# Building LLVM with CMake

```{contents}
:local:
```

## Introduction

[CMake](http://www.cmake.org/) 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 a new contributor**, please start with the {doc}`GettingStarted`
page. This page is geared for existing contributors moving from the legacy
configure/make system.

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

This page is geared towards users of the LLVM CMake build. If you're looking
for information about modifying the LLVM CMake build system, you may want to
see the {doc}`CMakePrimer` page. It has a basic overview of the CMake language.

(Quick start)=
## Quick start

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

1. [Download](http://www.cmake.org/cmake/resources/software.html) and install
   CMake. Version 3.20.0 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 build directory. Building LLVM in the source
   directory is not supported. `cd` to this directory:

   ``` console
   $ mkdir mybuilddir
   $ cd mybuilddir
   ```

4. Execute this command in the shell replacing `path/to/llvm/source/root` with
   the path to the root of your LLVM source tree:

   ``` console
   $ cmake path/to/llvm/source/root
   ```

   CMake will detect your development environment, perform a series of tests,
   and generate the files required for building LLVM. CMake will use default
   values for all build parameters. See the [Options and
   variables](#options-and-variables) section for a list of build parameters
   that you can modify.

   This can fail if CMake can't detect your toolset, or if it thinks that the
   environment is not sane enough. In 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 your 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; for instructions, see the [Usage](#Usage) section,
   below. You may also wish to control which targets LLVM enables, or which
   LLVM components are built; see the [Frequently Used LLVM-related
   variables](#frequently-used-llvm-related-variables) below.

5. After CMake has finished running, use IDE project files, or start
   the build from the build directory:

   ``` console
   $ cmake --build .
   ```

   The `--build` option tells `cmake` to invoke the underlying build
   tool (`make`, `ninja`, `xcodebuild`, `msbuild`, etc.)

   The underlying build tool can be invoked directly, of course, but
   the `--build` option is portable.

6. After LLVM has finished building, install it from the build directory:

   ``` console
   $ cmake --build . --target install
   ```

   The `--target` option with `install` parameter in addition to
   the `--build` option tells `cmake` to build the `install` target.

   It is possible to set a different install prefix at installation time by
   invoking the `cmake_install.cmake` script generated in the build directory:

   ``` console
   $ cmake -DCMAKE_INSTALL_PREFIX=/tmp/llvm -P cmake_install.cmake
   ```

(Basic CMake usage)=
(Usage)=
## Basic CMake usage

This section explains basic aspects of CMake for daily use.

CMake comes with extensive documentation, in the form of HTML files, and as
online help accessible via the `cmake` executable itself. Execute `cmake
--help` for further help options.

CMake allows you to specify a build tool (e.g., GNU make, Visual Studio,
or Xcode). If not specified on the command line, CMake tries to guess which
build tool to use based on your environment. Once it has identified your
build tool, CMake uses the corresponding *Generator* to create files for your
build tool (e.g., Makefiles or Visual Studio or Xcode project files). You can
explicitly specify the generator with the command line option `-G "Name of the
generator"`. To see a list of the available generators on your system, execute:

``` console
$ cmake --help
```

This will list the generator names at the end of the help text.

Generators' names are case-sensitive and may contain spaces. For this reason,
you should enter them exactly as they are listed in the `cmake --help`
output, in quotes. For example, to generate project files specifically for
Visual Studio 12, you can execute:

``` console
$ cmake -G "Visual Studio 12" path/to/llvm/source/root
```

A given development platform can have 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 most specific generator
supported by your development environment. If you want an alternative generator,
you must specify this to CMake with the `-G` option.

```{todo}
Explain variables and cache. Move explanation here from #options section.
```

(Options and variables)=
## 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:

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

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

``` console
$ cmake -UVARIABLE path/to/llvm/source
```

Variables are stored in the CMake cache. This is a file named `CMakeCache.txt`
stored at the root of your build directory that is generated by `cmake`.
Editing it yourself is not recommended.

Variables are listed in the CMake cache and later in this document with
the variable name and type separated by a colon. You can also specify the
variable and type on the CMake command line:

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

(cmake_frequently_used_variables)=
### Frequently-used CMake variables

Here are some of the CMake variables that are used often, along with a brief
explanation. For full documentation, consult the CMake manual, or execute
`cmake --help-variable VARIABLE_NAME`. See [Frequently Used LLVM-related
Variables](#frequently-used-llvm-related-variables) below for information about
commonly used variables that control features of LLVM and enabled subprojects.

(cmake_build_type)=

**CMAKE_BUILD_TYPE**:STRING

:   This configures the optimization level for `make` or `ninja` builds.

    Possible values:

    | Build Type         | Optimizations | Debug Info | Assertions | Best suited for            |
    |--------------------|---------------|------------|------------|----------------------------|
    | **Release**        | For Speed     | No         | No         | Users of LLVM and Clang    |
    | **Debug**          | None          | Yes        | Yes        | Developers of LLVM         |
    | **RelWithDebInfo** | For Speed     | Yes        | No         | Users that also need Debug |
    | **MinSizeRel**     | For Size      | No         | No         | When disk space matters    |

    - Optimizations make LLVM/Clang run faster but can be an impediment for
      step-by-step debugging.
    - Builds with debug information can use a lot of RAM and disk space and are
      usually slower to run. You can improve RAM usage by using `lld`, see
      the {ref}`LLVM_USE_LINKER <llvm_use_linker>` option.
    - Assertions are internal checks to help you find bugs. They typically
      slow down LLVM and Clang when enabled but can be useful during
      development. You can manually set {ref}`LLVM_ENABLE_ASSERTIONS
      <llvm_enable_assertions>` to override the default from
      `CMAKE_BUILD_TYPE`.

    If you are using an IDE such as Visual Studio or Xcode, use the IDE
    settings to set the build type.

