docs/HowToCrossCompileLLVM.rst - llvm - Git at Google

 ===================================================================
 How To Cross-Compile Clang/LLVM using Clang/LLVM
 ===================================================================

 Introduction
 ============

 This document contains information about building LLVM and
 Clang on host machine, targeting another platform.

 For more information on how to use Clang as a cross-compiler,
 please check http://clang.llvm.org/docs/CrossCompilation.html.

 TODO: Add MIPS and other platforms to this document.

 Cross-Compiling from x86_64 to ARM
 ==================================

 In this use case, we'll be using CMake and Ninja, on a Debian-based Linux
 system, cross-compiling from an x86_64 host (most Intel and AMD chips
 nowadays) to a hard-float ARM target (most ARM targets nowadays).

 The packages you'll need are:

  * ``cmake``
  * ``ninja-build`` (from backports in Ubuntu)
  * ``gcc-4.7-arm-linux-gnueabihf``
  * ``gcc-4.7-multilib-arm-linux-gnueabihf``
  * ``binutils-arm-linux-gnueabihf``
  * ``libgcc1-armhf-cross``
  * ``libsfgcc1-armhf-cross``
  * ``libstdc++6-armhf-cross``
  * ``libstdc++6-4.7-dev-armhf-cross``

 Configuring CMake
 -----------------

 For more information on how to configure CMake for LLVM/Clang,
 see :doc:`CMake`.

 The CMake options you need to add are:
  * ``-DCMAKE_CROSSCOMPILING=True``
  * ``-DCMAKE_INSTALL_PREFIX=<install-dir>``
  * ``-DLLVM_TABLEGEN=<path-to-host-bin>/llvm-tblgen``
  * ``-DCLANG_TABLEGEN=<path-to-host-bin>/clang-tblgen``
  * ``-DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf``
  * ``-DLLVM_TARGET_ARCH=ARM``
  * ``-DLLVM_TARGETS_TO_BUILD=ARM``
  * ``-DCMAKE_CXX_FLAGS='-target armv7a-linux-gnueabihf -mcpu=cortex-a9 -I/usr/arm-linux-gnueabihf/include/c++/4.7.2/arm-linux-gnueabihf/ -I/usr/arm-linux-gnueabihf/include/ -mfloat-abi=hard -ccc-gcc-name arm-linux-gnueabihf-gcc'``

 The TableGen options are required to compile it with the host compiler,
 so you'll need to compile LLVM (or at least ``llvm-tblgen``) to your host
 platform before you start. The CXX flags define the target, cpu (which
 defaults to ``fpu=VFP3`` with NEON), and forcing the hard-float ABI. If you're
 using Clang as a cross-compiler, you will *also* have to set ``-ccc-gcc-name``,
 to make sure it picks the correct linker.

 Most of the time, what you want is to have a native compiler to the
 platform itself, but not others. It might not even be feasible to
 produce x86 binaries from ARM targets, so there's no point in compiling
 all back-ends. For that reason, you should also set the
 ``TARGETS_TO_BUILD`` to only build the ARM back-end.

 You must set the ``CMAKE_INSTALL_PREFIX``, otherwise a ``ninja install``
 will copy ARM binaries to your root filesystem, which is not what you
 want.

 Hacks
 -----

 There are some bugs in current LLVM, which require some fiddling before
 running CMake:

 #. If you're using Clang as the cross-compiler, there is a problem in
    the LLVM ARM back-end that is producing absolute relocations on
    position-independent code (``R_ARM_THM_MOVW_ABS_NC``), so for now, you
    should disable PIC:

    .. code-block:: bash

       -DLLVM_ENABLE_PIC=False

    This is not a problem, since Clang/LLVM libraries are statically
    linked anyway, it shouldn't affect much.

 #. The ARM libraries won't be installed in your system, and possibly
    not easily installable anyway, so you'll have to build/download
    them separately. But the CMake prepare step, which checks for
    dependencies, will check the *host* libraries, not the *target*
    ones.

    A quick way of getting the libraries is to download them from
    a distribution repository, like Debian (http://packages.debian.org/wheezy/),
    and download the missing libraries. Note that the ``libXXX``
    will have the shared objects (``.so``) and the ``libXXX-dev`` will
    give you the headers and the static (``.a``) library. Just in
    case, download both.

    The ones you need for ARM are: ``libtinfo``, ``zlib1g``,
    ``libxml2`` and ``liblzma``. In the Debian repository you'll
    find downloads for all architectures.

