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.
Here's the short story for getting up and running quickly with LLVM:
Specify the full pathname of where the LLVM GCC frontend is installed.
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:
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:
There are some additional tools that you may want to have when working with LLVM:
If you want to make changes to the configure scripts, you will need GNU autoconf (2.57 or higher), and consequently, GNU M4 (version 1.4 or higher). You will also need automake. Any old version of automake from 1.4p5 on should work; we only use aclocal from that package.
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:
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 3.3.2: This version of GCC suffered from a serious bug which causes it to crash in the "convert_from_eh_region_ranges_1" GCC function.
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 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.
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:
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
Please note that you must follow these instructions to successfully build the LLVM C front-end.
If the main CVS server is overloaded or inaccessible, you can try one of these user-hosted mirrors:
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:
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.
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:
To configure LLVM, follow these steps:
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.
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 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:
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.
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.cs.uiuc.edu/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 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 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 regression tests and source code that is used to test the LLVM infrastructure.
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:
- 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.
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.
#include <stdio.h>
int main() {
printf("hello world\n");
return 0;
}
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.
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: