Getting Started with the LLVM System

Written by: John Criswell, Chris Lattner, Misha Brukman, Vikram Adve, and Guochun Shi.


Welcome to LLVM! In order to get started, you first need to know some basic information.

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 contains an assembler, disassembler, bitcode analyzer and bitcode optimizer. 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 bitcode. Currently, the GCC front end uses the GCC parser to convert code to LLVM. Once compiled into LLVM bitcode, a program can be manipulated with the LLVM tools from the LLVM suite.

There is a third, optional piece called llvm-test. It is a suite of programs with a testing harness that can be used to further test LLVM's functionality and performance.

Getting Started Quickly (A Summary)

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

  1. Read the documentation.
  2. Read the documentation.
  3. Remember that you were warned twice about reading the documentation.
  4. Install the llvm-gcc-4.2 front end if you intend to compile C or C++ (see Install the GCC Front End for details):
    1. cd where-you-want-the-C-front-end-to-live
    2. gunzip --stdout llvm-gcc-4.2-version-platform.tar.gz | tar -xvf -
    3. install-binutils-binary-from-MinGW (Windows only)
    4. Note: If the binary extension is ".bz" use bunzip2 instead of gunzip.
    5. Note: On Windows, use 7-Zip or a similar archiving tool.
    6. Add llvm-gcc's "bin" directory to your PATH environment variable.
  5. Get the LLVM Source Code
    • With the distributed files (or use SVN):
      1. cd where-you-want-llvm-to-live
      2. gunzip --stdout llvm-version.tar.gz | tar -xvf -
  6. [Optional] Get the Test Suite Source Code
    • With the distributed files (or use SVN):
      1. cd where-you-want-llvm-to-live
      2. cd llvm/projects
      3. gunzip --stdout llvm-test-version.tar.gz | tar -xvf -
  7. Configure the LLVM Build Environment
    1. cd where-you-want-to-build-llvm
    2. /path/to/llvm/configure [options]
      Some common options:
      • --prefix=directory

        Specify for directory the full pathname of where you want the LLVM tools and libraries to be installed (default /usr/local).

      • --with-llvmgccdir=directory

        Optionally, specify for directory the full pathname of the C/C++ front end installation to use with this LLVM configuration. If not specified, the PATH will be searched. This is only needed if you want to run the testsuite or do some special kinds of LLVM builds.

      • --enable-spec2000=directory

        Enable the SPEC2000 benchmarks for testing. The SPEC2000 benchmarks should be available in directory.

  8. Build the LLVM Suite:
    1. gmake -k |& tee gnumake.out    # this is csh or tcsh syntax
    2. If you get an "internal compiler error (ICE)" or test failures, see below.

Consult the Getting Started with LLVM section for detailed information on configuring and compiling LLVM. See Setting Up Your Environment for tips that simplify working with the GCC front end and LLVM tools. Go to Program Layout to learn about the layout of the source code tree.


Before you begin to use the LLVM system, review the requirements given below. This may save you some trouble by knowing ahead of time what hardware and software you will need.


LLVM is known to work on the following platforms:

OS Arch Compilers
AuroraUX x861 GCC
Linux x861 GCC
Linux amd64 GCC
Solaris V9 (Ultrasparc) GCC
FreeBSD x861 GCC
MacOS X2 PowerPC GCC
MacOS X2,9 x86 GCC
Cygwin/Win32 x861,8, 11 GCC 3.4.X, binutils 2.20
MinGW/Win32 x861,6, 8, 10 GCC 3.4.X, binutils 2.20

LLVM has partial support for the following platforms:

OS Arch Compilers
Windows x861 Visual Studio 2005 SP1 or higher4,5
AIX3,4 PowerPC GCC
Linux3,5 PowerPC GCC
Linux7 Alpha GCC
Linux7 Itanium (IA-64) GCC
HP-UX7 Itanium (IA-64) HP aCC


  1. Code generation supported for Pentium processors and up
  2. Code generation supported for 32-bit ABI only
  3. No native code generation
  4. Build is not complete: one or more tools do not link or function
  5. The GCC-based C/C++ frontend does not build
  6. The port is done using the MSYS shell.
  7. Native code generation exists but is not complete.
  8. Binutils 2.20 or later is required to build the assembler generated by LLVM properly.
  9. XCode 2.5 and gcc 4.0.1 (Apple Build 5370) will trip internal LLVM assert messages when compiled for Release at optimization levels greater than 0 (i.e., "-O1" and higher). Add OPTIMIZE_OPTION="-O0" to the build command line if compiling for LLVM Release or bootstrapping the LLVM toolchain.
  10. For MSYS/MinGW on Windows, be sure to install the MSYS version of the perl package, and be sure it appears in your path before any Windows-based versions such as Strawberry Perl and ActivePerl, as these have Windows-specifics that will cause the build to fail.
  11. In general, LLVM modules requiring dynamic linking can not be built on Windows. However, you can build LLVM tools using "make tools-only".

Note that you will need about 1-3 GB of space for a full LLVM build in Debug mode, depending on the system (it is so large because of all the debugging information and the fact that the libraries are statically linked into multiple tools). If you do not need many of the tools and you are space-conscious, you can pass ONLY_TOOLS="tools you need" to make. The Release build requires considerably less space.

The LLVM suite may compile on other platforms, but it is not guaranteed to do so. If compilation is successful, the LLVM utilities should be able to assemble, disassemble, analyze, and optimize LLVM bitcode. Code generation should work as well, although the generated native code may not work on your platform.

The GCC front end is not very portable at the moment. If you want to get it to work on another platform, you can download a copy of the source and try to compile it on your platform.


Compiling LLVM requires that you have several software packages installed. The table below lists those required packages. The Package column is the usual name for the software package that LLVM depends on. The Version column provides "known to work" versions of the package. The Notes column describes how LLVM uses the package and provides other details.

GNU Make 3.79, 3.79.1 Makefile/build processor
GCC 3.4.2 C/C++ compiler1
TeXinfo 4.5 For building the CFE
SVN ≥1.3 Subversion access to LLVM2
DejaGnu 1.4.2 Automated test suite3
tcl 8.3, 8.4 Automated test suite3
expect 5.38.0 Automated test suite3
perl ≥5.6.0 Nightly tester, utilities
GNU M4 1.4 Macro processor for configuration4
GNU Autoconf 2.60 Configuration script builder4
GNU Automake 1.9.6 aclocal macro generator4
libtool 1.5.22 Shared library manager4


  1. Only the C and C++ languages are needed so there's no need to build the other languages for LLVM's purposes. See below for specific version info.
  2. You only need Subversion if you intend to build from the latest LLVM sources. If you're working from a release distribution, you don't need Subversion.
  3. Only needed if you want to run the automated test suite in the llvm/test directory.
  4. If you want to make changes to the configure scripts, you will need GNU autoconf (2.60), and consequently, GNU M4 (version 1.4 or higher). You will also need automake (1.9.6). We only use aclocal from that package.

Additionally, your compilation host is expected to have the usual plethora of Unix utilities. Specifically:

Broken versions of GCC and other tools

LLVM is very demanding of the host C++ compiler, and as such tends to expose bugs in the compiler. In particular, several versions of GCC crash when trying to compile LLVM. We routinely use GCC 3.3.3, 3.4.0, and Apple 4.0.1 successfully with them (however, see important notes below). Other versions of GCC will probably work as well. GCC versions listed here are known to not work. If you are using one of these versions, please try to upgrade your GCC to something more recent. If you run into a problem with a version of GCC not listed here, please let us know. Please use the "gcc -v" command to find out which version of GCC you are using.

GCC versions prior to 3.0: GCC 2.96.x and before had several problems in the STL that effectively prevent it from compiling LLVM.

GCC 3.2.2 and 3.2.3: These versions of GCC fails to compile LLVM with a bogus template error. This was fixed in later GCCs.

GCC 3.3.2: This version of GCC suffered from a serious bug which causes it to crash in the "convert_from_eh_region_ranges_1" GCC function.

Cygwin GCC 3.3.3: The version of GCC 3.3.3 commonly shipped with Cygwin does not work. Please upgrade to a newer version if possible.

