utils/cvsupdate is the preferred way of updating the tree.

emacs/

The emacs directory contains syntax-highlighting files which will work with Emacs and XEmacs editors, providing syntax highlighting support for LLVM assembly files and TableGen description files. For information on how to use the syntax files, consult the README file in that directory.

getsrcs.sh

The getsrcs.sh script finds and outputs all non-generated source files, which is useful if one wishes to do a lot of development across directories and does not want to individually find each file. One way to use it is to run, for example: xemacs `utils/getsources.sh` from the top of your LLVM source tree.

makellvm

The makellvm script compiles all files in the current directory and then compiles and links the tool that is the first argument. For example, assuming you are in the directory llvm/lib/Target/Sparc, if makellvm is in your path, simply running makellvm llc will make a build of the current directory, switch to directory llvm/tools/llc and build it, causing a re-linking of LLC.

NightlyTest.pl and NightlyTestTemplate.html

These files are used in a cron script to generate nightly status reports of the functionality of tools, and the results can be seen by following the appropriate link on the LLVM homepage.

TableGen/

The TableGen directory contains the tool used to generate register descriptions, instruction set descriptions, and even assemblers from common TableGen description files.

vim/

The vim directory contains syntax-highlighting files which will work with the VIM editor, providing syntax highlighting support for LLVM assembly files and TableGen description files. For information on how to use the syntax files, consult the README file in that directory.

Compiling the LLVM GCC Front End

An Example Using the LLVM Tool Chain

Common Problems

When I run configure, it finds the wrong C compiler.

The configure script attempts to locate first gcc and then cc, unless it finds compiler paths set in CC and CXX for the C and C++ compiler, respectively. If configure finds the wrong compiler, either adjust your PATH environment variable or set CC and CXX explicitly.

I compile the code, and I get some error about /localhome.

There are several possible causes for this. The first is that you didn't set a pathname properly when using configure, and it defaulted to a pathname that we use on our research machines.

Another possibility is that we hardcoded a path in our Makefiles. If you see this, please email the LLVM bug mailing list with the name of the offending Makefile and a description of what is wrong with it.

The configure script finds the right C compiler, but it uses the LLVM linker from a previous build. What do I do?

The configure script uses the PATH to find executables, so if it's grabbing the wrong linker/assembler/etc, there are two ways to fix it:

1. Adjust your PATH environment variable so that the correct program appears first in the PATH. This may work, but may not be convenient when you want them first in your path for other work.

2. Run configure with an alternative PATH that is correct. In a Borne compatible shell, the syntax would be:

   PATH= ./configure ...

   This is still somewhat inconvenient, but it allows configure to do its work without having to adjust your PATH permanently.

I've upgraded to a new version of LLVM, and I get strange build errors.

Sometimes changes to the LLVM source code alters how the build system works. Changes in libtool, autoconf, or header file dependencies are especially prone to this sort of problem.

The best thing to try is to remove the old files and re-build. In most cases, this takes care of the problem. To do this, just type make clean and then make in the directory that fails to build.

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:

• LLVM homepage
• LLVM doxygen tree
• Starting a Project that Uses LLVM