Getting Started with the LLVM System

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. 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
  2. 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:
      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
  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 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.

  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

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:

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:

There are some additional tools that you may want to have when working 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. A complete guide to installation is provided in the next section.

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

Getting Started with LLVM
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-1.2.tar.gz
This is the source code to the LLVM suite.

cfrontend-1.2.sparc-sun-solaris2.8.tar.gz
This is the binary release of the GCC front end for Solaris/Sparc.

cfrontend-1.2.i686-redhat-linux-gnu.tar.gz
This is the binary release of the GCC front end for Linux/x86.

cfrontend-1.2.i386-unknown-freebsd5.1.tar.gz
This is the binary release of the GCC front end for FreeBSD/x86.

cfrontend-1.2.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:

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:

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
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 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.

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:

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:

  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 store their header files here.
  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

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/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 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.
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.

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.

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

    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:

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The LLVM Compiler Infrastructure
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