Getting Started with the LLVM System

- Overview
- Getting Started Quickly (A Summary)
- Requirements
  1. Hardware
  2. Software
  3. Broken versions of GCC
- 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 GCC Front End
  6. Local LLVM Configuration
  7. Compiling the LLVM Suite Source Code
  8. The Location of LLVM Object Files
  9. Optional Configuration Items
- Program layout
  1. `CVS` directories
  2. `llvm/include`
  3. `llvm/lib`
  4. `llvm/runtime`
  5. `llvm/test`
  6. `llvm-test`
  7. `llvm/tools`
  8. `llvm/utils`
- An Example Using the LLVM Tool Chain
- Common Problems
- Links

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

Overview

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, bytecode analyzer, and bytecode 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 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.

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 GCC front end:
   1. `cd where-you-want-the-C-front-end-to-live`
   2. `gunzip --stdout cfrontend.platform.tar.gz | tar -xvf -`
   3. Sparc and MacOS X Only:
      `cd cfrontend/platform
      ./fixheaders`
5. Get the Source Code
   - With the distributed files:
     1. `cd where-you-want-llvm-to-live`
     2. `gunzip --stdout llvm-version.tar.gz | tar -xvf -`
     3. `cd llvm`
   - With anonymous CVS access (or use a mirror):
     1. `cd where-you-want-llvm-to-live`
     2. `cvs -d :pserver:anon@llvm-cvs.cs.uiuc.edu:/var/cvs/llvm login`
     3. Hit the return key when prompted for the password.
     4. `cvs -z3 -d :pserver:anon@llvm-cvs.cs.uiuc.edu:/var/cvs/llvm co llvm`
     5. `cd llvm`
6. 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 the full pathname of where the LLVM GCC frontend is installed.

      - `--enable-spec2000=directory`

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

7. 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`
   3. If you get an "internal compiler error (ICE)" 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.

Requirements

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.

Hardware

LLVM is known to work on the following platforms:

OS | Arch | Compilers
--- | --- | ---
Linux | x861 | GCC
Solaris | V9 (Ultrasparc) | GCC
FreeBSD | x861 | GCC
MacOS X2 | PowerPC | GCC
Cygwin/Win32 | x861 | GCC

LLVM has partial support for the following platforms:

OS | Arch | Compilers
--- | --- | ---
Windows | x861 | Visual Studio .NET4,5, MinGW
AIX3,4 | PowerPC | GCC
Linux3,5 | PowerPC | GCC

Notes:
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 don't link
5 The GCC-based C/C++ frontend does not build

Note that you will need about 1-3 GB of space for a full LLVM build in Debug mode, depending on the system (because of all the debug info), and the libraries appear in more than one of the tools that get linked, so there is some duplication. If you do not need many of the tools and you are space-conscious, you can disable them individually in `llvm/tools/Makefile`. 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 bytecode. 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.

Software

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.

Package | Version | Notes
--- | --- | ---
GNU Make | 3.79, 3.79.1 | Makefile/build processor
GCC | 3.4.2 | C/C++ compiler (Note 4)
Flex | 2.5.4 | LEX compiler
Bison | 1.35 | YACC compiler
DejaGnu | 1.4.2 | Automated test suite (Note 2)
tcl | 8.3, 8.4 | Automated test suite (Note 2)
expect | 5.38.0 | Automated test suite (Note 2)
GNU M4 | 1.4 | Macro processor for configuration (Note 1)
GNU Autoconf | 2.59 | Configuration script builder (Note 1)
GNU Automake | 2.59 | aclocal macro generator (Note 1)
perl | >5.6.0 | Nightly tester, utilities
libtool | 1.5.10 | Shared library manager (Note 1)
QMTest | 2.0.3 | Automated test suite (Note 2, Note 3)
Python | 2.3 | Automated test suite (Note 2, Note 3)
CVS | >1.11 | CVS access to LLVM (Note 5)

Notes:

