Creating an LLVM Project

1. Overview
2. Create a project from the Sample Project
3. Source tree layout
4. Writing LLVM-style Makefiles
   1. Required Variables
   2. Variables for Building Subdirectories
   3. Variables for Building Libraries
   4. Variables for Building Programs
   5. Miscellaneous Variables
5. Placement of object code
6. Further help

Written by John Criswell

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:

1. 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.
2. Include `Makefile.config` from `$(LLVM_OBJ_ROOT)`.
3. Include `Makefile.rules` from `$(LLVM_SRC_ROOT)`.

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

1. You can write your own Makefiles which hard-code these values.
2. 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:

1. 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.
2. Add your source code and Makefiles to your source tree.
3. 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.
4. 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.

5. 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 basic support for tests. The LLVM system provides the following:

  - LLVM provides a tcl procedure that is used by Dejagnu to run tests. It can be found in `llvm/lib/llvm-dg.exp`. This test procedure uses RUN lines in the actual test case to determine how to run the test. See the TestingGuide for more details. You can easily write Makefile support similar to the Makefiles in `llvm/test` to use Dejagnu to run your project's tests.

  - LLVM contains an optional package called `llvm-test` which provides benchmarks and programs that 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.

    Currently, there is no way to hook your tests directly into the `llvm/test` 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/tools/<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.