Building LLVM with CMake


CMake is a cross-platform build-generator tool. CMake does not build the project, it generates the files needed by your build tool (GNU make, Visual Studio, etc) for building LLVM.

If you are really anxious about getting a functional LLVM build, go to the Quick start section. If you are a CMake novice, start on Basic CMake usage and then go back to the Quick start once you know what you are doing. The Options and variables section is a reference for customizing your build. If you already have experience with CMake, this is the recommended starting point.

Quick start

We use here the command-line, non-interactive CMake interface.

  1. Download and install CMake. Version 2.8 is the minimum required.

  2. Open a shell. Your development tools must be reachable from this shell through the PATH environment variable.

  3. Create a directory for containing the build. It is not supported to build LLVM on the source directory. cd to this directory:

    $ mkdir mybuilddir
    $ cd mybuilddir
  4. Execute this command on the shell replacing path/to/llvm/source/root with the path to the root of your LLVM source tree:

    $ cmake path/to/llvm/source/root

    CMake will detect your development environment, perform a series of test and generate the files required for building LLVM. CMake will use default values for all build parameters. See the Options and variables section for fine-tuning your build

    This can fail if CMake can’t detect your toolset, or if it thinks that the environment is not sane enough. On this case make sure that the toolset that you intend to use is the only one reachable from the shell and that the shell itself is the correct one for you development environment. CMake will refuse to build MinGW makefiles if you have a POSIX shell reachable through the PATH environment variable, for instance. You can force CMake to use a given build tool, see the Usage section.

Basic CMake usage

This section explains basic aspects of CMake, mostly for explaining those options which you may need on your day-to-day usage.

CMake comes with extensive documentation in the form of html files and on the cmake executable itself. Execute cmake --help for further help options.

CMake requires to know for which build tool it shall generate files (GNU make, Visual Studio, Xcode, etc). If not specified on the command line, it tries to guess it based on you environment. Once identified the build tool, CMake uses the corresponding Generator for creating files for your build tool. You can explicitly specify the generator with the command line option -G "Name of the generator". For knowing the available generators on your platform, execute

$ cmake --help

This will list the generator’s names at the end of the help text. Generator’s names are case-sensitive. Example:

$ cmake -G "Visual Studio 11" path/to/llvm/source/root

For a given development platform there can be more than one adequate generator. If you use Visual Studio “NMake Makefiles” is a generator you can use for building with NMake. By default, CMake chooses the more specific generator supported by your development environment. If you want an alternative generator, you must tell this to CMake with the -G option.

Options and variables

Variables customize how the build will be generated. Options are boolean variables, with possible values ON/OFF. Options and variables are defined on the CMake command line like this:

$ cmake -DVARIABLE=value path/to/llvm/source

You can set a variable after the initial CMake invocation for changing its value. You can also undefine a variable:

$ cmake -UVARIABLE path/to/llvm/source

Variables are stored on the CMake cache. This is a file named CMakeCache.txt on the root of the build directory. Do not hand-edit it.

Variables are listed here appending its type after a colon. It is correct to write the variable and the type on the CMake command line:

$ cmake -DVARIABLE:TYPE=value path/to/llvm/source

Frequently-used CMake variables

Here are some of the CMake variables that are used often, along with a brief explanation and LLVM-specific notes. For full documentation, check the CMake docs or execute cmake --help-variable VARIABLE_NAME.

Sets the build type for make based generators. Possible values are Release, Debug, RelWithDebInfo and MinSizeRel. On systems like Visual Studio the user sets the build type with the IDE settings.
Path where LLVM will be installed if “make install” is invoked or the “INSTALL” target is built.
Extra suffix to append to the directory where libraries are to be installed. On a 64-bit architecture, one could use -DLLVM_LIBDIR_SUFFIX=64 to install libraries to /usr/lib64.
Extra flags to use when compiling C source files.
Extra flags to use when compiling C++ source files.
Flag indicating if shared libraries will be built. Its default value is OFF. Shared libraries are not supported on Windows and not recommended on the other OSes.

LLVM-specific variables

Semicolon-separated list of targets to build, or all for building all targets. Case-sensitive. Defaults to all. Example: -DLLVM_TARGETS_TO_BUILD="X86;PowerPC".
Build LLVM tools. Defaults to ON. Targets for building each tool are generated in any case. You can build an tool separately by invoking its target. For example, you can build llvm-as with a makefile-based system executing make llvm-as on the root of your build directory.
Generate build targets for the LLVM tools. Defaults to ON. You can use that option for disabling the generation of build targets for the LLVM tools.
Build LLVM examples. Defaults to OFF. Targets for building each example are generated in any case. See documentation for LLVM_BUILD_TOOLS above for more details.
Generate build targets for the LLVM examples. Defaults to ON. You can use that option for disabling the generation of build targets for the LLVM examples.
Build LLVM unit tests. Defaults to OFF. Targets for building each unit test are generated in any case. You can build a specific unit test with the target UnitTestNameTests (where at this time UnitTestName can be ADT, Analysis, ExecutionEngine, JIT, Support, Transform, VMCore; see the subdirectories of unittests for an updated list.) It is possible to build all unit tests with the target UnitTests.
Generate build targets for the LLVM unit tests. Defaults to ON. You can use that option for disabling the generation of build targets for the LLVM unit tests.
Append version control revision info (svn revision number or Git revision id) to LLVM version string (stored in the PACKAGE_VERSION macro). For this to work cmake must be invoked before the build. Defaults to OFF.
Build with threads support, if available. Defaults to ON.
Build in C++1y mode, if available. Defaults to OFF.
Enables code assertions. Defaults to OFF if and only if CMAKE_BUILD_TYPE is Release.
Build LLVM with exception handling support. This is necessary if you wish to link against LLVM libraries and make use of C++ exceptions in your own code that need to propagate through LLVM code. Defaults to OFF.
Add the -fPIC flag for the compiler command-line, if the compiler supports this flag. Some systems, like Windows, do not need this flag. Defaults to ON.
Build LLVM with run time type information. Defaults to OFF.
Enable all compiler warnings. Defaults to ON.
Enable pedantic mode. This disable compiler specific extensions, is possible. Defaults to ON.
Stop and fail build, if a compiler warning is triggered. Defaults to OFF.
Build 32-bits executables and libraries on 64-bits systems. This option is available only on some 64-bits unix systems. Defaults to OFF.
LLVM target to use for native code generation. This is required for JIT generation. It defaults to “host”, meaning that it shall pick the architecture of the machine where LLVM is being built. If you are cross-compiling, set it to the target architecture name.
Full path to a native TableGen executable (usually named tblgen). This is intended for cross-compiling: if the user sets this variable, no native TableGen will be created.
Arguments given to lit. make check and make clang-test are affected. By default, '-sv --no-progress-bar' on Visual C++ and Xcode, '-sv' on others.
The path to GnuWin32 tools for tests. Valid on Windows host. Defaults to “”, then Lit seeks tools according to %PATH%. Lit can find tools(eg. grep, sort, &c) on LLVM_LIT_TOOLS_DIR at first, without specifying GnuWin32 to %PATH%.
Indicates whether LLVM Interpreter will be linked with Foreign Function Interface library. If the library or its headers are installed on a custom location, you can set the variables FFI_INCLUDE_DIR and FFI_LIBRARY_DIR. Defaults to OFF.
Path to {Clang,lld,Polly}‘s source directory. Defaults to tools/{clang,lld,polly}. {Clang,lld,Polly} will not be built when it is empty or it does not point to a valid path.
Enable building OProfile JIT support. Defaults to OFF
Enable building support for Intel JIT Events API. Defaults to OFF
Build with zlib to support compression/uncompression in LLVM tools. Defaults to ON.
Define the sanitizer used to build LLVM binaries and tests. Possible values are Address, Memory and MemoryWithOrigins. Defaults to empty string.
Enables all enabled documentation targets (i.e. Doxgyen and Sphinx targets) to be built as part of the normal build. If the install target is run then this also enables all built documentation targets to be installed. Defaults to OFF.
Enables the generation of browsable HTML documentation using doxygen. Defaults to OFF.
Enables the generation of a Qt Compressed Help file. Defaults to OFF. This affects the make target doxygen-llvm. When enabled, apart from the normal HTML output generated by doxygen, this will produce a QCH file named org.llvm.qch. You can then load this file into Qt Creator. This option is only useful in combination with -DLLVM_ENABLE_DOXYGEN=ON; otherwise this has no effect.
The filename of the Qt Compressed Help file that will be genrated when -DLLVM_ENABLE_DOXYGEN=ON and -DLLVM_ENABLE_DOXYGEN_QT_HELP=ON are given. Defaults to org.llvm.qch. This option is only useful in combination with -DLLVM_ENABLE_DOXYGEN_QT_HELP=ON; otherwise this has no effect.
Namespace under which the intermediate Qt Help Project file lives. See Qt Help Project for more information. Defaults to “org.llvm”. This option is only useful in combination with -DLLVM_ENABLE_DOXYGEN_QT_HELP=ON; otherwise this has no effect.
See Qt Help Project for more information. Defaults to the CMake variable ${PACKAGE_STRING} which is a combination of the package name and version string. This filter can then be used in Qt Creator to select only documentation from LLVM when browsing through all the help files that you might have loaded. This option is only useful in combination with -DLLVM_ENABLE_DOXYGEN_QT_HELP=ON; otherwise this has no effect.
The path to the qhelpgenerator executable. Defaults to whatever CMake’s find_program() can find. This option is only useful in combination with -DLLVM_ENABLE_DOXYGEN_QT_HELP=ON; otherwise this has no effect.
If enabled CMake will search for the sphinx-build executable and will make the SPHINX_OUTPUT_HTML and SPHINX_OUTPUT_MAN CMake options available. Defaults to OFF.
The path to the sphinx-build executable detected by CMake.
If enabled (and LLVM_ENABLE_SPHINX is enabled) then the targets for building the documentation as html are added (but not built by default unless LLVM_BUILD_DOCS is enabled). There is a target for each project in the source tree that uses sphinx (e.g. docs-llvm-html, docs-clang-html and docs-lld-html). Defaults to ON.
If enabled (and LLVM_ENABLE_SPHINX is enabled) the targets for building the man pages are added (but not built by default unless LLVM_BUILD_DOCS is enabled). Currently the only target added is docs-llvm-man. Defaults to ON.

