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.

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

Download and install CMake. Version 2.6.2 is the minimum required.
Open a shell. Your development tools must be reachable from this shell through the PATH environment variable.
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
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.

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 8 2005" 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.

TODO: explain variables and cache. Move explanation here from #options section.

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

Here are listed 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.

CMAKE_BUILD_TYPE:STRING: Sets the build type for make based generators. Possible values are Release, Debug, RelWithDebInfo and MiniSizeRel. On systems like Visual Studio the user sets the build type with the IDE settings.
CMAKE_INSTALL_PREFIX:PATH: Path where LLVM will be installed if "make install" is invoked or the "INSTALL" target is built.
CMAKE_C_FLAGS:STRING: Extra flags to use when compiling C source files.
CMAKE_CXX_FLAGS:STRING: Extra flags to use when compiling C++ source files.
BUILD_SHARED_LIBS:BOOL: Flag indicating is shared libraries will be built. Its default value is OFF. Shared libraries are not supported on Windows and not recommended in the other OSes.

LLVM_TARGETS_TO_BUILD:STRING: Semicolon-separated list of targets to build, or all for building all targets. Case-sensitive. For Visual C++ defaults to X86. On the other cases defaults to all. Example: -DLLVM_TARGETS_TO_BUILD="X86;PowerPC;Alpha".
LLVM_ENABLE_THREADS:BOOL: Build with threads support, if available. Defaults to ON.
LLVM_ENABLE_PIC:BOOL: Add the -fPIC flag to the compiler command-line, if the compiler supports this flag. Some systems, like Windows, does not need this flag. Defaults to OFF.
LLVM_BUILD_32_BITS:BOOL: 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_PLO_FLAGS:STRING: Extra flags for creating partially linked objects. Visual C++ does not use this.
LLVM_TABLEGEN:STRING: Full path to a native TableGen executable (usually named tblgen). This is intented for cross-compiling: if the user sets this variable, no native TableGen will be created.