1. If you want to make changes to the configure scripts, you will need GNU autoconf (2.59), and consequently, GNU M4 (version 1.4 or higher). You will also need automake. We only use aclocal from that package.
2. Only needed if you want to run the automated test suite in the `test` directory.
3. These are needed to use the LLVM test suite. Please note that newer versions of QMTest may not work with the LLVM test suite. QMTest 2.0.3 can be retrieved from the QMTest CVS repository using the following commands:
   - `cvs -d :pserver:anoncvs@cvs.codesourcery.com:/home/qm/Repository login`
   - When prompted, use `anoncvs` as the password.
   - `cvs -d :pserver:anoncvs@cvs.codesourcery.com:/home/qm/Repository co -r release-2-0-3 qm`
4. 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.
5. You only need CVS if you intend to build from the latest LLVM sources. If you're working from a release distribution, you don't need CVS.

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

- ar - archive library builder
- bzip2* - bzip2 command for distribution generation
- bunzip2* - bunzip2 command for distribution checking
- chmod - change permissions on a file
- cat - output concatentation utility
- cp - copy files
- date - print the current date/time
- echo - print to standard output
- egrep - extended regular expression search utility
- etags - C/C++ tag file creator for vim/emacs
- find - find files/dirs in a file system
- grep - regular expression search utility
- gzip* - gzip command for distribution generation
- gunzip* - gunzip command for distribution checking
- install - install directories/files
- mkdir - create a directory
- mv - move (rename) files
- ranlib - symbol table builder for archive libraries
- rm - remove (delete) files and directories
- sed - stream editor for transforming output
- sh - Bourne shell for make build scripts
- tar - tape archive for distribution generation
- test - test things in file system
- unzip* - unzip command for distribution checking
- zip* - zip command for distribution generation

Broken versions of GCC

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 and GCC 3.4.0 and have had success with them. 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: This version of GCC fails to compile LLVM.

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.

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:

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 `cfrontend/platform/llvm-gcc`.

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

alias llvmgcc LLVMGCCDIR`/bin/gcc`
alias llvmg++ LLVMGCCDIR`/bin/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 two files: the LLVM suite and the LLVM GCC front end compiled for your platform. Each file is a TAR archive that is compressed with the gzip program.

The files are as follows:

llvm-1.3.tar.gz
  This is the source code to the LLVM suite.

cfrontend-1.3.source.tar.gz
  This is the source release of the GCC front end.

cfrontend-1.3.sparc-sun-solaris2.8.tar.gz
  This is the binary release of the GCC front end for Solaris/Sparc.

cfrontend-1.3.i686-redhat-linux-gnu.tar.gz
  This is the binary release of the GCC front end for Linux/x86.

cfrontend-1.3.i386-unknown-freebsd5.1.tar.gz
  This is the binary release of the GCC front end for FreeBSD/x86.

cfrontend-1.3.powerpc-apple-darwin7.0.0.tar.gz
  This is the binary release of the GCC front end for MacOS X/PPC.

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 :pserver:anon@llvm-cvs.cs.uiuc.edu:/var/cvs/llvm login`
- Hit the return key when prompted for the password.
- `cvs -z3 -d :pserver:anon@llvm-cvs.cs.uiuc.edu:/var/cvs/llvm co 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.

If you want to get a specific release (as opposed to the most recent revision), you can specify a label. The following releases have the following label:

- Release 1.3: RELEASE_13
- Release 1.2: RELEASE_12
- Release 1.1: RELEASE_11
- Release 1.0: RELEASE_1

If you would like to get the GCC front end source code, you can also get it from the CVS repository:

      cvs -z3 -d :pserver:anon@llvm-cvs.cs.uiuc.edu:/var/cvs/llvm co llvm-gcc

Please note that you must follow these instructions to successfully build the LLVM C front-end.

LLVM CVS Mirrors

If the main CVS server is overloaded or inaccessible, you can try one of these user-hosted mirrors:

- Mirror hosted by eXtensible Systems Inc.

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-version.platform.tar.gz | tar -xvf -`

If you are using Solaris/Sparc or MacOS X/PPC, you will need to fix the header files:

`cd cfrontend/platform
./fixheaders`

The binary versions of the 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. This is not for the faint of heart, so be forewarned.