SuSE GCC 3.3.3: The version of GCC 3.3.3 shipped with SuSE 9.1 (and possibly others) does not compile LLVM correctly (it appears that exception handling is broken in some cases). Please download the FSF 3.3.3 or upgrade to a newer version of GCC.

GCC 3.4.0 on linux/x86 (32-bit): GCC miscompiles portions of the code generator, causing an infinite loop in the llvm-gcc build when built with optimizations enabled (i.e. a release build).

GCC 3.4.2 on linux/x86 (32-bit): GCC miscompiles portions of the code generator at -O3, as with 3.4.0. However gcc 3.4.2 (unlike 3.4.0) correctly compiles LLVM at -O2. A work around is to build release LLVM builds with "make ENABLE_OPTIMIZED=1 OPTIMIZE_OPTION=-O2 ..."

GCC 3.4.x on X86-64/amd64: GCC miscompiles portions of LLVM.

GCC 3.4.4 (CodeSourcery ARM 2005q3-2): this compiler miscompiles LLVM when building with optimizations enabled. It appears to work with "make ENABLE_OPTIMIZED=1 OPTIMIZE_OPTION=-O1" or build a debug build.

IA-64 GCC 4.0.0: The IA-64 version of GCC 4.0.0 is known to miscompile LLVM.

Apple Xcode 2.3: GCC crashes when compiling LLVM at -O3 (which is the default with ENABLE_OPTIMIZED=1. To work around this, build with "ENABLE_OPTIMIZED=1 OPTIMIZE_OPTION=-O2".

GCC 4.1.1: GCC fails to build LLVM with template concept check errors compiling some files. At the time of this writing, GCC mainline (4.2) did not share the problem.

GCC 4.1.1 on X86-64/amd64: GCC miscompiles portions of LLVM when compiling llvm itself into 64-bit code. LLVM will appear to mostly work but will be buggy, e.g. failing portions of its testsuite.

GCC 4.1.2 on OpenSUSE: Seg faults during libstdc++ build and on x86_64 platforms compiling md5.c gets a mangled constant.

GCC 4.1.2 (20061115 (prerelease) (Debian 4.1.1-21)) on Debian: Appears to miscompile parts of LLVM 2.4. One symptom is ValueSymbolTable complaining about symbols remaining in the table on destruction.

GCC 4.1.2 20071124 (Red Hat 4.1.2-42): Suffers from the same symptoms as the previous one. It appears to work with ENABLE_OPTIMIZED=0 (the default).

Cygwin GCC 4.3.2 20080827 (beta) 2: Users reported various problems related with link errors when using this GCC version.

Debian GCC 4.3.2 on X86: Crashes building some files in LLVM 2.6.

GCC 4.3.3 (Debian 4.3.3-10) on ARM: Miscompiles parts of LLVM 2.6 when optimizations are turned on. The symptom is an infinite loop in FoldingSetImpl::RemoveNode while running the code generator.

GNU ld 2.16.X. Some 2.16.X versions of the ld linker will produce very long warning messages complaining that some ".gnu.linkonce.t.*" symbol was defined in a discarded section. You can safely ignore these messages as they are erroneous and the linkage is correct. These messages disappear using ld 2.17.

GNU binutils 2.17: Binutils 2.17 contains a bug which causes huge link times (minutes instead of seconds) when building LLVM. We recommend upgrading to a newer version ( or later).

GNU Binutils 2.19.1 Gold: This version of Gold contained a bug which causes intermittent failures when building LLVM with position independent code. The symptom is an error about cyclic dependencies. We recommend upgrading to a newer version of Gold.

Getting Started with LLVM

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 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.

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:

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

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).

This is where the LLVM GCC Front End is installed.

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

Setting Up Your Environment

In order to compile and use LLVM, you may need to set some environment variables.

[Optional] This environment variable helps LLVM linking tools find the locations of your bitcode libraries. It is provided only as a convenience since you can specify the paths using the -L options of the tools and the C/C++ front-end will automatically use the bitcode files installed in its lib directory.
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 two files: the LLVM suite and the LLVM GCC front end compiled for your platform. There is an additional test suite that is optional. Each file is a TAR archive that is compressed with the gzip program.

