llvm/docs/HowToCrossCompileLLVM.rst - rust-lang/llvm-project - 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 https://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_SYSTEM_NAME=<target-system>``
  * ``-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``

 Note: ``CMAKE_CROSSCOMPILING`` is always set automatically when ``CMAKE_SYSTEM_NAME`` is set. Don't put ``-DCMAKE_CROSSCOMPILING=TRUE`` in your options.

 If you're compiling with GCC, you can use architecture options for your target,
 and the compiler driver will detect everything that it needs:

  * ``-DCMAKE_CXX_FLAGS='-march=armv7-a -mcpu=cortex-a9 -mfloat-abi=hard'``

 However, if you're using Clang, the driver might not be up-to-date with your
 specific Linux distribution, version or GCC layout, so you'll need to fudge.

 In addition to the ones above, you'll also need:

  * ``--target=arm-linux-gnueabihf`` or whatever is the triple of your cross GCC.
  * ``'--sysroot=/usr/arm-linux-gnueabihf'``, ``'--sysroot=/opt/gcc/arm-linux-gnueabihf'``
    or whatever is the location of your GCC's sysroot (where /lib, /bin etc are).
  * Appropriate use of ``-I`` and ``-L``, depending on how the cross GCC is installed,
    and where are the libraries and headers.

 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 in this case
 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 ``--sysroot``
 to make sure it picks the correct linker.

 When using Clang, it's important that you choose the triple to be *identical*
 to the GCC triple and the sysroot. This will make it easier for Clang to
 find the correct tools and include headers. But that won't mean all headers and
 libraries will be found. You'll still need to use ``-I`` and ``-L`` to locate
 those extra ones, depending on your distribution.

 Most of the time, what you want is to have a native compiler to the
 platform itself, but not others. So there's rarely a point in compiling
 all back-ends. For that reason, you should also set the
 ``TARGETS_TO_BUILD`` to only build the back-end you're targeting to.

 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.
    But the CMake prepare step, which checks for
    dependencies, will check the *host* libraries, not the *target*
    ones. Below there's a list of some dependencies, but your project could
    have more, or this document could be outdated. You'll see the errors
    while linking as an indication of that.

    Debian based distros have a way to add ``multiarch``, which adds
    a new architecture and allows you to install packages for those
    systems. See https://wiki.debian.org/Multiarch/HOWTO for more info.

    But not all distros will have that, and possibly not an easy way to
    install them in any anyway, so you'll have to build/download
    them separately.

    A quick way of getting the libraries is to download them from
    a distribution repository, like Debian (http://packages.debian.org/jessie/),
    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> -DCMAKE_BUILD_TYPE=<type> <options above>

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

    .. code-block:: bash

      $ CC='clang' CXX='clang++' cmake -G Ninja <source-dir> -DCMAKE_BUILD_TYPE=<type> <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
 https://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 https://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_SYSTEM_NAME=<target-system>``
	* ``-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``

	Note: ``CMAKE_CROSSCOMPILING`` is always set automatically when ``CMAKE_SYSTEM_NAME`` is set. Don't put ``-DCMAKE_CROSSCOMPILING=TRUE`` in your options.

	If you're compiling with GCC, you can use architecture options for your target,
	and the compiler driver will detect everything that it needs:

	* ``-DCMAKE_CXX_FLAGS='-march=armv7-a -mcpu=cortex-a9 -mfloat-abi=hard'``

	However, if you're using Clang, the driver might not be up-to-date with your
	specific Linux distribution, version or GCC layout, so you'll need to fudge.

	In addition to the ones above, you'll also need:

	* ``--target=arm-linux-gnueabihf`` or whatever is the triple of your cross GCC.
	* ``'--sysroot=/usr/arm-linux-gnueabihf'``, ``'--sysroot=/opt/gcc/arm-linux-gnueabihf'``
	or whatever is the location of your GCC's sysroot (where /lib, /bin etc are).
	* Appropriate use of ``-I`` and ``-L``, depending on how the cross GCC is installed,
	and where are the libraries and headers.

	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 in this case
	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 ``--sysroot``
	to make sure it picks the correct linker.

	When using Clang, it's important that you choose the triple to be identical
	to the GCC triple and the sysroot. This will make it easier for Clang to
	find the correct tools and include headers. But that won't mean all headers and
	libraries will be found. You'll still need to use ``-I`` and ``-L`` to locate
	those extra ones, depending on your distribution.

	Most of the time, what you want is to have a native compiler to the
	platform itself, but not others. So there's rarely a point in compiling
	all back-ends. For that reason, you should also set the
	``TARGETS_TO_BUILD`` to only build the back-end you're targeting to.

	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.
	But the CMake prepare step, which checks for
	dependencies, will check the host libraries, not the target
	ones. Below there's a list of some dependencies, but your project could
	have more, or this document could be outdated. You'll see the errors
	while linking as an indication of that.

	Debian based distros have a way to add ``multiarch``, which adds
	a new architecture and allows you to install packages for those
	systems. See https://wiki.debian.org/Multiarch/HOWTO for more info.

	But not all distros will have that, and possibly not an easy way to
	install them in any anyway, so you'll have to build/download
	them separately.

	A quick way of getting the libraries is to download them from
	a distribution repository, like Debian (http://packages.debian.org/jessie/),
	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> -DCMAKE_BUILD_TYPE=<type> <options above>

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

	.. code-block:: bash

	$ CC='clang' CXX='clang++' cmake -G Ninja <source-dir> -DCMAKE_BUILD_TYPE=<type> <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
	https://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.