    After you download and unpack all ``.deb`` packages, copy all
    ``.so`` and ``.a`` to a directory, make the appropriate
    symbolic links (if necessary), and add the relevant ``-L``
    and ``-I`` paths to ``-DCMAKE_CXX_FLAGS`` above.


 Running CMake and Building
 --------------------------

 Finally, if you're using your platform compiler, run:

    .. code-block:: bash

      $ cmake -G Ninja <source-dir> <options above>

 If you're using Clang as the cross-compiler, run:

    .. code-block:: bash

      $ CC='clang' CXX='clang++' cmake -G Ninja <source-dir> <options above>

 If you have ``clang``/``clang++`` on the path, it should just work, and special
 Ninja files will be created in the build directory. I strongly suggest
 you to run ``cmake`` on a separate build directory, *not* inside the
 source tree.

 To build, simply type:

    .. code-block:: bash

      $ ninja

 It should automatically find out how many cores you have, what are
 the rules that needs building and will build the whole thing.

 You can't run ``ninja check-all`` on this tree because the created
 binaries are targeted to ARM, not x86_64.

 Installing and Using
 --------------------

 After the LLVM/Clang has built successfully, you should install it
 via:

    .. code-block:: bash

      $ ninja install

 which will create a sysroot on the install-dir. You can then tar
 that directory into a binary with the full triple name (for easy
 identification), like:

    .. code-block:: bash

      $ ln -sf <install-dir> arm-linux-gnueabihf-clang
      $ tar zchf arm-linux-gnueabihf-clang.tar.gz arm-linux-gnueabihf-clang

 If you copy that tarball to your target board, you'll be able to use
 it for running the test-suite, for example. Follow the guidelines at
 http://llvm.org/docs/lnt/quickstart.html, unpack the tarball in the
 test directory, and use options:

    .. code-block:: bash

      $ ./sandbox/bin/python sandbox/bin/lnt runtest nt \
          --sandbox sandbox \
          --test-suite `pwd`/test-suite \
          --cc `pwd`/arm-linux-gnueabihf-clang/bin/clang \
          --cxx `pwd`/arm-linux-gnueabihf-clang/bin/clang++

 Remember to add the ``-jN`` options to ``lnt`` to the number of CPUs
 on your board. Also, the path to your clang has to be absolute, so
 you'll need the `pwd` trick above.
	===================================================================
	How To Cross-Compile Clang/LLVM using Clang/LLVM
	===================================================================

	Introduction
	============

	This document contains information about building LLVM and
	Clang on host machine, targeting another platform.

	For more information on how to use Clang as a cross-compiler,
	please check http://clang.llvm.org/docs/CrossCompilation.html.

	TODO: Add MIPS and other platforms to this document.

	Cross-Compiling from x86_64 to ARM
	==================================

	In this use case, we'll be using CMake and Ninja, on a Debian-based Linux
	system, cross-compiling from an x86_64 host (most Intel and AMD chips
	nowadays) to a hard-float ARM target (most ARM targets nowadays).

	The packages you'll need are:

	* ``cmake``
	* ``ninja-build`` (from backports in Ubuntu)
	* ``gcc-4.7-arm-linux-gnueabihf``
	* ``gcc-4.7-multilib-arm-linux-gnueabihf``
	* ``binutils-arm-linux-gnueabihf``
	* ``libgcc1-armhf-cross``
	* ``libsfgcc1-armhf-cross``
	* ``libstdc++6-armhf-cross``
	* ``libstdc++6-4.7-dev-armhf-cross``

	Configuring CMake
	-----------------

	For more information on how to configure CMake for LLVM/Clang,
	see :doc:`CMake`.

	The CMake options you need to add are:
	* ``-DCMAKE_CROSSCOMPILING=True``
	* ``-DCMAKE_INSTALL_PREFIX=<install-dir>``
	* ``-DLLVM_TABLEGEN=<path-to-host-bin>/llvm-tblgen``
	* ``-DCLANG_TABLEGEN=<path-to-host-bin>/clang-tblgen``
	* ``-DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf``
	* ``-DLLVM_TARGET_ARCH=ARM``
	* ``-DLLVM_TARGETS_TO_BUILD=ARM``
	* ``-DCMAKE_CXX_FLAGS='-target armv7a-linux-gnueabihf -mcpu=cortex-a9 -I/usr/arm-linux-gnueabihf/include/c++/4.7.2/arm-linux-gnueabihf/ -I/usr/arm-linux-gnueabihf/include/ -mfloat-abi=hard -ccc-gcc-name arm-linux-gnueabihf-gcc'``

	The TableGen options are required to compile it with the host compiler,
	so you'll need to compile LLVM (or at least ``llvm-tblgen``) to your host
	platform before you start. The CXX flags define the target, cpu (which
	defaults to ``fpu=VFP3`` with NEON), and forcing the hard-float ABI. If you're
	using Clang as a cross-compiler, you will also have to set ``-ccc-gcc-name``,
	to make sure it picks the correct linker.

	Most of the time, what you want is to have a native compiler to the
	platform itself, but not others. It might not even be feasible to
	produce x86 binaries from ARM targets, so there's no point in compiling
	all back-ends. For that reason, you should also set the
	``TARGETS_TO_BUILD`` to only build the ARM back-end.

	You must set the ``CMAKE_INSTALL_PREFIX``, otherwise a ``ninja install``
	will copy ARM binaries to your root filesystem, which is not what you
	want.

	Hacks
	-----

	There are some bugs in current LLVM, which require some fiddling before
	running CMake:

	#. If you're using Clang as the cross-compiler, there is a problem in
	the LLVM ARM back-end that is producing absolute relocations on
	position-independent code (``R_ARM_THM_MOVW_ABS_NC``), so for now, you
	should disable PIC:

	.. code-block:: bash

	-DLLVM_ENABLE_PIC=False

	This is not a problem, since Clang/LLVM libraries are statically
	linked anyway, it shouldn't affect much.

	#. The ARM libraries won't be installed in your system, and possibly
	not easily installable anyway, so you'll have to build/download
	them separately. But the CMake prepare step, which checks for
	dependencies, will check the host libraries, not the target
	ones.

	A quick way of getting the libraries is to download them from
	a distribution repository, like Debian (http://packages.debian.org/wheezy/),
	and download the missing libraries. Note that the ``libXXX``
	will have the shared objects (``.so``) and the ``libXXX-dev`` will
	give you the headers and the static (``.a``) library. Just in
	case, download both.

	The ones you need for ARM are: ``libtinfo``, ``zlib1g``,
	``libxml2`` and ``liblzma``. In the Debian repository you'll
	find downloads for all architectures.

	After you download and unpack all ``.deb`` packages, copy all
	``.so`` and ``.a`` to a directory, make the appropriate
	symbolic links (if necessary), and add the relevant ``-L``
	and ``-I`` paths to ``-DCMAKE_CXX_FLAGS`` above.


	Running CMake and Building
	--------------------------

	Finally, if you're using your platform compiler, run:

	.. code-block:: bash

	$ cmake -G Ninja <source-dir> <options above>

	If you're using Clang as the cross-compiler, run:

	.. code-block:: bash

	$ CC='clang' CXX='clang++' cmake -G Ninja <source-dir> <options above>

	If you have ``clang``/``clang++`` on the path, it should just work, and special
	Ninja files will be created in the build directory. I strongly suggest
	you to run ``cmake`` on a separate build directory, not inside the
	source tree.

	To build, simply type:

	.. code-block:: bash

	$ ninja

	It should automatically find out how many cores you have, what are
	the rules that needs building and will build the whole thing.

	You can't run ``ninja check-all`` on this tree because the created
	binaries are targeted to ARM, not x86_64.

	Installing and Using
	--------------------

	After the LLVM/Clang has built successfully, you should install it
	via:

	.. code-block:: bash

	$ ninja install

	which will create a sysroot on the install-dir. You can then tar
	that directory into a binary with the full triple name (for easy
	identification), like:

	.. code-block:: bash

	$ ln -sf <install-dir> arm-linux-gnueabihf-clang
	$ tar zchf arm-linux-gnueabihf-clang.tar.gz arm-linux-gnueabihf-clang

	If you copy that tarball to your target board, you'll be able to use
	it for running the test-suite, for example. Follow the guidelines at
	http://llvm.org/docs/lnt/quickstart.html, unpack the tarball in the
	test directory, and use options:

	.. code-block:: bash

	$ ./sandbox/bin/python sandbox/bin/lnt runtest nt \
	--sandbox sandbox \
	--test-suite `pwd`/test-suite \
	--cc `pwd`/arm-linux-gnueabihf-clang/bin/clang \
	--cxx `pwd`/arm-linux-gnueabihf-clang/bin/clang++

	Remember to add the ``-jN`` options to ``lnt`` to the number of CPUs
	on your board. Also, the path to your clang has to be absolute, so
	you'll need the `pwd` trick above.