The files are as follows, with x.y marking the version number:

Source release for the LLVM libraries and tools.
Source release for the LLVM test suite.
Source release of the llvm-gcc-4.2 front end. See README.LLVM in the root directory for build instructions.
Binary release of the llvm-gcc-4.2 front end for a specific platform.
Checkout LLVM from Subversion

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

If you want to get a specific release (as opposed to the most recent revision), you can checkout it from the 'tags' directory (instead of 'trunk'). The following releases are located in the following subdirectories of the 'tags' directory:

If you would like to get the LLVM test suite (a separate package as of 1.4), you get it from the Subversion repository:

% cd llvm/projects
% svn co llvm-test

By placing it in the llvm/projects, it will be automatically configured by the LLVM configure script as well as automatically updated when you run svn update.

If you would like to get the GCC front end source code, you can also get it and build it yourself. Please follow these instructions to successfully get and build the LLVM GCC front-end.

Install the GCC Front End

Before configuring and compiling the LLVM suite (or if you want to use just the LLVM GCC front end) you can optionally extract the front end from the binary distribution. It is used for running the llvm-test testsuite and for compiling C/C++ programs. Note that you can optionally build llvm-gcc yourself after building the main LLVM repository.

To install the GCC front end, do the following (on Windows, use an archival tool like 7-zip that understands gzipped tars):

  1. cd where-you-want-the-front-end-to-live
  2. gunzip --stdout llvm-gcc-4.2-version-platform.tar.gz | tar -xvf -

Once the binary is uncompressed, if you're using a *nix-based system, add a symlink for llvm-gcc and llvm-g++ to some directory in your path. If you're using a Windows-based system, add the bin subdirectory of your front end installation directory to your PATH environment variable. For example, if you uncompressed the binary to c:\llvm-gcc, add c:\llvm-gcc\bin to your PATH.

If you now want to build LLVM from source, when you configure LLVM, it will automatically detect llvm-gcc's presence (if it is in your path) enabling its use in llvm-test. Note that you can always build or install llvm-gcc at any point after building the main LLVM repository: just reconfigure llvm and llvm-test will pick it up.

As a convenience for Windows users, the front end binaries for MinGW/x86 include versions of the required w32api and mingw-runtime binaries. The last remaining step for Windows users is to simply uncompress the binary binutils package from MinGW into your front end installation directory. While the front end installation steps are not quite the same as a typical manual MinGW installation, they should be similar enough to those who have previously installed MinGW on Windows systems.

To install binutils on Windows:

  1. download GNU Binutils from MinGW Downloads
  2. cd where-you-uncompressed-the-front-end
  3. uncompress archived binutils directories (not the tar file) into the current directory

The binary versions of the LLVM GCC front end may not suit all of your needs. For example, the binary distribution may include an old version of a system header file, not "fix" a header file that needs to be fixed for GCC, or it may be linked with libraries not available on your system. In cases like these, you may want to try building the GCC front end from source. Thankfully, this is much easier now than it was in the past.

We also do not currently support updating of the GCC front end by manually overlaying newer versions of the w32api and mingw-runtime binary packages that may become available from MinGW. At this time, it's best to think of the MinGW LLVM GCC front end binary as a self-contained convenience package that requires Windows users to simply download and uncompress the GNU Binutils binary package from the MinGW project.

Regardless of your platform, if you discover that installing the LLVM GCC front end binaries is not as easy as previously described, or you would like to suggest improvements, please let us know how you would like to see things improved by dropping us a note on our mailing list.

Local LLVM Configuration

Once checked out from the Subversion repository, the LLVM suite source code must be configured via the configure script. This script sets variables in the various *.in files, most notably llvm/Makefile.config and llvm/include/Config/config.h. It also populates OBJ_ROOT with the Makefiles needed to begin building LLVM.

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

CC Tells configure which C compiler to use. By default, configure will look for the first GCC C compiler in PATH. Use this variable to override configure's default behavior.
CXX Tells configure which C++ compiler to use. By default, configure will look for the first GCC C++ compiler in PATH. Use this variable to override configure's default behavior.

