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+.. _makefile_guide:
+
+===================
+LLVM Makefile Guide
+===================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This document provides *usage* information about the LLVM makefile system. While
+loosely patterned after the BSD makefile system, LLVM has taken a departure from
+BSD in order to implement additional features needed by LLVM.  Although makefile
+systems, such as ``automake``, were attempted at one point, it has become clear
+that the features needed by LLVM and the ``Makefile`` norm are too great to use
+a more limited tool. Consequently, LLVM requires simply GNU Make 3.79, a widely
+portable makefile processor. LLVM unabashedly makes heavy use of the features of
+GNU Make so the dependency on GNU Make is firm. If you're not familiar with
+``make``, it is recommended that you read the `GNU Makefile Manual
+<http://www.gnu.org/software/make/manual/make.html>`_.
+
+While this document is rightly part of the `LLVM Programmer's
+Manual <ProgrammersManual.html>`_, it is treated separately here because of the
+volume of content and because it is often an early source of bewilderment for
+new developers.
+
+General Concepts
+================
+
+The LLVM Makefile System is the component of LLVM that is responsible for
+building the software, testing it, generating distributions, checking those
+distributions, installing and uninstalling, etc. It consists of a several files
+throughout the source tree. These files and other general concepts are described
+in this section.
+
+Projects
+--------
+
+The LLVM Makefile System is quite generous. It not only builds its own software,
+but it can build yours too. Built into the system is knowledge of the
+``llvm/projects`` directory. Any directory under ``projects`` that has both a
+``configure`` script and a ``Makefile`` is assumed to be a project that uses the
+LLVM Makefile system.  Building software that uses LLVM does not require the
+LLVM Makefile System nor even placement in the ``llvm/projects``
+directory. However, doing so will allow your project to get up and running
+quickly by utilizing the built-in features that are used to compile LLVM. LLVM
+compiles itself using the same features of the makefile system as used for
+projects.
+
+For complete details on setting up your projects configuration, simply mimic the
+``llvm/projects/sample`` project. Or for further details, consult the
+`Projects <Projects.html>`_ page.
+
+Variable Values
+---------------
+
+To use the makefile system, you simply create a file named ``Makefile`` in your
+directory and declare values for certain variables.  The variables and values
+that you select determine what the makefile system will do. These variables
+enable rules and processing in the makefile system that automatically Do The
+Right Thing&trade;.
+
+Including Makefiles
+-------------------
+
+Setting variables alone is not enough. You must include into your Makefile
+additional files that provide the rules of the LLVM Makefile system. The various
+files involved are described in the sections that follow.
+
+``Makefile``
+^^^^^^^^^^^^
+
+Each directory to participate in the build needs to have a file named
+``Makefile``. This is the file first read by ``make``. It has three
+sections:
+
+#. Settable Variables --- Required that must be set first.
+#. ``include $(LEVEL)/Makefile.common`` --- include the LLVM Makefile system.
+#. Override Variables --- Override variables set by the LLVM Makefile system.
+
+.. _$(LEVEL)/Makefile.common:
+
+``Makefile.common``
+^^^^^^^^^^^^^^^^^^^
+
+Every project must have a ``Makefile.common`` file at its top source
+directory. This file serves three purposes:
+
+#. It includes the project's configuration makefile to obtain values determined
+   by the ``configure`` script. This is done by including the
+   `$(LEVEL)/Makefile.config`_ file.
+
+#. It specifies any other (static) values that are needed throughout the
+   project. Only values that are used in all or a large proportion of the
+   project's directories should be placed here.
+
+#. It includes the standard rules for the LLVM Makefile system,
+   `$(LLVM_SRC_ROOT)/Makefile.rules`_.  This file is the *guts* of the LLVM
+   ``Makefile`` system.
+
+.. _$(LEVEL)/Makefile.config:
+
+``Makefile.config``
+^^^^^^^^^^^^^^^^^^^
+
+Every project must have a ``Makefile.config`` at the top of its *build*
+directory. This file is **generated** by the ``configure`` script from the
+pattern provided by the ``Makefile.config.in`` file located at the top of the
+project's *source* directory. The contents of this file depend largely on what
+configuration items the project uses, however most projects can get what they
+need by just relying on LLVM's configuration found in
+``$(LLVM_OBJ_ROOT)/Makefile.config``.
+
+.. _$(LLVM_SRC_ROOT)/Makefile.rules:
+
+``Makefile.rules``
+^^^^^^^^^^^^^^^^^^
+
+This file, located at ``$(LLVM_SRC_ROOT)/Makefile.rules`` is the heart of the
+LLVM Makefile System. It provides all the logic, dependencies, and rules for
+building the targets supported by the system. What it does largely depends on
+the values of ``make`` `variables`_ that have been set *before*
+``Makefile.rules`` is included.
+
+Comments
+^^^^^^^^
+
+User ``Makefile``\s need not have comments in them unless the construction is
+unusual or it does not strictly follow the rules and patterns of the LLVM
+makefile system. Makefile comments are invoked with the pound (``#``) character.
+The ``#`` character and any text following it, to the end of the line, are
+ignored by ``make``.
+
+Tutorial
+========
+
+This section provides some examples of the different kinds of modules you can
+build with the LLVM makefile system. In general, each directory you provide will
+build a single object although that object may be composed of additionally
+compiled components.
+
+Libraries
+---------
+
+Only a few variable definitions are needed to build a regular library.
+Normally, the makefile system will build all the software into a single
+``libname.o`` (pre-linked) object. This means the library is not searchable and
+that the distinction between compilation units has been dissolved. Optionally,
+you can ask for a shared library (.so) or archive library (.a) built.  Archive
+libraries are the default. For example:
+
+.. code-block:: makefile
+
+  LIBRARYNAME = mylib
+  SHARED_LIBRARY = 1
+  ARCHIVE_LIBRARY = 1
+
+says to build a library named ``mylib`` with both a shared library
+(``mylib.so``) and an archive library (``mylib.a``) version. The contents of all
+the libraries produced will be the same, they are just constructed differently.
+Note that you normally do not need to specify the sources involved. The LLVM
+Makefile system will infer the source files from the contents of the source
+directory.
+
+The ``LOADABLE_MODULE=1`` directive can be used in conjunction with
+``SHARED_LIBRARY=1`` to indicate that the resulting shared library should be
+openable with the ``dlopen`` function and searchable with the ``dlsym`` function
+(or your operating system's equivalents). While this isn't strictly necessary on
+Linux and a few other platforms, it is required on systems like HP-UX and
+Darwin. You should use ``LOADABLE_MODULE`` for any shared library that you
+intend to be loaded into an tool via the ``-load`` option. See the
+`WritingAnLLVMPass.html <WritingAnLLVMPass.html#makefile>`_ document for an
+example of why you might want to do this.
+
+Bitcode Modules
+^^^^^^^^^^^^^^^
+
+In some situations, it is desirable to build a single bitcode module from a
+variety of sources, instead of an archive, shared library, or bitcode
+library. Bitcode modules can be specified in addition to any of the other types
+of libraries by defining the `MODULE_NAME`_ variable. For example:
+
+.. code-block:: makefile
+
+  LIBRARYNAME = mylib
+  BYTECODE_LIBRARY = 1
+  MODULE_NAME = mymod
+
+will build a module named ``mymod.bc`` from the sources in the directory. This
+module will be an aggregation of all the bitcode modules derived from the
+sources. The example will also build a bitcode archive containing a bitcode
+module for each compiled source file. The difference is subtle, but important
+depending on how the module or library is to be linked.
+
+Loadable Modules
+^^^^^^^^^^^^^^^^
+
+In some situations, you need to create a loadable module. Loadable modules can
+be loaded into programs like ``opt`` or ``llc`` to specify additional passes to
+run or targets to support.  Loadable modules are also useful for debugging a
+pass or providing a pass with another package if that pass can't be included in
+LLVM.
+
+LLVM provides complete support for building such a module. All you need to do is
+use the ``LOADABLE_MODULE`` variable in your ``Makefile``. For example, to build
+a loadable module named ``MyMod`` that uses the LLVM libraries ``LLVMSupport.a``
+and ``LLVMSystem.a``, you would specify:
+
+.. code-block:: makefile
+
+  LIBRARYNAME := MyMod
+  LOADABLE_MODULE := 1
+  LINK_COMPONENTS := support system
+
+Use of the ``LOADABLE_MODULE`` facility implies several things:
+
+#. There will be no "``lib``" prefix on the module. This differentiates it from
+    a standard shared library of the same name.
+
+#. The `SHARED_LIBRARY`_ variable is turned on.
+
+#. The `LINK_LIBS_IN_SHARED`_ variable is turned on.
+
+A loadable module is loaded by LLVM via the facilities of libtool's libltdl
+library which is part of ``lib/System`` implementation.
+
+Tools
+-----
+
+For building executable programs (tools), you must provide the name of the tool
+and the names of the libraries you wish to link with the tool. For example:
+
+.. code-block:: makefile
+
+  TOOLNAME = mytool
+  USEDLIBS = mylib
+  LINK_COMPONENTS = support system
+
+says that we are to build a tool name ``mytool`` and that it requires three
+libraries: ``mylib``, ``LLVMSupport.a`` and ``LLVMSystem.a``.
+
+Note that two different variables are use to indicate which libraries are
+linked: ``USEDLIBS`` and ``LLVMLIBS``. This distinction is necessary to support
+projects. ``LLVMLIBS`` refers to the LLVM libraries found in the LLVM object
+directory. ``USEDLIBS`` refers to the libraries built by your project. In the
+case of building LLVM tools, ``USEDLIBS`` and ``LLVMLIBS`` can be used
+interchangeably since the "project" is LLVM itself and ``USEDLIBS`` refers to
+the same place as ``LLVMLIBS``.
+
+Also note that there are two different ways of specifying a library: with a
+``.a`` suffix and without. Without the suffix, the entry refers to the re-linked
+(.o) file which will include *all* symbols of the library.  This is
+useful, for example, to include all passes from a library of passes.  If the
+``.a`` suffix is used then the library is linked as a searchable library (with
+the ``-l`` option). In this case, only the symbols that are unresolved *at
+that point* will be resolved from the library, if they exist. Other
+(unreferenced) symbols will not be included when the ``.a`` syntax is used. Note
+that in order to use the ``.a`` suffix, the library in question must have been
+built with the ``ARCHIVE_LIBRARY`` option set.
+
+JIT Tools
+^^^^^^^^^
+
+Many tools will want to use the JIT features of LLVM.  To do this, you simply
+specify that you want an execution 'engine', and the makefiles will
+automatically link in the appropriate JIT for the host or an interpreter if none
+is available:
+
+.. code-block:: makefile
+
+  TOOLNAME = my_jit_tool
+  USEDLIBS = mylib
+  LINK_COMPONENTS = engine
+
+Of course, any additional libraries may be listed as other components.  To get a
+full understanding of how this changes the linker command, it is recommended
+that you:
+
+.. code-block:: bash
+
+  % cd examples/Fibonacci
+  % make VERBOSE=1
+
+Targets Supported
+=================
+
+This section describes each of the targets that can be built using the LLVM
+Makefile system. Any target can be invoked from any directory but not all are
+applicable to a given directory (e.g. "check", "dist" and "install" will always
+operate as if invoked from the top level directory).
+
+================= ===============      ==================
+Target Name       Implied Targets      Target Description
+================= ===============      ==================
+``all``           \                    Compile the software recursively. Default target.
+``all-local``     \                    Compile the software in the local directory only.
+``check``         \                    Change to the ``test`` directory in a project and run the test suite there.
+``check-local``   \                    Run a local test suite. Generally this is only defined in the  ``Makefile`` of the project's ``test`` directory.
+``clean``         \                    Remove built objects recursively.
+``clean-local``   \                    Remove built objects from the local directory only.
+``dist``          ``all``              Prepare a source distribution tarball.
+``dist-check``    ``all``              Prepare a source distribution tarball and check that it builds.
+``dist-clean``    ``clean``            Clean source distribution tarball temporary files.
+``install``       ``all``              Copy built objects to installation directory.
+``preconditions`` ``all``              Check to make sure configuration and makefiles are up to date.
+``printvars``     ``all``              Prints variables defined by the makefile system (for debugging).
+``tags``          \                    Make C and C++ tags files for emacs and vi.
+``uninstall``     \                    Remove built objects from installation directory.
+================= ===============      ==================
+
+.. _all:
+
+``all`` (default)
+-----------------
+
+When you invoke ``make`` with no arguments, you are implicitly instructing it to
+seek the ``all`` target (goal). This target is used for building the software
+recursively and will do different things in different directories.  For example,
+in a ``lib`` directory, the ``all`` target will compile source files and
+generate libraries. But, in a ``tools`` directory, it will link libraries and
+generate executables.
+
+``all-local``
+-------------
+
+This target is the same as `all`_ but it operates only on the current directory
+instead of recursively.
+
+``check``
+---------
+
+This target can be invoked from anywhere within a project's directories but
+always invokes the `check-local`_ target in the project's ``test`` directory, if
+it exists and has a ``Makefile``. A warning is produced otherwise.  If
+`TESTSUITE`_ is defined on the ``make`` command line, it will be passed down to
+the invocation of ``make check-local`` in the ``test`` directory. The intended
+usage for this is to assist in running specific suites of tests. If
+``TESTSUITE`` is not set, the implementation of ``check-local`` should run all
+normal tests.  It is up to the project to define what different values for
+``TESTSUTE`` will do. See the `Testing Guide <TestingGuide.html>`_ for further
+details.
+
+``check-local``
+---------------
+
+This target should be implemented by the ``Makefile`` in the project's ``test``
+directory. It is invoked by the ``check`` target elsewhere.  Each project is
+free to define the actions of ``check-local`` as appropriate for that
+project. The LLVM project itself uses dejagnu to run a suite of feature and
+regresson tests. Other projects may choose to use dejagnu or any other testing
+mechanism.
+
+``clean``
+---------
+
+This target cleans the build directory, recursively removing all things that the
+Makefile builds. The cleaning rules have been made guarded so they shouldn't go
+awry (via ``rm -f $(UNSET_VARIABLE)/*`` which will attempt to erase the entire
+directory structure.
+
+``clean-local``
+---------------
+
+This target does the same thing as ``clean`` but only for the current (local)
+directory.
+
+``dist``
+--------
+
+This target builds a distribution tarball. It first builds the entire project
+using the ``all`` target and then tars up the necessary files and compresses
+it. The generated tarball is sufficient for a casual source distribution, but
+probably not for a release (see ``dist-check``).
+
+``dist-check``
+--------------
+
+This target does the same thing as the ``dist`` target but also checks the
+distribution tarball. The check is made by unpacking the tarball to a new
+directory, configuring it, building it, installing it, and then verifying that
+the installation results are correct (by comparing to the original build).  This
+target can take a long time to run but should be done before a release goes out
+to make sure that the distributed tarball can actually be built into a working
+release.
+
+``dist-clean``
+--------------
+
+This is a special form of the ``clean`` clean target. It performs a normal
+``clean`` but also removes things pertaining to building the distribution.
+
+``install``
+-----------
+
+This target finalizes shared objects and executables and copies all libraries,
+headers, executables and documentation to the directory given with the
+``--prefix`` option to ``configure``.  When completed, the prefix directory will
+have everything needed to **use** LLVM.
+
+The LLVM makefiles can generate complete **internal** documentation for all the
+classes by using ``doxygen``. By default, this feature is **not** enabled
+because it takes a long time and generates a massive amount of data (>100MB). If
+you want this feature, you must configure LLVM with the --enable-doxygen switch
+and ensure that a modern version of doxygen (1.3.7 or later) is available in
+your ``PATH``. You can download doxygen from `here
+<http://www.stack.nl/~dimitri/doxygen/download.html#latestsrc>`_.
+
+``preconditions``
+-----------------
+
+This utility target checks to see if the ``Makefile`` in the object directory is
+older than the ``Makefile`` in the source directory and copies it if so. It also
+reruns the ``configure`` script if that needs to be done and rebuilds the
+``Makefile.config`` file similarly. Users may overload this target to ensure
+that sanity checks are run *before* any building of targets as all the targets
+depend on ``preconditions``.
+
+``printvars``
+-------------
+
+This utility target just causes the LLVM makefiles to print out some of the
+makefile variables so that you can double check how things are set.
+
+``reconfigure``
+---------------
+
+This utility target will force a reconfigure of LLVM or your project. It simply
+runs ``$(PROJ_OBJ_ROOT)/config.status --recheck`` to rerun the configuration
+tests and rebuild the configured files. This isn't generally useful as the
+makefiles will reconfigure themselves whenever its necessary.
+
+``spotless``
+------------
+
+.. warning::
+
+  Use with caution!
+
+This utility target, only available when ``$(PROJ_OBJ_ROOT)`` is not the same as
+``$(PROJ_SRC_ROOT)``, will completely clean the ``$(PROJ_OBJ_ROOT)`` directory
+by removing its content entirely and reconfiguring the directory. This returns
+the ``$(PROJ_OBJ_ROOT)`` directory to a completely fresh state. All content in
+the directory except configured files and top-level makefiles will be lost.
+
+``tags``
+--------
+
+This target will generate a ``TAGS`` file in the top-level source directory. It
+is meant for use with emacs, XEmacs, or ViM. The TAGS file provides an index of
+symbol definitions so that the editor can jump you to the definition
+quickly.
+
+``uninstall``
+-------------
+
+This target is the opposite of the ``install`` target. It removes the header,
+library and executable files from the installation directories. Note that the
+directories themselves are not removed because it is not guaranteed that LLVM is
+the only thing installing there (e.g. ``--prefix=/usr``).
+
+.. _variables:
+
+Variables
+=========
+
+Variables are used to tell the LLVM Makefile System what to do and to obtain
+information from it. Variables are also used internally by the LLVM Makefile
+System. Variable names that contain only the upper case alphabetic letters and
+underscore are intended for use by the end user. All other variables are
+internal to the LLVM Makefile System and should not be relied upon nor
+modified. The sections below describe how to use the LLVM Makefile
+variables.
+
+Control Variables
+-----------------
+
+Variables listed in the table below should be set *before* the inclusion of
+`$(LEVEL)/Makefile.common`_.  These variables provide input to the LLVM make
+system that tell it what to do for the current directory.
+
+``BUILD_ARCHIVE``
+    If set to any value, causes an archive (.a) library to be built.
+
+``BUILT_SOURCES``
+    Specifies a set of source files that are generated from other source
+    files. These sources will be built before any other target processing to
+    ensure they are present.
+
+``BYTECODE_LIBRARY``
+    If set to any value, causes a bitcode library (.bc) to be built.
+
+``CONFIG_FILES``
+    Specifies a set of configuration files to be installed.
+
+``DEBUG_SYMBOLS``
+    If set to any value, causes the build to include debugging symbols even in
+    optimized objects, libraries and executables. This alters the flags
+    specified to the compilers and linkers. Debugging isn't fun in an optimized
+    build, but it is possible.
+
+``DIRS``
+    Specifies a set of directories, usually children of the current directory,
+    that should also be made using the same goal. These directories will be
+    built serially.
+
+``DISABLE_AUTO_DEPENDENCIES``
+    If set to any value, causes the makefiles to **not** automatically generate
+    dependencies when running the compiler. Use of this feature is discouraged
+    and it may be removed at a later date.
+
+``ENABLE_OPTIMIZED``
+    If set to 1, causes the build to generate optimized objects, libraries and
+    executables. This alters the flags specified to the compilers and
+    linkers. Generally debugging won't be a fun experience with an optimized
+    build.
+
+``ENABLE_PROFILING``
+    If set to 1, causes the build to generate both optimized and profiled
+    objects, libraries and executables. This alters the flags specified to the
+    compilers and linkers to ensure that profile data can be collected from the
+    tools built. Use the ``gprof`` tool to analyze the output from the profiled
+    tools (``gmon.out``).
+
+``DISABLE_ASSERTIONS``
+    If set to 1, causes the build to disable assertions, even if building a
+    debug or profile build.  This will exclude all assertion check code from the
+    build. LLVM will execute faster, but with little help when things go
+    wrong.
+
+``EXPERIMENTAL_DIRS``
+    Specify a set of directories that should be built, but if they fail, it
+    should not cause the build to fail. Note that this should only be used
+    temporarily while code is being written.
+
+``EXPORTED_SYMBOL_FILE``
+    Specifies the name of a single file that contains a list of the symbols to
+    be exported by the linker. One symbol per line.
+
+``EXPORTED_SYMBOL_LIST``
+    Specifies a set of symbols to be exported by the linker.
+
+``EXTRA_DIST``
+    Specifies additional files that should be distributed with LLVM. All source
+    files, all built sources, all Makefiles, and most documentation files will
+    be automatically distributed. Use this variable to distribute any files that
+    are not automatically distributed.
+
+``KEEP_SYMBOLS``
+    If set to any value, specifies that when linking executables the makefiles
+    should retain debug symbols in the executable. Normally, symbols are
+    stripped from the executable.
+
+``LEVEL`` (required)
+    Specify the level of nesting from the top level. This variable must be set
+    in each makefile as it is used to find the top level and thus the other
+    makefiles.
+
+``LIBRARYNAME``
+    Specify the name of the library to be built. (Required For Libraries)
+
+``LINK_COMPONENTS``
+    When specified for building a tool, the value of this variable will be
+    passed to the ``llvm-config`` tool to generate a link line for the
+    tool. Unlike ``USEDLIBS`` and ``LLVMLIBS``, not all libraries need to be
+    specified. The ``llvm-config`` tool will figure out the library dependencies
+    and add any libraries that are needed. The ``USEDLIBS`` variable can still
+    be used in conjunction with ``LINK_COMPONENTS`` so that additional
+    project-specific libraries can be linked with the LLVM libraries specified
+    by ``LINK_COMPONENTS``.
+
+.. _LINK_LIBS_IN_SHARED:
+
+``LINK_LIBS_IN_SHARED``
+    By default, shared library linking will ignore any libraries specified with
+    the `LLVMLIBS`_ or `USEDLIBS`_. This prevents shared libs from including
+    things that will be in the LLVM tool the shared library will be loaded
+    into. However, sometimes it is useful to link certain libraries into your
+    shared library and this option enables that feature.
+
+.. _LLVMLIBS:
+
+``LLVMLIBS``
+    Specifies the set of libraries from the LLVM ``$(ObjDir)`` that will be
+    linked into the tool or library.
+
+``LOADABLE_MODULE``
+    If set to any value, causes the shared library being built to also be a
+    loadable module. Loadable modules can be opened with the dlopen() function
+    and searched with dlsym (or the operating system's equivalent). Note that
+    setting this variable without also setting ``SHARED_LIBRARY`` will have no
+    effect.
+
+.. _MODULE_NAME:
+
+``MODULE_NAME``
+    Specifies the name of a bitcode module to be created. A bitcode module can
+    be specified in conjunction with other kinds of library builds or by
+    itself. It constructs from the sources a single linked bitcode file.
+
+``NO_INSTALL``
+    Specifies that the build products of the directory should not be installed
+    but should be built even if the ``install`` target is given.  This is handy
+    for directories that build libraries or tools that are only used as part of
+    the build process, such as code generators (e.g.  ``tblgen``).
+
+``OPTIONAL_DIRS``
+    Specify a set of directories that may be built, if they exist, but its not
+    an error for them not to exist.
+
+``PARALLEL_DIRS``
+    Specify a set of directories to build recursively and in parallel if the
+    ``-j`` option was used with ``make``.
+
+.. _SHARED_LIBRARY:
+
+``SHARED_LIBRARY``
+    If set to any value, causes a shared library (``.so``) to be built in
+    addition to any other kinds of libraries. Note that this option will cause
+    all source files to be built twice: once with options for position
+    independent code and once without. Use it only where you really need a
+    shared library.
+
+``SOURCES`` (optional)
+    Specifies the list of source files in the current directory to be
+    built. Source files of any type may be specified (programs, documentation,
+    config files, etc.). If not specified, the makefile system will infer the
+    set of source files from the files present in the current directory.
+
+``SUFFIXES``
+    Specifies a set of filename suffixes that occur in suffix match rules.  Only
+    set this if your local ``Makefile`` specifies additional suffix match
+    rules.
+
+``TARGET``
+    Specifies the name of the LLVM code generation target that the current
+    directory builds. Setting this variable enables additional rules to build
+    ``.inc`` files from ``.td`` files. 
+
+.. _TESTSUITE:
+
+``TESTSUITE``
+    Specifies the directory of tests to run in ``llvm/test``.
+
+``TOOLNAME``
+    Specifies the name of the tool that the current directory should build.
+
+``TOOL_VERBOSE``
+    Implies ``VERBOSE`` and also tells each tool invoked to be verbose. This is
+    handy when you're trying to see the sub-tools invoked by each tool invoked
+    by the makefile. For example, this will pass ``-v`` to the GCC compilers
+    which causes it to print out the command lines it uses to invoke sub-tools
+    (compiler, assembler, linker).
+
+.. _USEDLIBS:
+
+``USEDLIBS``
+    Specifies the list of project libraries that will be linked into the tool or
+    library.
+
+``VERBOSE``
+    Tells the Makefile system to produce detailed output of what it is doing
+    instead of just summary comments. This will generate a LOT of output.
+
+Override Variables
+------------------
+
+Override variables can be used to override the default values provided by the
+LLVM makefile system. These variables can be set in several ways:
+
+* In the environment (e.g. setenv, export) --- not recommended.
+* On the ``make`` command line --- recommended.
+* On the ``configure`` command line.
+* In the Makefile (only *after* the inclusion of `$(LEVEL)/Makefile.common`_).
+
+The override variables are given below:
+
+``AR`` (defaulted)
+    Specifies the path to the ``ar`` tool.
+
+``PROJ_OBJ_DIR``
+    The directory into which the products of build rules will be placed.  This
+    might be the same as `PROJ_SRC_DIR`_ but typically is not.
+
+.. _PROJ_SRC_DIR:
+
+``PROJ_SRC_DIR``
+    The directory which contains the source files to be built.
+
+``BUILD_EXAMPLES``
+    If set to 1, build examples in ``examples`` and (if building Clang)
+    ``tools/clang/examples`` directories.
+
+``BZIP2`` (configured)
+    The path to the ``bzip2`` tool.
+
+``CC`` (configured)
+    The path to the 'C' compiler.
+
+``CFLAGS``
+    Additional flags to be passed to the 'C' compiler.
+
+``CXX``
+    Specifies the path to the C++ compiler.
+
+``CXXFLAGS``
+    Additional flags to be passed to the C++ compiler.
+
+``DATE`` (configured)
+    Specifies the path to the ``date`` program or any program that can generate
+    the current date and time on its standard output.
+
+``DOT`` (configured)
+    Specifies the path to the ``dot`` tool or ``false`` if there isn't one.
+
+``ECHO`` (configured)
+    Specifies the path to the ``echo`` tool for printing output.
+
+``EXEEXT`` (configured)
+    Provides the extension to be used on executables built by the makefiles.
+    The value may be empty on platforms that do not use file extensions for
+    executables (e.g. Unix).
+
+``INSTALL`` (configured)
+    Specifies the path to the ``install`` tool.
+
+``LDFLAGS`` (configured)
+    Allows users to specify additional flags to pass to the linker.
+
+``LIBS`` (configured)
+    The list of libraries that should be linked with each tool.
+
+``LIBTOOL`` (configured)
+    Specifies the path to the ``libtool`` tool. This tool is renamed ``mklib``
+    by the ``configure`` script.
+
+``LLVMAS`` (defaulted)
+    Specifies the path to the ``llvm-as`` tool.
+
+``LLVMCC``
+    Specifies the path to the LLVM capable compiler.
+
+``LLVMCXX``
+    Specifies the path to the LLVM C++ capable compiler.
+
+``LLVMGCC`` (defaulted)
+    Specifies the path to the LLVM version of the GCC 'C' Compiler.
+
+``LLVMGXX`` (defaulted)
+    Specifies the path to the LLVM version of the GCC C++ Compiler.
+
+``LLVMLD`` (defaulted)
+    Specifies the path to the LLVM bitcode linker tool
+
+``LLVM_OBJ_ROOT`` (configured)
+    Specifies the top directory into which the output of the build is placed.
+
+``LLVM_SRC_ROOT`` (configured)
+    Specifies the top directory in which the sources are found.
+
+``LLVM_TARBALL_NAME`` (configured)
+    Specifies the name of the distribution tarball to create. This is configured
+    from the name of the project and its version number.
+
+``MKDIR`` (defaulted)
+    Specifies the path to the ``mkdir`` tool that creates directories.
+
+``ONLY_TOOLS``
+    If set, specifies the list of tools to build.
+
+``PLATFORMSTRIPOPTS``
+    The options to provide to the linker to specify that a stripped (no symbols)
+    executable should be built.
+
+``RANLIB`` (defaulted)
+    Specifies the path to the ``ranlib`` tool.
+
+``RM`` (defaulted)
+    Specifies the path to the ``rm`` tool.
+
+``SED`` (defaulted)
+    Specifies the path to the ``sed`` tool.
+
+``SHLIBEXT`` (configured)
+    Provides the filename extension to use for shared libraries.
+
+``TBLGEN`` (defaulted)
+    Specifies the path to the ``tblgen`` tool.
+
+``TAR`` (defaulted)
+    Specifies the path to the ``tar`` tool.
+
+``ZIP`` (defaulted)
+    Specifies the path to the ``zip`` tool.
+
+Readable Variables
+------------------
+
+Variables listed in the table below can be used by the user's Makefile but
+should not be changed. Changing the value will generally cause the build to go
+wrong, so don't do it.
+
+``bindir``
+    The directory into which executables will ultimately be installed. This
+    value is derived from the ``--prefix`` option given to ``configure``.
+
+``BuildMode``
+    The name of the type of build being performed: Debug, Release, or
+    Profile.
+
+``bytecode_libdir``
+    The directory into which bitcode libraries will ultimately be installed.
+    This value is derived from the ``--prefix`` option given to ``configure``.
+
+``ConfigureScriptFLAGS``
+    Additional flags given to the ``configure`` script when reconfiguring.
+
+``DistDir``
+    The *current* directory for which a distribution copy is being made.
+
+.. _Echo:
+
+``Echo``
+    The LLVM Makefile System output command. This provides the ``llvm[n]``
+    prefix and starts with ``@`` so the command itself is not printed by
+    ``make``.
+
+``EchoCmd``
+    Same as `Echo`_ but without the leading ``@``.
+
+``includedir``
+    The directory into which include files will ultimately be installed.  This
+    value is derived from the ``--prefix`` option given to ``configure``.
+
+``libdir``
+    The directory into which native libraries will ultimately be installed.
+    This value is derived from the ``--prefix`` option given to
+    ``configure``.
+
+``LibDir``
+    The configuration specific directory into which libraries are placed before
+    installation.
+
+``MakefileConfig``
+    Full path of the ``Makefile.config`` file.
+
+``MakefileConfigIn``
+    Full path of the ``Makefile.config.in`` file.
+
+``ObjDir``
+    The configuration and directory specific directory where build objects
+    (compilation results) are placed.
+
+``SubDirs``
+    The complete list of sub-directories of the current directory as
+    specified by other variables.
+
+``Sources``
+    The complete list of source files.
+
+``sysconfdir``
+    The directory into which configuration files will ultimately be
+    installed. This value is derived from the ``--prefix`` option given to
+    ``configure``.
+
+``ToolDir``
+    The configuration specific directory into which executables are placed
+    before they are installed.
+
+``TopDistDir``
+    The top most directory into which the distribution files are copied.
+
+``Verb``
+    Use this as the first thing on your build script lines to enable or disable
+    verbose mode. It expands to either an ``@`` (quiet mode) or nothing (verbose
+    mode).
+
+Internal Variables
+------------------
+
+Variables listed below are used by the LLVM Makefile System and considered
+internal. You should not use these variables under any circumstances.
+
+.. code-block:: makefile
+
+    Archive
+    AR.Flags
+    BaseNameSources
+    BCCompile.C
+    BCCompile.CXX
+    BCLinkLib
+    C.Flags
+    Compile.C
+    CompileCommonOpts
+    Compile.CXX
+    ConfigStatusScript
+    ConfigureScript
+    CPP.Flags
+    CPP.Flags 
+    CXX.Flags
+    DependFiles
+    DestArchiveLib
+    DestBitcodeLib
+    DestModule
+    DestSharedLib
+    DestTool
+    DistAlways
+    DistCheckDir
+    DistCheckTop
+    DistFiles
+    DistName
+    DistOther
+    DistSources
+    DistSubDirs
+    DistTarBZ2
+    DistTarGZip
+    DistZip
+    ExtraLibs
+    FakeSources
+    INCFiles
+    InternalTargets
+    LD.Flags
+    LibName.A
+    LibName.BC
+    LibName.LA
+    LibName.O
+    LibTool.Flags
+    Link
+    LinkModule
+    LLVMLibDir
+    LLVMLibsOptions
+    LLVMLibsPaths
+    LLVMToolDir
+    LLVMUsedLibs
+    LocalTargets
+    Module
+    ObjectsBC
+    ObjectsLO
+    ObjectsO
+    ObjMakefiles
+    ParallelTargets
+    PreConditions
+    ProjLibsOptions
+    ProjLibsPaths
+    ProjUsedLibs
+    Ranlib
+    RecursiveTargets
+    SrcMakefiles
+    Strip
+    StripWarnMsg
+    TableGen
+    TDFiles
+    ToolBuildPath
+    TopLevelTargets
+    UserTargets