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 begin building LLVM.

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

Variable | Purpose
--- | ---
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:

--with-llvmgccdir=LLVMGCCDIR
  Path to the location where the LLVM GCC front end binaries and associated libraries were installed. This must be specified as an absolute pathname.

--enable-optimized
  Enables optimized compilation by default (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) 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.

--enable-spec2000
--enable-spec2000=<`directory`>
  Enable the use of SPEC2000 when testing LLVM. This is disabled by default (unless `configure` finds SPEC2000 installed). By specifying `directory`, you can tell configure where to find the SPEC2000 benchmarks. If `directory` is left unspecified, `configure` uses the default value `/home/vadve/shared/benchmarks/speccpu2000/benchspec`.

--enable-spec95
--enable-spec95=<`directory`>
  Enable the use of SPEC95 when testing LLVM. It is similar to the --enable-spec2000 option.

--enable-povray
--enable-povray=<`directory`>
  Enable the use of Povray as an external test. Versions of Povray written in C should work. This option is similar to the --enable-spec2000 option.

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`

In addition to running `configure`, you must set the `LLVM_LIB_SEARCH_PATH` environment variable in your startup scripts. This environment variable is used to locate "system" libraries like "`-lc`" and "`-lm`" when linking. This variable should be set to the absolute path of the `bytecode-libs` subdirectory of the GCC front end, or LLVMGCCDIR/`bytecode-libs`. For example, one might set `LLVM_LIB_SEARCH_PATH` to `/home/vadve/lattner/local/x86/llvm-gcc/bytecode-libs` for the x86 version of the GCC front end on our research machines.

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 types `gmake` (unless the `--enable-optimized` option was used during configuration). The build system will 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. 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.

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 known broken version of GCC to compile LLVM with.

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 libraries and tools in a heirarchy under $PREFIX, specified with `./configure --prefix=[dir]`, defaults to `/usr/local`.

`gmake -C runtime install`
  Assuming you built LLVM into $OBJDIR, when this command is run, it will 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.

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

`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.

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.

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`

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 bytecode 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::llvm::/path/to/lli:' > /proc/sys/fs/binfmt_misc/register
       $ chmod u+x hello.bc                (if needed)
       $ ./hello.bc

This allows you to execute LLVM bytecode files directly. Thanks to Jack Cummings for pointing this out!

Program Layout

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

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

`llvm/include`

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

`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...
`llvm/include/llvm/Support`
  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.
`llvm/include/llvm/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`

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.

`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/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 Invariant 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/SparcV9` directory holds the Sparc machine description while `llvm/lib/Target/CBackend` implements the LLVM-to-C converter
`llvm/lib/CodeGen/`
  This directory contains the major parts of the code generator: Instruction Selector, Instruction Scheduling, and Register Allocation.
`llvm/lib/Debugger/`
  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.
`llvm/lib/ExecutionEngine/`
  This directory contains libraries for executing LLVM bytecode directly at runtime in both interpreted and JIT compiled fashions.
`llvm/lib/Support/`
  This directory contains the source code that corresponds to the header files located in `llvm/include/Support/`.
`llvm/lib/System/`
  This directory contains the operating system abstraction layer that shields LLVM from platform-specific coding.

`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 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.

`llvm-test`

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

`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. More detailed information is in the Command Guide.

`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`.
`llvmc`
  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 bytecode. This reduces the need to get the traditional `-l<name>` options right on the command line.
`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.
`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, or to C code (with the -march=c option).
`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`

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.

`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 -z3 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.

`llvmgrep`
  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.
`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.

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`

   Note that you should have already built the tools and they have to be in your path, at least `gccas` and `gccld`.

   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. Note that all LLVM optimizations are enabled by default, so there is no need for a "-O3" switch.

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

   `% ./hello`

   or

   `% lli hello.bc`

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

   `% llvm-dis < hello.bc | less`

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

   `% llc hello.bc -o hello.s`

6. 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`

7. Execute the native code program:

   `% ./hello.native`

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.

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