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

Path to the LLVM C/C++ FrontEnd to be used with this LLVM configuration. The value of this option should specify the full pathname of the C/C++ Front End to be used. If this option is not provided, the PATH will be searched for a program named llvm-gcc and the C/C++ FrontEnd install directory will be inferred from the path found. If the option is not given, and no llvm-gcc can be found in the path then a warning will be produced by configure indicating this situation. LLVM may still be built with the tools-only target but attempting to build the runtime libraries will fail as these libraries require llvm-gcc and llvm-g++. See Install the GCC Front End for details on installing the C/C++ Front End. See Bootstrapping the LLVM C/C++ Front-End for details on building the C/C++ Front End.
Path to the tcl include directory under which tclsh can be found. Use this if you have multiple tcl installations on your machine and you want to use a specific one (8.x) for LLVM. LLVM only uses tcl for running the dejagnu based test suite in llvm/test. If you don't specify this option, the LLVM configure script will search for the tcl 8.4 and 8.3 releases.

Enables optimized compilation (debugging symbols are removed and GCC optimization flags are enabled). Note that this is the default setting if you are using the LLVM distribution. The default behavior of an Subversion checkout is to use an unoptimized build (also known as a debug build).

Enables debug symbols in the runtime libraries. The default is to strip debug symbols from the runtime libraries.
Compile the Just In Time (JIT) compiler 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.

Controls which targets will be built and linked into llc. The default value for target_options is "all" which builds and links all available targets. The value "host-only" can be specified to build only a native compiler (no cross-compiler targets available). The "native" target is selected as the target of the build host. You can also specify a comma separated list of target names that you want available in llc. The target names use all lower case. The current set of targets is:
alpha, ia64, powerpc, skeleton, sparc, x86.

Look for the doxygen program and enable construction of doxygen based documentation from the source code. This is disabled by default because generating the documentation can take a long time and producess 100s of megabytes of output.
LLVM can use external disassembler library for various purposes (now it's used only for examining code produced by JIT). This option will enable usage of udis86 x86 (both 32 and 64 bits) disassembler library.

To configure LLVM, follow these steps:

  1. Change directory into the object root directory:

    % cd OBJ_ROOT
  2. Run the configure script located in the LLVM source tree:

    % SRC_ROOT/configure --prefix=/install/path [other options]
Compiling the LLVM Suite Source Code

Once you have configured LLVM, you can build it. There are three types of builds:

Debug Builds
These builds are the default when one is using an Subversion checkout and types gmake (unless the --enable-optimized option was used during configuration). The build system will compile the tools and libraries with debugging information. To get a Debug Build using the LLVM distribution the --disable-optimized option must be passed to configure.

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. For these builds, the build system will compile the tools and libraries with GCC optimizations enabled and strip debugging information from the libraries and executables it generates. Note that Release Builds are default when using an LLVM distribution.

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.

Once you have LLVM configured, you can build it by entering the OBJ_ROOT directory and issuing the following command:

% gmake

If the build fails, please check here to see if you are using a version of GCC that is known not to compile LLVM.

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 dist-clean
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 header files, libraries, tools, and documentation in a hierarchy under $PREFIX, specified with ./configure --prefix=[dir], which defaults to /usr/local.

gmake -C runtime install-bytecode
Assuming you built LLVM into $OBJDIR, when this command is run, it will install bitcode libraries into the GCC front end's bitcode library directory. If you need to update your bitcode libraries, this is the target to use once you've built them.

Please see the Makefile Guide for further details on these make targets and descriptions of other targets available.

It is also possible to override default values from configure by declaring variables on the command line. The following are some examples:

Perform a Release (Optimized) build.

Perform a Release (Optimized) build without assertions enabled.

Perform a Debug build.

Perform a Profiling build.

gmake VERBOSE=1
Print what gmake is doing on standard output.

Ask each tool invoked by the makefiles to print out what it is doing on the standard output. This also implies VERBOSE=1.

Every directory in the LLVM object tree includes a Makefile to build it and any subdirectories that it contains. Entering any directory inside the LLVM object tree and typing gmake should rebuild anything in or below that directory that is out of date.

Cross-Compiling LLVM

It is possible to cross-compile LLVM itself. That is, you can create LLVM executables and libraries to be hosted on a platform different from the platform where they are build (a Canadian Cross build). To configure a cross-compile, supply the configure script with --build and --host options that are different. The values of these options must be legal target triples that your GCC compiler supports.

The result of such a build is executables that are not runnable on on the build host (--build option) but can be executed on the compile host (--host option).

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:

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

Debug Builds with assertions enabled (the default)

Release Builds

Profile Builds
Optional Configuration Items

If you're running on a Linux system that supports the "binfmt_misc" module, and you have root access on the system, you can set your system up to execute LLVM bitcode files directly. To do this, use commands like this (the first command may not be required if you are already using the module):

$ mount -t binfmt_misc none /proc/sys/fs/binfmt_misc
$ echo ':llvm:M::BC::/path/to/lli:' > /proc/sys/fs/binfmt_misc/register
$ chmod u+x hello.bc   (if needed)
$ ./hello.bc

This allows you to execute LLVM bitcode files directly. On Debian, you can also use this command instead of the 'echo' command above:

$ sudo update-binfmts --install llvm /path/to/lli --magic 'BC'
Program Layout

One useful source of information about the LLVM source base is the LLVM doxygen documentation available at The following is a brief introduction to code layout:


This directory contains some simple examples of how to use the LLVM IR and JIT.


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

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...
This directory contains generic support libraries that are provided with LLVM but not necessarily specific to LLVM. For example, some C++ STL utilities and a Command Line option processing library store their header files here.
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.

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

This directory holds the core LLVM source files that implement core classes like Instruction and BasicBlock.
This directory holds the source code for the LLVM assembly language parser library.
This directory holds code for reading and write LLVM bitcode.
This directory contains a variety of different program analyses, such as Dominator Information, Call Graphs, Induction Variables, Interval Identification, Natural Loop Identification, etc.
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 Invariant Code Motion, Dead Global Elimination, and many others.
This directory contains files that describe various target architectures for code generation. For example, the llvm/lib/Target/X86 directory holds the X86 machine description while llvm/lib/Target/CBackend implements the LLVM-to-C converter.
This directory contains the major parts of the code generator: Instruction Selector, Instruction Scheduling, and Register Allocation.
This directory contains the source level debugger library that makes it possible to instrument LLVM programs so that a debugger could identify source code locations at which the program is executing.
This directory contains libraries for executing LLVM bitcode directly at runtime in both interpreted and JIT compiled fashions.
This directory contains the source code that corresponds to the header files located in llvm/include/Support/.
This directory contains the operating system abstraction layer that shields LLVM from platform-specific coding.

This directory contains projects that are not strictly part of LLVM but are shipped with LLVM. This is also the directory where you should create your own LLVM-based projects. See llvm/projects/sample for an example of how to set up your own project.


This directory contains libraries which are compiled into LLVM bitcode 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.


This directory contains feature and regression tests and other basic sanity checks on the LLVM infrastructure. These are intended to run quickly and cover a lot of territory without being exhaustive.


This is not a directory in the normal llvm module; it is a separate Subversion module that must be checked out (usually to projects/test-suite). This module contains a comprehensive correctness, performance, and benchmarking test suite for LLVM. It is a separate Subversion module because not every LLVM user is interested in downloading or building such a comprehensive test suite. For further details on this test suite, please see the Testing Guide document.


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. More detailed information is in the Command Guide.

