Creating an LLVM Project

Overview

The LLVM build system is designed to facilitate the building of third party projects that use LLVM header files, libraries, and tools. In order to use these facilities, a Makefile from a project must do the following things:

Set environment variables.
There are several environment variables that a Makefile needs to set to use the LLVM build system:

LLVM_SRC_ROOT
The root of the LLVM source tree.

LLVM_OBJ_ROOT
The root of the LLVM object tree.

BUILD_SRC_ROOT
The root of the project's source tree.

BUILD_OBJ_ROOT
The root of the project's object tree.

BUILD_SRC_DIR
The directory containing the current source to be compiled.

BUILD_OBJ_DIR
The directory where the current source will place the new object files. This should always be the current directory.

LEVEL
The relative path from the current directory to the root of the object tree.
Include the LLVM Makefile.config from $(LLVM_OBJ_ROOT).
Include the LLVM Makefile.rules from $(LLVM_SRC_ROOT).

There are two ways that you can set all of these variables:

You can write your own Makefiles which hard-code these values.
You can use the pre-made LLVM sample project. This sample project includes Makefiles, a configure script that can be used to configure the location of LLVM, and the ability to support multiple object directories from a single source directory.

This document assumes that you will base your project off of the LLVM sample project found in llvm/projects/sample. If you want to devise your own build system, studying the sample project and LLVM Makefiles will probably provide enough information on how to write your own Makefiles.

Create a Project from the Sample Project

Follow these simple steps to start your project:

Copy the llvm/projects/sample directory to any place of your choosing. You can place it anywhere you like. Rename the directory to match the name of your project.
Add your source code and Makefiles to your source tree.
If you want your Makefiles to be configured by the configure script, or if you want to support multiple object directories, add your Makefiles to the configure script by adding them into the autoconf/configure.ac file. The macro AC_CONFIG_MAKEFILE will copy a file, unmodified, from the source directory to the object directory.
After updating autoconf/configure.ac, regenerate the configure script with these commands:
cd autoconf autoconf -o ../configure
You must be using Autoconf version 2.57 or higher.
Run configure in the directory in which you want to place object code. Use the following options to tell your project where it can find LLVM:

--with-llvmsrc=<directory>
Tell your project where the LLVM source tree is located.

--with-llvmobj=<directory>
Tell your project where the LLVM object tree is located.

That's it! Now all you have to do is type gmake in the root of your object directory, and your project should build.

Source Tree Layout

In order to use the LLVM build system, you will want to organize your source code so that it can benefit from the build system's features. Mainly, you want your source tree layout to look similar to the LLVM source tree layout. The best way to do this is to just copy the project tree from llvm/projects/sample and modify it to meet your needs, but you can certainly add to it if you want. Underneath your top level directory, you should have the following directories:

lib

This subdirectory should contain all of your library source code. For each library that you build, you will have one directory in lib that will contain that library's source code.

Libraries can be object files, archives, or dynamic libraries. The lib directory is just a convenient place for libraries as it places them all in a directory from which they can be linked later.

include

This subdirectory should contain any header files that are global to your project. By global, we mean that they are used by more than one library or executable of your project.

By placing your header files in include, they will be found automatically by the LLVM build system. For example, if you have a file include/jazz/note.h, then your source files can include it simply with #include "jazz/note.h".

tools

This subdirectory should contain all of your source code for executables. For each program that you build, you will have one directory in tools that will contain that program's source code.

test

This subdirectory should contain tests that verify that your code works correctly. Automated tests are especially useful.

Currently, the LLVM build system provides little support for tests, although some exists. Expanded support for tests will hopefully occur in the future. In the meantime, the LLVM system does provide the following:

LLVM provides several QMTest test classes that can be used to create tests. They can be found in llvm/test/QMTest/llvm.py. These test classes perform a variety of functions, including code optimization tests, assembly tests, and code analysis tests. The Makefile in llvm/test provides the QMTest context needed by LLVM test classes.
The LLVM source tree provides benchmarks and programs which are known to compile with the LLVM GCC front ends. You can use these programs to test your code, gather statistics information, and compare it to the current LLVM performance statistics. These programs are found in the llvm/test/Programs directory.
Currently, there is no way to hook your tests directly into the llvm/test/Programs testing harness. You will simply need to find a way to use the source provided within that directory on your own.

Typically, you will want to build your lib directory first followed by your tools directory.

Writing LLVM Style Makefiles

The LLVM build system provides a convenient way to build libraries and executables. Most of your project Makefiles will only need to define a few variables. Below is a list of the variables one can set and what they can do:

Required Variables

LEVEL: This variable is the relative path from this Makefile to the top directory of your project's source code. For example, if your source code is in /tmp/src, then the Makefile in /tmp/src/jump/high would set LEVEL to "../..".

Variables for Building Subdirectories

DIRS: This is a space separated list of subdirectories that should be built. They will be built, one at a time, in the order specified.
PARALLEL_DIRS: This is a list of directories that can be built in parallel. These will be built after the directories in DIRS have been built.
OPTIONAL_DIRS: This is a list of directories that can be built if they exist, but will not cause an error if they do not exist. They are built serially in the order in which they are listed.

Variables for Building Libraries

LIBRARYNAME: This variable contains the base name of the library that will be built. For example, to build a library named libsample.a, LIBRARYNAME should be set to sample.
BUILD_ARCHIVE: By default, a library is a .o file that is linked directly into a program. To build an archive (also known as a static library), set the BUILD_ARCHIVE variable.
SHARED_LIBRARY: If SHARED_LIBRARY is defined in your Makefile, a shared (or dynamic) library will be built.

Variables for Building Programs

TOOLNAME

This variable contains the name of the program that will be built. For example, to build an executable named sample, TOOLNAME should be set to sample.

USEDLIBS

This variable holds a space separated list of libraries that should be linked into the program. These libraries must either be LLVM libraries or libraries that come from your lib directory. The libraries must be specified by their base name. For example, to link libsample.a, you would set USEDLIBS to sample.

Note that this works only for statically linked libraries.

LIBS

To link dynamic libraries, add -l<library base name> to the LIBS variable. The LLVM build system will look in the same places for dynamic libraries as it does for static libraries.

For example, to link libsample.so, you would have the following line in your Makefile:

LIBS+=-lsample

Miscellaneous Variables

ExtraSource

This variable contains a space separated list of extra source files that need to be built. It is useful for including the output of Lex and Yacc programs.

CFLAGS

CPPFLAGS

This variable can be used to add options to the C and C++ compiler, respectively. It is typically used to add options that tell the compiler the location of additional directories to search for header files.

It is highly suggested that you append to CFLAGS and CPPFLAGS as opposed to overwriting them. The master Makefiles may already have useful options in them that you may not want to overwrite.

Placement of Object Code

The final location of built libraries and executables will depend upon whether you do a Debug, Release, or Profile build.

Libraries: All libraries (static and dynamic) will be stored in BUILD_OBJ_ROOT/lib/<type>, where type is Debug, Release, or Profile for a debug, optimized, or profiled build, respectively.
Executables: All executables will be stored in BUILD_OBJ_ROOT/lib/<type>, where type is Debug, Release, or Profile for a debug, optimized, or profiled build, respectively.

Further Help

If you have any questions or need any help creating an LLVM project, the LLVM team would be more than happy to help. You can always post your questions to the LLVM Developers Mailing List (llvmdev@cs.uiuc.edu).

Written by John Criswell.