Executing the test suite

Testing is performed when the check target is built. For instance, if you are using makefiles, execute this command while on the top level of your build directory:

$ make check

On Visual Studio, you may run tests to build the project “check”.

Cross compiling

See this wiki page for generic instructions on how to cross-compile with CMake. It goes into detailed explanations and may seem daunting, but it is not. On the wiki page there are several examples including toolchain files. Go directly to this section for a quick solution.

Also see the LLVM-specific variables section for variables used when cross-compiling.

Embedding LLVM in your project

From LLVM 3.5 onwards both the CMake and autoconf/Makefile build systems export LLVM libraries as importable CMake targets. This means that clients of LLVM can now reliably use CMake to develop their own LLVM based projects against an installed version of LLVM regardless of how it was built.

Here is a simple example of CMakeLists.txt file that imports the LLVM libraries and uses them to build a simple application simple-tool.

cmake_minimum_required(VERSION 2.8.8)


message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")

# Set your project compile flags.
# E.g. if using the C++ header files
# you will need to enable C++11 support
# for your compiler.


# Now build our tools
add_excutable(simple-tool tool.cpp)

# Find the libraries that correspond to the LLVM components
# that we wish to use
llvm_map_components_to_libnames(llvm_libs support core irreader)

# Link against LLVM libraries
target_link_libraries(simple-tool ${llvm_libs})

The find_package(...) directive when used in CONFIG mode (as in the above example) will look for the LLVMConfig.cmake file in various locations (see cmake manual for details). It creates a LLVM_DIR cache entry to save the directory where LLVMConfig.cmake is found or allows the user to specify the directory (e.g. by passing -DLLVM_DIR=/usr/share/llvm/cmake to the cmake command or by setting it directly in ccmake or cmake-gui).

This file is available in two different locations.

  • <INSTALL_PREFIX>/share/llvm/cmake/LLVMConfig.cmake where <INSTALL_PREFIX> is the install prefix of an installed version of LLVM. On Linux typically this is /usr/share/llvm/cmake/LLVMConfig.cmake.
  • <LLVM_BUILD_ROOT>/share/llvm/cmake/LLVMConfig.cmake where <LLVM_BUILD_ROOT> is the root of the LLVM build tree. Note this only available when building LLVM with CMake

If LLVM is installed in your operating system’s normal installation prefix (e.g. on Linux this is usually /usr/) find_package(LLVM ...) will automatically find LLVM if it is installed correctly. If LLVM is not installed or you wish to build directly against the LLVM build tree you can use LLVM_DIR as previously mentioned.

The LLVMConfig.cmake file sets various useful variables. Notable variables include

The path to the LLVM CMake directory (i.e. the directory containing LLVMConfig.cmake).
A list of preprocessor defines that should be used when building against LLVM.
This is set to ON if LLVM was built with assertions, otherwise OFF.
This is set to ON if LLVM was built with exception handling (EH) enabled, otherwise OFF.
This is set to ON if LLVM was built with run time type information (RTTI), otherwise OFF.
A list of include paths to directories containing LLVM header files.
The LLVM version. This string can be used with CMake conditionals. E.g. if (${LLVM_PACKAGE_VERSION} VERSION_LESS "3.5").
The path to the directory containing the LLVM tools (e.g. llvm-as).

Notice that in the above example we link simple-tool against several LLVM libraries. The list of libraries is determined by using the llvm_map_components_to_libnames() CMake function. For a list of available components look at the output of running llvm-config --components.

Note that for LLVM < 3.5 llvm_map_components_to_libraries() was used instead of llvm_map_components_to_libnames(). This is now deprecated and will be removed in a future version of LLVM.

Developing LLVM passes out of source

It is possible to develop LLVM passes out of LLVM’s source tree (i.e. against an installed or built LLVM). An example of a project layout is provided below.

<project dir>/
    <pass name>/

Contents of <project dir>/CMakeLists.txt:



add_subdirectory(<pass name>)

Contents of <project dir>/<pass name>/CMakeLists.txt:

add_library(LLVMPassname MODULE Pass.cpp)

Note if you intend for this pass to be merged into the LLVM source tree at some point in the future it might make more sense to use LLVM’s internal add_llvm_loadable_module function instead by...

Adding the following to <project dir>/CMakeLists.txt (after find_package(LLVM ...))


And then changing <project dir>/<pass name>/CMakeLists.txt to


When you are done developing your pass, you may wish to integrate it into LLVM source tree. You can achieve it in two easy steps:

  1. Copying <pass name> folder into <LLVM root>/lib/Transform directory.
  2. Adding add_subdirectory(<pass name>) line into <LLVM root>/lib/Transform/CMakeLists.txt.

Compiler/Platform-specific topics

Notes for specific compilers and/or platforms.

Microsoft Visual C++

Specifies the maximum number of parallell compiler jobs to use per project when building with msbuild or Visual Studio. Only supported for the Visual Studio 2010 CMake generator. 0 means use all processors. Default is 0.