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.
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.
Here's the short story for getting up and running quickly with LLVM:
Specify for directory the full pathname of where you want the LLVM tools and libraries to be installed (default /usr/local).
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.
Enable the SPEC2000 benchmarks for testing. The SPEC2000 benchmarks should be available in directory.
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 |
|---|---|---|
| Linux | x861 | GCC |
| Solaris | V9 (Ultrasparc) | GCC |
| FreeBSD | x861 | GCC |
| MacOS X2 | PowerPC | GCC |
| MacOS X2 | x86 | GCC |
| Cygwin/Win32 | x861,8 | GCC 3.4.X, binutils 2.15 |
| MinGW/Win32 | x861,6,8 | GCC 3.4.X, binutils 2.15 |
| Linux | amd643 | GCC |
LLVM has partial support for the following platforms:
| OS | Arch | Compilers |
|---|---|---|
| Windows | x861 | Visual Studio .NET4,5 |
| AIX3,4 | PowerPC | GCC |
| Linux3,5 | PowerPC | GCC |
| Linux7 | Alpha | GCC |
| Linux7 | Itanium (IA-64) | GCC |
| HP-UX7 | Itanium (IA-64) | HP aCC |
Notes:
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 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.
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++ compiler1 |
| TeXinfo | 4.5 | For building the CFE |
| Flex | 2.5.4 | LEX compiler |
| Bison | 1.28, 1.35, 1.75, 1.875d, 2.0, or 2.1 (not 1.85 or 1.875) |
YACC compiler |
| CVS | ≥1.11 | CVS 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 | 2.59 | Configuration script builder4 |
| GNU Automake | 1.9.2 | aclocal macro generator4 |
| libtool | 1.5.10 | Shared library manager4 |
Notes:
Additionally, your compilation host is expected to have the usual plethora of Unix utilities. Specifically:
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 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: 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.
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.
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.
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.
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.
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:
For the pre-built GCC front end binaries, the LLVMGCCDIR is cfrontend/platform/llvm-gcc.
In order to compile and use LLVM, you may need to set some environment variables.
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:
It is also possible to download the sources of the llvm-gcc4 front end from a read-only subversion mirror at svn://anonsvn.opensource.apple.com/svn/llvm/trunk.
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 labels:
If you would like to get the LLVM test suite (a separate package as of 1.4), you get it from the CVS repository:
cd llvm/projects cvs -z3 -d :pserver:anon@llvm.org:/var/cvs/llvm 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 cvs update.
If you would like to get the GCC 3.4 front end source code, you can also get it from the CVS repository:
cvs -z3 -d :pserver:anon@llvm.org:/var/cvs/llvm co llvm-gcc
Please note that you must follow these instructions to successfully build the LLVM 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:
Next, you will need to fix your system 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.
Once checked out from the CVS 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:
| 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:
To configure LLVM, follow these steps:
Once you have configured LLVM, you can build it. There are three types of builds:
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:
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:
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.
It is possible to cross-compile LLVM. That is, you can create LLVM executables and libraries for a platform different than the one one which you are compiling. To do this, a few additional steps are required. 1 To cross-compile LLVM, use these instructions:
The result of such a build will produce executables that are not executable on your build host (--build option) but can be executed on your compile host (--host option).
Notes:
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:
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!
One useful source of information about the LLVM source base is the LLVM doxygen documentation available at http://llvm.org/doxygen/. The following is a brief introduction to code layout:
Every directory checked out of CVS will contain a CVS directory; for the most part these can just be ignored.
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 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 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. See llvm/projects/Stacker for a fully functional example of a compiler front end.
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.
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 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 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.
- gccas
- This tool is invoked by the llvm-gcc 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 llvm-gcc -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.
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.
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.
This section gives an example of using LLVM. Since we are currently transitioning from llvm-gcc3 to llvm-gcc4, we include examples for both.
Note: The gcc4 frontend's invocation is considerably different from the previous gcc3 frontend. In particular, the gcc4 frontend does not create bytecode by default: gcc4 produces native code. As the example below illustrates, the '--emit-llvm' flag is needed to produce LLVM bytecode output. For makefiles and configure scripts, the CFLAGS variable needs '--emit-llvm' to produce bytecode output.
#include <stdio.h>
int main() {
printf("hello world\n");
return 0;
}
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).
Next, compile the C file into a LLVM bytecode 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 bytecode file.
Unlike llvm-gcc3, llvm-gcc4 correctly responds to -O[0123] arguments.
Run the program in both forms. To run the program, use:
% ./hello
and
% lli hello.bc
The second examples shows how to invoke the LLVM JIT, lli.
Use the llvm-dis utility to take a look at the LLVM assembly code:
% llvm-dis < hello.bc | less
Compile the program to native assembly using the LLC code generator:
% llc hello.bc -o hello.s
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
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.
#include <stdio.h>
int main() {
printf("hello world\n");
return 0;
}
Next, compile the C file into a LLVM bytecode file:
% llvm-gcc 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.
Run the program. To make sure the program ran, execute one of the following commands:
% ./hello
or
% lli hello.bc
Use the llvm-dis utility to take a look at the LLVM assembly code:
% llvm-dis < hello.bc | less
Compile the program to native assembly using the LLC code generator:
% llc hello.bc -o hello.s
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
Execute the native code program:
% ./hello.native
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 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: