llvm/docs/CMake.rst

========================
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 really anxious about getting a functional LLVM build, go to the
`Quick start`_ section. If you are a CMake novice, start with `Basic CMake usage`_
and then go back to the `Quick start`_ section 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:

Quick start
===========

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

#. `Download <http://www.cmake.org/cmake/resources/software.html>`_ and install
   CMake. Version 2.8.8 is the minimum required.

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

#. Create a build directory. Building LLVM in the source
   directory is not supported. cd to this directory:

   .. code-block:: console

     $ mkdir mybuilddir
     $ cd mybuilddir

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

   .. code-block:: 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`_ 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`_ section, below.

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

   .. code-block:: 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.

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

   .. code-block:: 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:

   .. code-block:: 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
which you may need in your day-to-day usage.

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

.. code-block:: 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:

.. code-block:: console

  $ cmake -G "Visual Studio 12" 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 most 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:

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:

.. code-block:: 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:

.. code-block:: 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:

.. code-block:: console

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

Frequently-used CMake variables
-------------------------------

Here are some of the CMake variables that are used often, along with a
brief explanation and LLVM-specific notes. For full documentation, consult the
CMake manual, 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. If you are using an IDE such as
  Visual Studio, you should use the IDE settings to set the build type.

**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 if shared libraries will be built. Its default value is
  OFF. This option is only recommended for use by LLVM developers.
  On Windows, shared libraries may be used when building with MinGW, including
  mingw-w64, but not when building with the Microsoft toolchain.

.. _LLVM-specific variables:

LLVM-specific variables
-----------------------

**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"``.

**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_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_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 this
  option to disable 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 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_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_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_CXX1Y**:BOOL
  Build in C++1y mode, if available. Defaults to OFF.

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

**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_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_RTTI**:BOOL
  Build LLVM with run-time type information. Defaults to OFF.

**LLVM_ENABLE_WARNINGS**:BOOL
  Enable all compiler warnings. Defaults to ON.

**LLVM_ENABLE_PEDANTIC**:BOOL
  Enable pedantic mode. This disables compiler-specific extensions, if
  possible. Defaults to ON.

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

**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_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_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 ``llvm-tblgen``). This is
  intended 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
  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_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_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_USE_OPROFILE**:BOOL
  Enable building OProfile JIT support. Defaults to OFF.

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

**LLVM_ENABLE_ZLIB**:BOOL
  Enable building with zlib to support compression/uncompression in LLVM tools.
  Defaults to ON.

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

**LLVM_PARALLEL_COMPILE_JOBS**:STRING
  Define the maximum number of concurrent compilation jobs.

**LLVM_PARALLEL_LINK_JOBS**:STRING
  Define the maximum number of concurrent link jobs.

**LLVM_BUILD_DOCS**:BOOL
  Enables all enabled documentation targets (i.e. Doxgyen and Sphinx targets) to
  be built as part of the normal build. If the ``install`` target is run then
  this also enables all built documentation targets to be installed. Defaults to
  OFF.

**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_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_QHP_NAMESPACE**:STRING
  Namespace under which the intermediate Qt Help Project file lives. See `Qt
  Help Project`_
  for more information. Defaults to "org.llvm". This option is only useful in
  combination with ``-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON``; otherwise
  it has no effect.

**LLVM_DOXYGEN_QHP_CUST_FILTER_NAME**:STRING
  See `Qt Help Project`_ for
  more information. Defaults to the CMake variable ``${PACKAGE_STRING}`` which
  is a combination of the package name and version string. This filter can then
  be used in Qt Creator to select only documentation from LLVM when browsing
  through all the help files that you might have loaded. This option is only
  useful in combination with ``-DLLVM_ENABLE_DOXYGEN_QT_HELP=ON``;
  otherwise it has no effect.

.. _Qt Help Project: http://qt-project.org/doc/qt-4.8/qthelpproject.html#custom-filters

**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_SVG**:BOOL
  Uses .svg files instead of .png files for graphs in the Doxygen output.
  Defaults to OFF.

**LLVM_ENABLE_SPHINX**:BOOL
  If enabled CMake will search for the ``sphinx-build`` executable and will make
  the ``SPHINX_OUTPUT_HTML`` and ``SPHINX_OUTPUT_MAN`` CMake options available.
  Defaults to OFF.

**SPHINX_EXECUTABLE**:STRING
  The path to the ``sphinx-build`` executable detected by CMake.

**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.

Executing the test suite
========================

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:

.. code-block:: 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 <http://www.vtk.org/Wiki/CMake_Cross_Compiling>`_ 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
<http://www.vtk.org/Wiki/CMake_Cross_Compiling#Information_how_to_set_up_various_cross_compiling_toolchains>`_
for a quick solution.

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

Embedding LLVM in your project
==============================

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

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

.. code-block:: cmake

  cmake_minimum_required(VERSION 2.8.8)
  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(${LLVM_INCLUDE_DIRS})
  add_definitions(${LLVM_DEFINITIONS})

  # 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 a ``LLVM_DIR`` cache entry to save the
directory where ``LLVMConfig.cmake`` is found or allows the user to specify the
directory (e.g. by passing ``-DLLVM_DIR=/usr/share/llvm/cmake`` to
the ``cmake`` command or by setting it directly in ``ccmake`` or ``cmake-gui``).

This file is available in two different locations.

* ``<INSTALL_PREFIX>/share/llvm/cmake/LLVMConfig.cmake`` where
  ``<INSTALL_PREFIX>`` is the install prefix of an installed version of LLVM.
  On Linux typically this is ``/usr/share/llvm/cmake/LLVMConfig.cmake``.

* ``<LLVM_BUILD_ROOT>/share/llvm/cmake/LLVMConfig.cmake`` where
  ``<LLVM_BUILD_ROOT>`` is the root of the LLVM build tree. **Note: this 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
------------------------------------

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

.. code-block:: none

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

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

.. code-block:: cmake

  find_package(LLVM REQUIRED CONFIG)

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

  add_subdirectory(<pass name>)

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

.. code-block:: 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_loadable_module`` function instead by...


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

.. code-block:: cmake

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

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

.. code-block:: cmake

  add_llvm_loadable_module(LLVMPassname
    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:

#. Copying ``<pass name>`` folder into ``<LLVM root>/lib/Transform`` directory.

#. 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 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.