    Note: on Windows (building with MSVC or clang-cl), CMake's
    **RelWithDebInfo** setting does not enable the same optimizations as
    **Release**. Using the **Release** build type with {ref}`LLVM_ENABLE_PDB
    <llvm_enable_pdb>` set may be a better option.

**CMAKE_INSTALL_PREFIX**:PATH

:   Path where LLVM will be installed when the "install" target is built.

**CMAKE\_{C,CXX}\_FLAGS**:STRING

:   Extra flags to use when compiling C and C++ source files respectively.

**CMAKE\_{C,CXX}\_COMPILER**:STRING

:   Specify the C and C++ compilers to use. If you have multiple compilers
    installed, CMake might not default to the one you wish to use.

(Frequently Used LLVM-related variables)=
### Frequently Used LLVM-related variables

The default configuration may not match your requirements. Here are LLVM
variables that are frequently used to control that. The full description is in
[LLVM-related variables](#llvm-related-variables) below.

**LLVM_ENABLE_PROJECTS**:STRING

:   Control which projects are enabled. For example, you may want to work on
    clang or lldb by specifying `-DLLVM_ENABLE_PROJECTS="clang;lldb"`.

**LLVM_ENABLE_RUNTIMES**:STRING

:   Control which runtimes are enabled. For example, you may want to work on
    libc++ or libc++abi by specifying
    `-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"`.

**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`.

**LLVM_PARALLEL\_{COMPILE,LINK}\_JOBS**:STRING

:   Building the llvm toolchain can use a lot of resources, particularly during
    linking. These options, when you use the Ninja generator, allow you to
    restrict the parallelism. For example, to avoid OOMs or going into swap,
    permit only one link job per 15 GB of RAM available on a 32 GB machine,
    specify `-G Ninja -DLLVM_PARALLEL_LINK_JOBS=2`.

**LLVM_TARGETS_TO_BUILD**:STRING

:   Control which targets are enabled. For example, you may only need to enable
    your native target with, for example, `-DLLVM_TARGETS_TO_BUILD=X86`.

(llvm_use_linker)=

**LLVM_USE_LINKER**:STRING

:   Override the system's default linker. For instance, use `lld` with
    `-DLLVM_USE_LINKER=lld`.

### Rarely-used CMake variables

Here are some of the CMake variables that are rarely used, along with a brief
explanation and LLVM-related notes. For full documentation, consult the CMake
manual, or execute `cmake --help-variable VARIABLE_NAME`.

**CMAKE_CXX_STANDARD**:STRING

:   Sets the C++ standard to conform to when building LLVM. LLVM requires C++17
    or higher. This defaults to 17.

**CMAKE_INSTALL_BINDIR**:PATH

:   The path to install executables, relative to the *CMAKE_INSTALL_PREFIX*.
    Defaults to "bin".

**CMAKE_INSTALL_DOCDIR**:PATH

:   The path to install documentation, relative to the *CMAKE_INSTALL_PREFIX*.
    Defaults to "share/doc".

**CMAKE_INSTALL_INCLUDEDIR**:PATH

:   The path to install header files, relative to the *CMAKE_INSTALL_PREFIX*.
    Defaults to "include".

**CMAKE_INSTALL_MANDIR**:PATH

:   The path to install manpage files, relative to the *CMAKE_INSTALL_PREFIX*.
    Defaults to "share/man".

(LLVM-related variables)=
### LLVM-related variables

These variables provide fine control over the build of LLVM and its enabled
sub-projects. Nearly all of these variable names begin with `LLVM_`.

(LLVM-related variables BUILD_SHARED_LIBS)=

**BUILD_SHARED_LIBS**:BOOL

:   Flag indicating if each LLVM component (e.g. Support) is built as a shared
    library (ON) or as a static library (OFF). Its default value is OFF. On
    Windows, shared libraries may be used when building with MinGW, including
    mingw-w64, but not when building with the Microsoft toolchain.

    ```{note}
    `BUILD_SHARED_LIBS` is only recommended for use by LLVM developers. If you want to build LLVM as a shared library, you should use the `LLVM_BUILD_LLVM_DYLIB` option.
    ```

**LLVM_ABI_BREAKING_CHECKS**:STRING

:   Used to decide if LLVM should be built with ABI breaking checks or not.
    Allowed values are `WITH_ASSERTS` (default), `FORCE_ON` and `FORCE_OFF`.
    `WITH_ASSERTS` turns on ABI breaking checks in an assertion-enabled build.
    `FORCE_ON` (`FORCE_OFF`) turns them on (off) irrespective of whether normal
    (`NDEBUG`-based) assertions are enabled or not. A version of LLVM built
    with ABI breaking checks is not ABI compatible with a version built without
    it.

**LLVM_ADDITIONAL_BUILD_TYPES**:LIST

:   Adding a semicolon-separated list of additional build types to this flag
    allows for them to be specified as values in `CMAKE_BUILD_TYPE` without
    encountering a fatal error during the configuration process.

**LLVM_APPEND_VC_REV**:BOOL

:   Embed version control revision info (Git revision id). The version info is
    provided by the `LLVM_REVISION` macro in
    `llvm/include/llvm/Support/VCSRevision.h`. Developers using git who don't
    need revision info can disable this option to avoid re-linking most
    binaries after a branch switch. Defaults to ON.

**LLVM_FORCE_VC_REPOSITORY**:STRING

:   Set the git repository to include in version info rather than calling git
    to determine it.

**LLVM_FORCE_VC_REVISION**:STRING

:   Force a specific Git revision id rather than calling git to determine it.
    This is useful in environments where git is not available or non-functional
    but the VC revision is available through other means.

**LLVM_BUILD_32_BITS**:BOOL

:   Build 32-bit executables and libraries on 64-bit systems. This option is
    available only on some 64-bit Unix systems. Defaults to OFF.

**LLVM_BUILD_BENCHMARKS**:BOOL

:   Adds benchmarks to the list of default targets. Defaults to OFF.

**LLVM_BUILD_DOCS**:BOOL

:   Adds all *enabled* documentation targets (i.e., Doxygen and Sphinx targets)
    as dependencies of the default build targets. This results in all of the
    (enabled) documentation targets being built as part of a normal build. If
    the `install` target is run, then this also enables all built documentation
    targets to be installed. Defaults to OFF. To enable a particular
    documentation target, see `LLVM_ENABLE_SPHINX` and `LLVM_ENABLE_DOXYGEN`.

**LLVM_BUILD_EXAMPLES**:BOOL

:   Include LLVM examples in the 'all' build target and install them as part of
    the `install` target. Defaults to OFF. Targets for building examples are
    still generated, this is controlled by *LLVM_INCLUDE_EXAMPLES*. Note that
    some examples might still be built as dependencies for tests.

**LLVM_BUILD_INSTRUMENTED_COVERAGE**:BOOL

:   If enabled, [source-based code
    coverage](https://clang.llvm.org/docs/SourceBasedCodeCoverage.html)
    instrumentation is enabled while building llvm. If CMake can locate the
    code coverage scripts and the llvm-cov and llvm-profdata tools that pair
    with your compiler, the build will also generate the
    `generate-coverage-report` target to generate the code coverage report for
    LLVM, and the `clear-profile-data` utility target to delete captured
    profile data. See documentation for *LLVM_CODE_COVERAGE_TARGETS* and
    *LLVM_COVERAGE_SOURCE_DIRS* for more information on configuring code
    coverage reports.

**LLVM_BUILD_LLVM_DYLIB**:BOOL

:   If enabled, the target for building the libLLVM shared library is added.
    This library contains all of LLVM's components in a single shared library.
    Defaults to OFF. This cannot be used in conjunction with
    `BUILD_SHARED_LIBS`. Tools will only be linked to the libLLVM shared
    library if `LLVM_LINK_LLVM_DYLIB` is also ON. The components in the library
    can be customised by setting `LLVM_DYLIB_COMPONENTS` to a list of the
    desired components. This option is not available on Windows.

**LLVM_BUILD_TESTS**:BOOL

:   Include LLVM unit tests in the 'all' build target. Defaults to OFF. Targets
    for building each unit test are generated in any case. You can build a
    specific unit test using the targets defined under *unittests*, such as
    ADTTests, IRTests, SupportTests, etc. (Search for `add_llvm_unittest` in
    the subdirectories of *unittests* for a complete list of unit tests.) It is
    possible to build all unit tests with the target *UnitTests*.

**LLVM_BUILD_TOOLS**:BOOL

:   Build LLVM tools. Defaults to ON. Targets for building each tool are
    generated in any case. You can build a tool separately by invoking its
    target. For example, you can build *llvm-as* with a Makefile-based system
    by executing *make llvm-as* at the root of your build directory.

**LLVM_CCACHE_BUILD**:BOOL

:   If enabled and the `ccache` program is available, then LLVM will be built
    using `ccache` to speed up rebuilds of LLVM and its components. Defaults to
    OFF. The size and location of the cache maintained by `ccache` can be
    adjusted via the `LLVM_CCACHE_MAXSIZE` and `LLVM_CCACHE_DIR` options, which
    are passed to the `CCACHE_MAXSIZE` and `CCACHE_DIR` environment variables,
    respectively.

**LLVM_CODE_COVERAGE_TARGETS**:STRING

:   If set to a semicolon-separated list of targets, those targets will be used
    to drive the code coverage reports. If unset, the target list will be
    constructed using the LLVM build's CMake export list.

**LLVM_COVERAGE_SOURCE_DIRS**:STRING

:   If set to a semicolon-separated list of directories, the coverage reports
    will limit code coverage summaries to just the listed directories. If
    unset, coverage reports will include all sources identified by the tooling.

**LLVM_CREATE_XCODE_TOOLCHAIN**:BOOL

:   macOS only: If enabled, CMake will generate a target named
    'install-xcode-toolchain'. This target will create a directory at
    `$CMAKE_INSTALL_PREFIX/Toolchains` containing an xctoolchain directory
    which can be used to override the default system tools.

**LLVM_DEFAULT_TARGET_TRIPLE**:STRING

:   LLVM target to use for code generation when no target is explicitly
    specified. It defaults to "host", meaning that it shall pick the
    architecture of the machine where LLVM is being built. If you are building
    a cross-compiler, set it to the target triple of your desired architecture.

**LLVM_DOXYGEN_QCH_FILENAME**:STRING

:   The filename of the Qt Compressed Help file that will be generated 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 it has no
    effect.

**LLVM_DOXYGEN_QHELPGENERATOR_PATH**:STRING

:   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 it has no effect.

**LLVM_DOXYGEN_QHP_CUST_FILTER_NAME**:STRING

:   See [Qt Help
    Project](http://qt-project.org/doc/qt-4.8/qthelpproject.html#custom-filters)
    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 it has no effect.

**LLVM_DOXYGEN_QHP_NAMESPACE**:STRING

:   Namespace under which the intermediate Qt Help Project file lives. See [Qt
    Help
    Project](http://qt-project.org/doc/qt-4.8/qthelpproject.html#custom-filters)
    for more information. Defaults to "org.llvm". This option is only useful
    in combination with `-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON`; otherwise it has no
    effect.

(llvm_enable_assertions)=

**LLVM_ENABLE_ASSERTIONS**:BOOL

:   Enables code assertions. Defaults to ON if and only if `CMAKE_BUILD_TYPE`
    is *Debug*.

**LLVM_ENABLE_BINDINGS**:BOOL

:   If disabled, do not try to build the OCaml bindings.

**LLVM_ENABLE_CURL**:

:   Used to decide if LLVM tools, should support downloading information
    (particularly debug info from `llvm-debuginfod`) over HTTP. Allowed values
    are `OFF` (default), `ON`, and `FORCE_ON` (error if libcurl is not found).

**LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING**:STRING

:   Enhances Debugify's ability to detect line number errors by storing extra
    information inside Instructions, removing false positives from Debugify's
    results at the cost of performance. Allowed values are `DISABLED`
    (default), `COVERAGE`, and `COVERAGE_AND_ORIGIN`. `COVERAGE` tracks whether
    and why a line number was intentionally dropped or not generated for an
    instruction, allowing Debugify to avoid reporting these as errors; this
    comes with a small performance cost of \~0.1%. `COVERAGE_AND_ORIGIN`
    additionally stores a stacktrace of the point where each DebugLoc is
    unintentionally dropped, allowing for much easier bug triaging at the cost
    of a \~10x performance slowdown. `COVERAGE` and `COVERAGE_AND_ORIGIN` are
    ABI-breaking options.

**LLVM_ENABLE_DIA_SDK**:BOOL

:   Enable building with MSVC DIA SDK for PDB debugging support. Available only
    with MSVC. Defaults to ON.

**LLVM_ENABLE_DOXYGEN**:BOOL

:   Enables the generation of browsable HTML documentation using doxygen.
    Defaults to OFF.

**LLVM_ENABLE_DOXYGEN_QT_HELP**:BOOL

:   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.

**LLVM_ENABLE_EH**:BOOL

:   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.

**LLVM_ENABLE_EXPENSIVE_CHECKS**:BOOL

:   Enable additional time/memory expensive checking. Defaults to OFF.

**LLVM_ENABLE_FFI**:BOOL

:   Indicates whether the LLVM Interpreter will be linked with the Foreign
    Function Interface library (libffi) in order to enable calling external
    functions. If the library or its headers are installed in a custom
    location, you can also set the variables `FFI_INCLUDE_DIR` and
    `FFI_LIBRARY_DIR` to the directories where `ffi.h` and `libffi.so` can be
    found, respectively. Defaults to OFF.

**LLVM_ENABLE_HTTPLIB**:BOOL

:   Enables the optional cpp-httplib dependency which is used by
    llvm-debuginfod to serve debug info over HTTP.
    [cpp-httplib](https://github.com/yhirose/cpp-httplib) must be installed, or
    `httplib_ROOT` must be set. Defaults to OFF.

**LLVM_ENABLE_IDE**:BOOL

:   Tell the build system that an IDE is being used. This in turn disables the
    creation of certain convenience build system targets, such as the various
    `install-*` and `check-*` targets, since IDEs don't always deal well with a
    large number of targets. This is usually autodetected, but it can be
    configured manually to explicitly control the generation of those targets.

**LLVM_ENABLE_LIBCXX**:BOOL

:   If the host compiler and linker support the stdlib flag, `-stdlib=libc++`
    is passed to invocations of both so that the project is built using libc++
    instead of stdlibc++. Defaults to OFF.

**LLVM_ENABLE_LIBEDIT**:BOOL

:   Controls whether to enable libedit support for command-line editing and
    history in LLVM tools. When `ON`, forces libedit support to be enabled and
    will cause a CMake configuration error if libedit cannot be found. When
    `OFF`, disables libedit support entirely. If not specified, LLVM will
    auto-detect libedit availability. Defaults to auto-detection.

**LLVM_ENABLE_LIBPFM**:BOOL

:   Enable building with libpfm to support hardware counter measurements in
    LLVM tools. Defaults to ON.

**LLVM_ENABLE_LLD**:BOOL

:   This option is equivalent to `-DLLVM_USE_LINKER=lld`, except during a
    2-stage build where a dependency is added from the first stage to the
    second ensuring that lld is built before stage2 begins.

**LLVM_ENABLE_LLVM_LIBC**: BOOL

:   If the LLVM libc overlay is installed in a location where the host linker
    can access it, all built executables will be linked against the LLVM libc
    overlay before linking against the system libc. Defaults to OFF.

**LLVM_ENABLE_LTO**:STRING

:   Add `-flto` or `-flto=` flags to the compile and link command lines,
    enabling link-time optimization. Possible values are `Off`, `On`, `Thin`
    and `Full`. Defaults to OFF.

**LLVM_ENABLE_MODULES**:BOOL

:   Compile with [Clang Header
    Modules](https://clang.llvm.org/docs/Modules.html).

(llvm_enable_pdb)=

**LLVM_ENABLE_PDB**:BOOL

:   For Windows builds using MSVC or clang-cl, generate PDB files when
    {ref}`CMAKE_BUILD_TYPE <cmake_build_type>` is set to Release.

**LLVM_ENABLE_PEDANTIC**:BOOL

:   Enable pedantic mode. This disables compiler-specific extensions, if
    possible. 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, do not need this flag. Defaults to
    ON.

**LLVM_ENABLE_PROJECTS**:STRING

:   Semicolon-separated list of projects to build, or *all* for building all
    (clang, lldb, lld, polly, etc) projects. This flag assumes that projects
    are checked out side-by-side and not nested, i.e. clang needs to be in
    parallel to llvm instead of nested in `llvm/tools`. This feature allows
    having one build for only LLVM and another for clang+llvm using the same
    source checkout.

    The full list is:

    `bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly`

    ```{note}
    Some projects listed here can also go in `LLVM_ENABLE_RUNTIMES`. They should only appear in one of the two lists. If a project is a valid possibility for both, prefer putting it in `LLVM_ENABLE_RUNTIMES`.
    ```

**LLVM_ENABLE_RTTI**:BOOL

:   Build LLVM with run-time type information. Defaults to OFF.

**LLVM_ENABLE_RUNTIMES**:STRING

:   Build libc++, libc++abi, libunwind or compiler-rt using the just-built
    compiler. This is the correct way to build runtimes when putting together a
    toolchain. It will build the builtins separately from the other runtimes to
    preserve correct dependency ordering. If you want to build the runtimes
    using a system compiler, see the [libc++
    documentation](https://libcxx.llvm.org/VendorDocumentation.html).

    ```{note}
    The list should not have duplicates with `LLVM_ENABLE_PROJECTS`.
    ```

    To list all possible runtimes, include an invalid name. For example
    `-DLLVM_ENABLE_RUNTIMES=notaruntime`. The resulting CMake error will list
    the possible runtime names.

    To enable all of the runtimes, use:

    `LLVM_ENABLE_RUNTIMES=all`

**LLVM_ENABLE_SPHINX**:BOOL

:   If specified, 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.

**LLVM_ENABLE_THREADS**:BOOL

:   Build with threads support, if available. Defaults to ON.

**LLVM_ENABLE_UNWIND_TABLES**:BOOL

:   Enable unwind tables in the binary. Disabling unwind tables can reduce the
    size of the libraries. Defaults to ON.

**LLVM_ENABLE_WARNINGS**:BOOL

:   Enable all compiler warnings. Defaults to ON.

**LLVM_ENABLE_WARNING_SUPPRESSIONS**:BOOL

:   Suppress specific compiler warnings. When disabled, this prevents
    suppressing warnings with flags such as MSVC's `-wd` or GCC/Clang's
    `-Wno-...`. Defaults to ON.

**LLVM_ENABLE_WERROR**:BOOL

:   Stop and fail the build, if a compiler warning is triggered. Defaults to
    OFF.

**LLVM_ENABLE_Z3_SOLVER**:BOOL

:   If enabled, the Z3 constraint solver is activated for the Clang static
    analyzer. A recent version of the z3 library must be available on the
    system.

**LLVM_ENABLE_ZLIB**:STRING

:   Used to decide if LLVM tools should support compression/decompression with
    zlib. Allowed values are `OFF`, `ON` (default, enable if zlib is found),
    and `FORCE_ON` (error if zlib is not found).

**LLVM_ENABLE_ZSTD**:STRING

:   Used to decide if LLVM tools should support compression/decompression with
    zstd. Allowed values are `OFF`, `ON` (default, enable if zstd is found),
    and `FORCE_ON` (error if zstd is not found).

**LLVM_EXPERIMENTAL_TARGETS_TO_BUILD**:STRING

:   Semicolon-separated list of experimental targets to build and linked into
    llvm. This will build the experimental target without needing it to add to
    the list of all the targets available in the LLVM's main `CMakeLists.txt`.

**LLVM_EXTERNAL_PROJECTS**:STRING

:   Semicolon-separated list of additional external projects to build as part
    of llvm. For each project, `LLVM_EXTERNAL_<NAME>_SOURCE_DIR` has to be
    specified with the path for the source code of the project. Example:
    `-DLLVM_EXTERNAL_PROJECTS="Foo;Bar" -DLLVM_EXTERNAL_FOO_SOURCE_DIR=/src/foo
    -DLLVM_EXTERNAL_BAR_SOURCE_DIR=/src/bar`.

**LLVM_EXTERNAL\_{CLANG,LLD,POLLY}\_SOURCE_DIR**:PATH

:   These variables specify the path to the source directory for the external
    LLVM projects Clang, lld, and Polly, respectively, relative to the
    top-level source directory. If the in-tree subdirectory for an external
    project exists (e.g., `llvm/tools/clang` for Clang), then the corresponding
    variable will not be used. If the variable for an external project does not
    point to a valid path, then that project will not be built.

**LLVM_EXTERNALIZE_DEBUGINFO**:BOOL

:   Generate dSYM files and strip executables and libraries (Darwin only).
    Defaults to OFF.

**LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES**:BOOL

:   When building executables, preserve symbol exports. Defaults to ON. You can
    use this option to disable exported symbols from all executables (Darwin
    Only).

**LLVM_FORCE_USE_OLD_TOOLCHAIN**:BOOL

:   If enabled, the compiler and standard library versions won't be checked.
    LLVM may not compile at all, or might fail at runtime due to known bugs in
    these toolchains.

**LLVM_INCLUDE_BENCHMARKS**:BOOL

:   Generate build targets for the LLVM benchmarks. Defaults to ON.

**LLVM_INCLUDE_EXAMPLES**:BOOL

:   Generate build targets for the LLVM examples. Defaults to ON. You can use
    this option to disable the generation of build targets for the LLVM
    examples.

**LLVM_INCLUDE_TESTS**:BOOL

:   Generate build targets for the LLVM unit tests. Defaults to ON. You can use
    this option to disable the generation of build targets for the LLVM unit
    tests.

**LLVM_INCLUDE_TOOLS**:BOOL

:   Generate build targets for the LLVM tools. Defaults to ON. You can use this
    option to disable the generation of build targets for the LLVM tools.

**LLVM_INDIVIDUAL_TEST_COVERAGE**:BOOL

:   Enable individual test case coverage. When set to ON, code coverage data
    for each test case will be generated and stored in a separate directory
    under the config.test_exec_root path. This feature allows code coverage
    analysis of each individual test case. Defaults to OFF.

**LLVM_INSTALL_BINUTILS_SYMLINKS**:BOOL

:   Install symlinks from the binutils tool names to the corresponding LLVM
    tools. For example, ar will be symlinked to llvm-ar.

**LLVM_INSTALL_CCTOOLS_SYMLINKS**:BOOL

:   Install symlinks from the cctools tool names to the corresponding LLVM
    tools. For example, lipo will be symlinked to llvm-lipo.

**LLVM_INSTALL_OCAMLDOC_HTML_DIR**:STRING

:   The path to install OCamldoc-generated HTML documentation to. This path can
    either be absolute or relative to the `CMAKE_INSTALL_PREFIX`. Defaults to
    `${CMAKE_INSTALL_DOCDIR}/llvm/ocaml-html`.

**LLVM_INSTALL_SPHINX_HTML_DIR**:STRING

:   The path to install Sphinx-generated HTML documentation to. This path can
    either be absolute or relative to the `CMAKE_INSTALL_PREFIX`. Defaults to
    `${CMAKE_INSTALL_DOCDIR}/llvm/html`.

**LLVM_INSTALL_UTILS**:BOOL

:   If enabled, utility binaries like `FileCheck` and `not` will be installed
    to `CMAKE_INSTALL_PREFIX`.

**LLVM_INSTALL_DOXYGEN_HTML_DIR**:STRING

:   The path to install Doxygen-generated HTML documentation to. This path can
    either be absolute or relative to the *CMAKE_INSTALL_PREFIX*. Defaults to
    `${CMAKE_INSTALL_DOCDIR}/llvm/doxygen-html`.

**LLVM_INTEGRATED_CRT_ALLOC**:PATH

:   On Windows, allows embedding a different C runtime allocator into the LLVM
    tools and libraries. Using a lock-free allocator such as the ones listed
    below greatly decreases ThinLTO link time by about an order of magnitude.
    It also mildly improves Clang build times, by about 5-10%. At the moment,
    rpmalloc, snmalloc and mimalloc are supported. Use the path to `git clone`
    to select the respective allocator, for example:

    ``` console
    $ D:\git> git clone https://github.com/mjansson/rpmalloc
    $ D:\llvm-project> cmake ... -DLLVM_INTEGRATED_CRT_ALLOC=D:\git\rpmalloc
    ```

    This option needs to be used along with the static CRT, i.e., if building
    the Release target, add `-DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded`. Note
    that rpmalloc is also supported natively in-tree, see option below.

**LLVM_ENABLE_RPMALLOC**:BOOL

:   Similar to LLVM_INTEGRATED_CRT_ALLOC, embeds the in-tree rpmalloc into the
    host toolchain as a C runtime allocator. The version currently used is
    rpmalloc 1.4.5. This option also implies linking with the static CRT,
    there's no need to provide CMAKE_MSVC_RUNTIME_LIBRARY.

**LLVM_LINK_LLVM_DYLIB**:BOOL

:   If enabled, tools will be linked with the libLLVM shared library. Defaults
    to OFF. Setting `LLVM_LINK_LLVM_DYLIB` to ON also sets
    `LLVM_BUILD_LLVM_DYLIB` to ON. This option is not available on Windows.

**LLVM\_\<target\>\_LINKER_FLAGS**:STRING

:   Defines the set of linker flags that should be applied to a \<target\>.

**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
    the empty string, in which case lit will look for tools needed for tests
    (e.g., `grep`, `sort`, etc.) in your `%PATH%`. If GnuWin32 is not in your
    `%PATH%`, then you can set this variable to the GnuWin32 directory so that
    lit can find tools needed for tests in that directory.

**LLVM_NATIVE_TOOL_DIR**:STRING

:   Full path to a directory containing executables for the build host
    (containing binaries such as `llvm-tblgen` and `clang-tblgen`). This is
    intended for cross-compiling: if the user sets this variable and the
    directory contains executables with the expected names, no separate native
    versions of those executables will be built.

**LLVM_NO_INSTALL_NAME_DIR_FOR_BUILD_TREE**:BOOL

:   Defaults to `OFF`. If set to `ON`, CMake's default logic for library IDs on
    Darwin in the build tree will be used. Otherwise the install-time library
    IDs will be used in the build tree as well. Mainly useful when other CMake
    library ID control variables (e.g., `CMAKE_INSTALL_NAME_DIR`) are being set
    to non-standard values.

**LLVM_VERSIONED_DYLIB_NAME_ON_DARWIN**:BOOL

:   Defaults to `ON`. If set to `ON`, Darwin shared libraries built through
    LLVM's CMake helpers use versioned dylib filenames and install names,
    matching the behavior on other Unix platforms more closely. If set to
    `OFF`, Darwin keeps the legacy unversioned dylib install name, for
    compatibility with existing consumers that expect `@rpath/libLLVM.dylib`.

**LLVM_UNVERSIONED\_{LIBLTO,LIBCLANG}\_ON_DARWIN**:BOOL

:   Default to `ON`. When `LLVM_VERSIONED_DYLIB_NAME_ON_DARWIN` is `ON`, these
    keep `libLTO` and `libclang`'s Darwin dylib identities unversioned. Set
    `LLVM_UNVERSIONED_LIBLTO_ON_DARWIN` to `OFF` to version `libLTO` using its
    Darwin `LTO_VERSION` policy instead. Set
    `LLVM_UNVERSIONED_LIBCLANG_ON_DARWIN` to `OFF` to version `libclang` using
    its existing ABI version policy instead.

**LLVM_OPTIMIZED_TABLEGEN**:BOOL

:   If enabled and building a debug or assert build, the CMake build system
    will generate a Release build tree to build a fully optimized tablegen for
    use during the build. Enabling this option can significantly speed up build
    times, especially when building LLVM in Debug configurations.

**LLVM_PARALLEL\_{COMPILE,LINK,TABLEGEN}\_JOBS**:STRING

:   Limit the maximum number of concurrent compilation, link or tablegen jobs
    respectively. The default total number of parallel jobs is determined by
    the number of logical CPUs.

**LLVM_PROFDATA_FILE**:PATH

:   Path to a profdata file to pass into clang's `-fprofile-instr-use` flag.
    This can only be specified if you're building with clang.

**LLVM_RAM_PER\_{COMPILE,LINK,TABLEGEN}\_JOB**:STRING

:   Limit the number of concurrent compile, link or tablegen jobs respectively,
    depending on available physical memory. The value specified is in MB. The
    respective `LLVM_PARALLEL_{COMPILE,LINK,TABLEGEN}_JOBS` variable is
    overwritten by computing the memory size divided by the specified value.
    The largest memory user is linking, but remember that jobs in the other
    categories might run in parallel with the link jobs, and you need to
    consider their memory requirements when in a memory-limited environment.
    Using a `-DLLVM_RAM_PER_LINK_JOB=10000` is a good approximation. On ELF
    platforms debug builds can reduce link-time memory pressure by also using
    `LLVM_USE_SPLIT_DWARF`.

**LLVM_REVERSE_ITERATION**:BOOL

:   If enabled, all supported unordered llvm containers would be iterated in
    reverse order. This is useful for uncovering non-determinism caused by
    iteration of unordered containers.

**LLVM_STATIC_LINK_CXX_STDLIB**:BOOL

:   Statically link to the C++ standard library if possible. This uses the flag
    `-static-libstdc++`, but a Clang host compiler will statically link to
    libc++ if used in conjunction with the **LLVM_ENABLE_LIBCXX** flag.
    Defaults to OFF.

**LLVM_TABLEGEN**:STRING

:   Full path to a native TableGen executable (usually named `llvm-tblgen`).
    This is intended for cross-compiling: if the user sets this variable, no
    native TableGen will be created.

**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_TARGETS_TO_BUILD**:STRING

:   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"`. The full list, as of August 2025,
    is:
    `AArch64;AMDGPU;ARM;AVR;BPF;Hexagon;Lanai;LoongArch;Mips;MSP430;NVPTX;PowerPC;RISCV;Sparc;SPIRV;SystemZ;VE;WebAssembly;X86;XCore`

    You can also specify `host` or `Native` to automatically detect and include
    the target corresponding to the host machine's architecture, or use `all`
    to include all available targets. For example, on an x86_64 machine,
    specifying `-DLLVM_TARGETS_TO_BUILD=host` will include the `X86` target.

**LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN**:BOOL

:   If enabled, the compiler version check will only warn when using a
    toolchain which is about to be deprecated, instead of emitting an error.

**LLVM_UBSAN_FLAGS**:STRING

:   Defines the set of compile flags used to enable UBSan. Only used if
    `LLVM_USE_SANITIZER` contains `Undefined`. This can be used to override the
    default set of UBSan flags.

**LLVM_UNREACHABLE_OPTIMIZE**:BOOL

:   This flag controls the behavior of `llvm_unreachable()` in a release build
    (when assertions are disabled in general). When ON (default) then
    `llvm_unreachable()` is considered "undefined behavior" and optimized as
    such. When OFF it is instead replaced with a guaranteed "trap".

**LLVM_USE_INTEL_JITEVENTS**:BOOL

:   Enable building support for Intel JIT Events API. Defaults to OFF.

**LLVM_USE_LINKER**:STRING

:   Add `-fuse-ld={name}` to the link invocation. The possible values depend on
    your compiler. For clang, the value can be an absolute path to your custom
    linker, otherwise clang will prefix the name with `ld.` and apply its usual
    search. For example, to link LLVM with the Gold linker, cmake can be
    invoked with `-DLLVM_USE_LINKER=gold`.

**LLVM_USE_OPROFILE**:BOOL

:   Enable building OProfile JIT support. Defaults to OFF.

**LLVM_USE_PERF**:BOOL

:   Enable building support for Perf (linux profiling tool) JIT support.
    Defaults to OFF.

**LLVM_USE_RELATIVE_PATHS_IN_FILES**:BOOL

:   Rewrite absolute source paths in sources and debug info to relative ones.
    The source prefix can be adjusted via the `LLVM_SOURCE_PREFIX` variable.

**LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO**:BOOL

:   Rewrite absolute source paths in debug info to relative ones. The source
    prefix can be adjusted via the `LLVM_SOURCE_PREFIX` variable.

**LLVM_USE_SANITIZER**:STRING

:   Define the sanitizer used to build LLVM binaries and tests. Possible values
    are `Address`, `HWAddress`, `Memory`, `MemoryWithOrigins`, `Undefined`,
    `Thread`, `DataFlow`, `Leaks`, and `Address;Undefined`. Defaults to empty
    string.

**LLVM_USE_SPLIT_DWARF**:BOOL

:   If enabled CMake will pass `-gsplit-dwarf` to the compiler. This option
    reduces link-time memory usage by reducing the amount of debug information
    that the linker needs to resolve. It is recommended for platforms using the
    ELF object format, like Linux systems when linker memory usage is too high.

**SPHINX_EXECUTABLE**:STRING

:   The path to the `sphinx-build` executable detected by CMake. For
    installation instructions, see
    <https://www.sphinx-doc.org/en/master/usage/installation.html>

**SPHINX_OUTPUT_HTML**:BOOL

:   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.

**SPHINX_OUTPUT_MAN**:BOOL

:   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.

**SPHINX_WARNINGS_AS_ERRORS**:BOOL

:   If enabled, then sphinx documentation warnings will be treated as errors.
    Defaults to ON.

#### Advanced variables

These are niche, and changing them from their defaults is more likely to cause
things to go wrong. They are also unstable across LLVM versions.

**LLVM_EXAMPLES_INSTALL_DIR**:STRING

:   The path for examples of using LLVM, relative to the
    *CMAKE_INSTALL_PREFIX*. Only matters if *LLVM_BUILD_EXAMPLES* is enabled.
    Defaults to "examples".

**LLVM_TOOLS_INSTALL_DIR**:STRING

:   The path to install the main LLVM tools, relative to the
    *CMAKE_INSTALL_PREFIX*. Defaults to *CMAKE_INSTALL_BINDIR*.

**LLVM_UTILS_INSTALL_DIR**:STRING

:   The path to install auxiliary LLVM utilities, relative to the
    *CMAKE_INSTALL_PREFIX*. Only matters if *LLVM_INSTALL_UTILS* is enabled.
    Defaults to *LLVM_TOOLS_INSTALL_DIR*.

## CMake Caches

Recently, LLVM and Clang have been adding some more complicated build system
features. Utilizing these new features often involves a complicated chain of
CMake variables passed on the command line. Clang provides a collection of CMake
cache scripts to make these features more approachable.

CMake cache files are utilized using CMake's `-C` flag:

``` console
$ cmake -C <path to cache file> <path to sources>
```

CMake cache scripts are processed in an isolated scope, only cached variables
remain set when the main configuration runs. CMake cached variables do not reset
variables that are already set unless the FORCE option is specified.

A few notes about CMake Caches:

- Order of command line arguments is important
  - `-D` arguments specified before `-C` are set before the cache is processed and
    can be read inside the cache file
  - `-D` arguments specified after `-C` are set after the cache is processed and
    are unset inside the cache file
- All `-D` arguments will override cache file settings
- CMAKE_TOOLCHAIN_FILE is evaluated after both the cache file and the command
  line arguments
- It is recommended that all `-D` options be specified *before* `-C`

For more information about some of the advanced build configurations supported
via Cache files see {doc}`AdvancedBuilds`.

## Executing the Tests

Testing is performed when the *check-all* target is built. For instance, if you are
using Makefiles, execute this command in the root of your build directory:

``` console
$ make check-all
```

On Visual Studio, you may run tests by building the project "check-all".
For more information about testing, see the {doc}`TestingGuide`.

## Cross compiling

See [this wiki
page](https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/CrossCompiling)
for generic instructions on how to cross-compile with CMake. It goes into
detailed explanations and may seem daunting, but it is not. The wiki page has
several examples including toolchain files. Go directly to the `Information how
to set up various cross compiling toolchains` section for a quick solution.

Also see the [LLVM-related variables](#llvm-related-variables) section for
variables used when cross-compiling.

## Embedding LLVM in your project

From LLVM 3.5 onward, the CMake build system exports 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 a `CMakeLists.txt` file that imports the LLVM libraries
and uses them to build a simple application `simple-tool`.

``` cmake
cmake_minimum_required(VERSION 3.20.0)
project(SimpleProject)

find_package(LLVM REQUIRED CONFIG)

message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
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.

include_directories(${LLVM_INCLUDE_DIRS})
separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
add_definitions(${LLVM_DEFINITIONS_LIST})

# Now build our tools
add_executable(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 an `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/lib/cmake/llvm` to
the `cmake` command or by setting it directly in `ccmake` or `cmake-gui`).

This file is available in two different locations.

- `<LLVM_INSTALL_PACKAGE_DIR>/LLVMConfig.cmake` where
  `<LLVM_INSTALL_PACKAGE_DIR>` is the location where LLVM CMake modules are
  installed as part of an installed version of LLVM. This is typically
  `cmake/llvm/` within the lib directory. On Linux, this is typically
  `/usr/lib/cmake/llvm/LLVMConfig.cmake`.
- `<LLVM_BUILD_ROOT>/lib/cmake/llvm/LLVMConfig.cmake` where
  `<LLVM_BUILD_ROOT>` is the root of the LLVM build tree. **Note: this is
  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:

`LLVM_CMAKE_DIR`

:   The path to the LLVM CMake directory (i.e., the directory containing
    `LLVMConfig.cmake`).

`LLVM_DEFINITIONS`

:   A list of preprocessor defines that should be used when building against
    LLVM.

`LLVM_ENABLE_ASSERTIONS`

:   This is set to ON if LLVM was built with assertions, otherwise OFF.

`LLVM_ENABLE_EH`

:   This is set to ON if LLVM was built with exception handling (EH) enabled,
    otherwise OFF.

`LLVM_ENABLE_RTTI`

:   This is set to ON if LLVM was built with run time type information (RTTI),
    otherwise OFF.

`LLVM_INCLUDE_DIRS`

:   A list of include paths to directories containing LLVM header files.

`LLVM_PACKAGE_VERSION`

:   The LLVM version. This string can be used with CMake conditionals, e.g.,
    `if (${LLVM_PACKAGE_VERSION} VERSION_LESS "3.5")`.

`LLVM_TOOLS_BINARY_DIR`

:   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.

(cmake-out-of-source-pass)=
### Developing LLVM passes out of source

You can 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.

``` none
<project dir>/
    |
    CMakeLists.txt
    <pass name>/
        |
        CMakeLists.txt
        Pass.cpp
        ...
```

Contents of `<project dir>/CMakeLists.txt`:

``` cmake
find_package(LLVM REQUIRED CONFIG)

separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
add_definitions(${LLVM_DEFINITIONS_LIST})
include_directories(${LLVM_INCLUDE_DIRS})

add_subdirectory(<pass name>)
```

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

``` cmake
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_library` function with the MODULE argument instead by adding the
following to `<project dir>/CMakeLists.txt` (after `find_package(LLVM ...)`):

``` cmake
list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")
include(AddLLVM)
```

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

``` cmake
add_llvm_library(LLVMPassname MODULE
  Pass.cpp
  )
```

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

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

## Compiler/Platform-specific topics

Notes for specific compilers and/or platforms.

### Windows

**LLVM_COMPILER_JOBS**:STRING

:   Specifies the maximum number of parallel 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.

**CMAKE_MT**:STRING

:   When compiling with clang-cl, CMake may use `llvm-mt` as the Manifest Tool
    when available. `llvm-mt` is only present when libxml2 is found at
    build-time. To ensure using Microsoft's Manifest Tool set `CMAKE_MT=mt`.

### Apple/OSX

**CMAKE_OSX_SYSROOT**:STRING

:   When compiling for OSX, in order for the test suite to find libSystem to
    link dylib tests you'll need to run CMake with `xcrun --show-sdk-path`
    as the string to pass in so that the testsuite can find your os
    libraries.

    This will show up as `ld: library not found for -lSystem` when
    running tests.
