# Building LLVM with CMake

- Introduction
- Quick start
- Basic CMake usage
- Options and variables
  - Frequently-used CMake variables
  - LLVM-specific variables
- Executing the test suite
- Cross compiling
- Embedding LLVM in your project
  - Developing LLVM pass out of source
- Compiler/Platform specific topics
  - Microsoft Visual C++

Written by Oscar Fuentes

## Introduction

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 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.

## 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 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 MinSizeRel. 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.
LLVM_LIBDIR_SUFFIX:STRING
  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.
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-specific variables

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_BUILD_TOOLS:BOOL
  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.
LLVM_INCLUDE_TOOLS:BOOL
  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.
LLVM_BUILD_EXAMPLES:BOOL
  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.
LLVM_INCLUDE_EXAMPLES:BOOL
  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.
LLVM_BUILD_TESTS:BOOL
  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.
LLVM_INCLUDE_TESTS:BOOL
  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.
LLVM_APPEND_VC_REV:BOOL
  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.
LLVM_ENABLE_THREADS:BOOL
  Build with threads support, if available. Defaults to ON.
LLVM_ENABLE_ASSERTIONS:BOOL
  Enables code assertions. Defaults to OFF if and only if CMAKE_BUILD_TYPE is Release.
LLVM_ENABLE_PIC:BOOL
  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.
LLVM_ENABLE_WARNINGS:BOOL
  Enable all compiler warnings. Defaults to ON.
LLVM_ENABLE_PEDANTIC:BOOL
  Enable pedantic mode. This disable compiler specific extensions, is possible. Defaults to ON.
LLVM_ENABLE_WERROR:BOOL
  Stop and fail build, if a compiler warning is triggered. 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_TARGET_ARCH:STRING
  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.
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.
LLVM_LIT_ARGS:STRING
  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.
LLVM_LIT_TOOLS_DIR:PATH
  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%.
LLVM_ENABLE_FFI:BOOL
  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.
LLVM_CLANG_SOURCE_DIR:PATH
  Path to Clang's source directory. Defaults to tools/clang. Clang will not be built when it is empty or it does not point valid path.

## 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

The most difficult part of adding LLVM to the build of a project is to determine the set of LLVM libraries corresponding to the set of required LLVM features. What follows is an example of how to obtain this information:

        # A convenience variable:
        set(LLVM_ROOT "" CACHE PATH "Root of LLVM install.")
        # A bit of a sanity check:
        if( NOT EXISTS ${LLVM_ROOT}/include/llvm )
        message(FATAL_ERROR "LLVM_ROOT (${LLVM_ROOT}) is not a valid LLVM install")
        endif()
        # We incorporate the CMake features provided by LLVM:
        set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${LLVM_ROOT}/share/llvm/cmake")
        include(LLVMConfig)
        # Now set the header and library paths:
        include_directories( ${LLVM_INCLUDE_DIRS} )
        link_directories( ${LLVM_LIBRARY_DIRS} )
        add_definitions( ${LLVM_DEFINITIONS} )
        # Let's suppose we want to build a JIT compiler with support for
        # binary code (no interpreter):
        llvm_map_components_to_libraries(REQ_LLVM_LIBRARIES jit native)
        # Finally, we link the LLVM libraries to our executable:
        target_link_libraries(mycompiler ${REQ_LLVM_LIBRARIES})

This assumes that LLVM_ROOT points to an install of LLVM. The procedure works too for uninstalled builds although we need to take care to add an include_directories for the location of the headers on the LLVM source directory (if we are building out-of-source.)

Alternativaly, you can utilize CMake's find_package functionality. Here is an equivalent variant of snippet shown above:

        find_package(LLVM)

        if( NOT LLVM_FOUND )
          message(FATAL_ERROR "LLVM package can't be found. Set CMAKE_PREFIX_PATH variable to LLVM's installation prefix.")
        endif()

        include_directories( ${LLVM_INCLUDE_DIRS} )
        link_directories( ${LLVM_LIBRARY_DIRS} )

        llvm_map_components_to_libraries(REQ_LLVM_LIBRARIES jit native)

        target_link_libraries(mycompiler ${REQ_LLVM_LIBRARIES})

### Developing LLVM pass out of source

It is possible to develop LLVM passes against installed LLVM. An example of project layout provided below:

          <project dir>/
              |
              CMakeLists.txt
              <pass name>/
                  |
                  CMakeLists.txt
                  Pass.cpp
                  ...

Contents of <project dir>/CMakeLists.txt:

        find_package(LLVM)

        # Define add_llvm_* macro's.
        include(AddLLVM)

        add_definitions(${LLVM_DEFINITIONS})
        include_directories(${LLVM_INCLUDE_DIRS})
        link_directories(${LLVM_LIBRARY_DIRS})

        add_subdirectory(<pass name>)

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

        add_llvm_loadable_module(LLVMPassname
          Pass.cpp
          )

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++

LLVM_COMPILER_JOBS:STRING
  Specifies the maximum number of parallell compiler jobs to use per project when building with msbuild or Visual Studio. Only supported for Visual Studio 2008 and Visual Studio 2010 CMake generators. 0 means use all processors. Default is 0.