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.
The LLVM Compiler Driver. This program can be configured to utilize both LLVM and non-LLVM compilation tools to enable pre-processing, translation, optimization, assembly, and linking of programs all from one command line. llvmc also takes care of processing the dependent libraries found in bitcode. This reduces the need to get the traditional -l<name> options right on the command line. Please note that this tool, while functional, is still experimental and not feature complete.
The archiver produces an archive containing the given LLVM bitcode files, optionally with an index for faster lookup.
The assembler transforms the human readable LLVM assembly to LLVM bitcode.
The disassembler transforms the LLVM bitcode to human readable LLVM assembly.
llvm-ld is a general purpose and extensible linker for LLVM. This is the linker invoked by llvmc. It performsn standard link time optimizations and allows optimization modules to be loaded and run so that language specific optimizations can be applied at link time.
llvm-link, not surprisingly, links multiple LLVM modules into a single program.
lli is the LLVM interpreter, which can directly execute LLVM bitcode (although very slowly...). For architectures that support it (currently x86, Sparc, and PowerPC), 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 is the LLVM backend compiler, which translates LLVM bitcode to a native code assembly file or to C code (with the -march=c option).
llvm-gcc is a GCC-based C frontend that has been retargeted to use LLVM as its backend instead of GCC's RTL backend. It can also emit LLVM bitcode or assembly (with the -emit-llvm option) instead of the usual machine code output. It works just like any other GCC compiler, taking the typical -c, -S, -E, -o options that are typically used. Additionally, the the source code for llvm-gcc is available as a separate Subversion module.
opt reads LLVM bitcode, applies a series of LLVM to LLVM transformations (which are specified on the command line), and then outputs the resultant bitcode. The 'opt -help' command is a good way to get a list of the program transformations available in LLVM.
opt can also be used to run a specific analysis on an input LLVM bitcode file and print out the results. It is primarily useful for debugging analyses, or familiarizing yourself with what an analysis does.

This directory contains utilities for working with LLVM source code, and some of the utilities are actually required as part of the build process because they are code generators for parts of LLVM infrastructure.

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'.

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.
The 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/` from the top of your LLVM source tree.

This little tool performs an "egrep -H -n" on each source file in LLVM and passes to it a regular expression provided on llvmgrep's command line. This is a very efficient way of searching the source base for a particular regular expression.
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. 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.

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

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.


This directory contains build scripts and project files for use with Visual C++. This allows developers on Windows to build LLVM without the need for Cygwin. The contents of this directory should be considered experimental at this time.

An Example Using the LLVM Tool Chain

This section gives an example of using LLVM. llvm-gcc3 is now obsolete, so we only include instructions for llvm-gcc4.

Note: The gcc4 frontend's invocation is considerably different from the previous gcc3 frontend. In particular, the gcc4 frontend does not create bitcode by default: gcc4 produces native code. As the example below illustrates, the '--emit-llvm' flag is needed to produce LLVM bitcode output. For makefiles and configure scripts, the CFLAGS variable needs '--emit-llvm' to produce bitcode output.

Example with llvm-gcc4
  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 native executable:

    % llvm-gcc hello.c -o hello

    Note that llvm-gcc works just like GCC by default. The standard -S and -c arguments work as usual (producing a native .s or .o file, respectively).

  3. Next, compile the C file into a LLVM bitcode file:

    % llvm-gcc -O3 -emit-llvm hello.c -c -o hello.bc

    The -emit-llvm option can be used with the -S or -c options to emit an LLVM ".ll" or ".bc" file (respectively) for the code. This allows you to use the standard LLVM tools on the bitcode file.

    Unlike llvm-gcc3, llvm-gcc4 correctly responds to -O[0123] arguments.

  4. Run the program in both forms. To run the program, use:

    % ./hello


    % lli hello.bc

    The second examples shows how to invoke the LLVM JIT, lli.

  5. Use the llvm-dis utility to take a look at the LLVM assembly code:

    llvm-dis < hello.bc | less
  6. Compile the program to native assembly using the LLC code generator:

    % llc hello.bc -o hello.s
  7. Assemble the native assembly language file into a program:

    Solaris: % /opt/SUNWspro/bin/cc -xarch=v9 hello.s -o hello.native
    Others:  % gcc hello.s -o hello.native
  8. Execute the native code program:

    % ./hello.native

    Note that using llvm-gcc to compile directly to native code (i.e. when the -emit-llvm option is not present) does steps 6/7/8 for you.

Common Problems

If you are having problems building or using LLVM, or if you have any other general questions about LLVM, please consult the Frequently Asked Questions page.


This document is just an introduction on 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:

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Reid Spencer
The LLVM Compiler Infrastructure
Last modified: $Date: 2010-07-08 03:27:18 -0500 (Thu, 08 Jul 2010) $