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 make variables. There are several variables that a Makefile needs to set to use the LLVM build system:
    • PROJECT_NAME - The name by which your project is known.
    • LLVM_SRC_ROOT - The root of the LLVM source tree.
    • LLVM_OBJ_ROOT - The root of the LLVM object tree.
    • PROJ_SRC_ROOT - The root of the project’s source tree.
    • PROJ_OBJ_ROOT - The root of the project’s object tree.
    • PROJ_INSTALL_ROOT - The root installation directory.
    • LEVEL - The relative path from the current directory to the project’s root ($PROJ_OBJ_ROOT).
  • Include Makefile.config from $(LLVM_OBJ_ROOT).
  • Include 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.

If you want to devise your own build system, studying other projects and LLVM Makefiles will probably provide enough information on how to write your own Makefiles.

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.

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 contains regression tests in llvm/test. These tests are run by the Lit testing tool. This test procedure uses RUN lines in the actual test case to determine how to run the test. See the LLVM Testing Infrastructure Guide for more details.

  • LLVM contains an optional package called llvm-test, which provides benchmarks and programs that are known to compile with the Clang front end. You can use these programs to test your code, gather statistical 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 be libraries that come from your lib directory. The libraries must be specified without their lib prefix. For example, to link libsample.a, you would set USEDLIBS to sample.a.

Note that this works only for statically linked libraries.

LLVMLIBS

This variable holds a space separated list of libraries that should be linked into the program. These libraries must be LLVM libraries. The libraries must be specified without their lib prefix. For example, to link with a driver that performs an IR transformation you might set LLVMLIBS to this minimal set of libraries LLVMSupport.a LLVMCore.a LLVMBitReader.a LLVMAsmParser.a LLVMAnalysis.a LLVMTransformUtils.a LLVMScalarOpts.a LLVMTarget.a.

Note that this works only for statically linked libraries. LLVM is split into a large number of static libraries, and the list of libraries you require may be much longer than the list above. To see a full list of libraries use: llvm-config --libs all. Using LINK_COMPONENTS as described below, obviates the need to set LLVMLIBS.

LINK_COMPONENTS

This variable holds a space separated list of components that the LLVM Makefiles pass to the llvm-config tool to generate a link line for the program. For example, to link with all LLVM libraries use LINK_COMPONENTS = all.

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

Note that LIBS must occur in the Makefile after the inclusion of Makefile.common.

Miscellaneous Variables

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 PROJ_OBJ_ROOT/<type>/lib, where type is Debug, Release, or Profile for a debug, optimized, or profiled build, respectively.

Executables

All executables will be stored in PROJ_OBJ_ROOT/<type>/bin, 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.