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7.5 KiB
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206 lines
7.5 KiB
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===================================================================
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How To Cross-Compile Clang/LLVM using Clang/LLVM
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===================================================================
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Introduction
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============
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This document contains information about building LLVM and
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Clang on host machine, targeting another platform.
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For more information on how to use Clang as a cross-compiler,
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please check https://clang.llvm.org/docs/CrossCompilation.html.
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TODO: Add MIPS and other platforms to this document.
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Cross-Compiling from x86_64 to ARM
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==================================
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In this use case, we'll be using CMake and Ninja, on a Debian-based Linux
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system, cross-compiling from an x86_64 host (most Intel and AMD chips
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nowadays) to a hard-float ARM target (most ARM targets nowadays).
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The packages you'll need are:
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* ``cmake``
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* ``ninja-build`` (from backports in Ubuntu)
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* ``gcc-4.7-arm-linux-gnueabihf``
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* ``gcc-4.7-multilib-arm-linux-gnueabihf``
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* ``binutils-arm-linux-gnueabihf``
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* ``libgcc1-armhf-cross``
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* ``libsfgcc1-armhf-cross``
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* ``libstdc++6-armhf-cross``
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* ``libstdc++6-4.7-dev-armhf-cross``
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Configuring CMake
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-----------------
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For more information on how to configure CMake for LLVM/Clang,
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see :doc:`CMake`.
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The CMake options you need to add are:
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* ``-DCMAKE_CROSSCOMPILING=True``
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* ``-DCMAKE_INSTALL_PREFIX=<install-dir>``
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* ``-DLLVM_TABLEGEN=<path-to-host-bin>/llvm-tblgen``
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* ``-DCLANG_TABLEGEN=<path-to-host-bin>/clang-tblgen``
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* ``-DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf``
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* ``-DLLVM_TARGET_ARCH=ARM``
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* ``-DLLVM_TARGETS_TO_BUILD=ARM``
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If you're compiling with GCC, you can use architecture options for your target,
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and the compiler driver will detect everything that it needs:
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* ``-DCMAKE_CXX_FLAGS='-march=armv7-a -mcpu=cortex-a9 -mfloat-abi=hard'``
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However, if you're using Clang, the driver might not be up-to-date with your
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specific Linux distribution, version or GCC layout, so you'll need to fudge.
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In addition to the ones above, you'll also need:
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* ``'-target arm-linux-gnueabihf'`` or whatever is the triple of your cross GCC.
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* ``'--sysroot=/usr/arm-linux-gnueabihf'``, ``'--sysroot=/opt/gcc/arm-linux-gnueabihf'``
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or whatever is the location of your GCC's sysroot (where /lib, /bin etc are).
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* Appropriate use of ``-I`` and ``-L``, depending on how the cross GCC is installed,
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and where are the libraries and headers.
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The TableGen options are required to compile it with the host compiler,
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so you'll need to compile LLVM (or at least ``llvm-tblgen``) to your host
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platform before you start. The CXX flags define the target, cpu (which in this case
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defaults to ``fpu=VFP3`` with NEON), and forcing the hard-float ABI. If you're
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using Clang as a cross-compiler, you will *also* have to set ``--sysroot``
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to make sure it picks the correct linker.
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When using Clang, it's important that you choose the triple to be *identical*
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to the GCC triple and the sysroot. This will make it easier for Clang to
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find the correct tools and include headers. But that won't mean all headers and
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libraries will be found. You'll still need to use ``-I`` and ``-L`` to locate
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those extra ones, depending on your distribution.
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Most of the time, what you want is to have a native compiler to the
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platform itself, but not others. So there's rarely a point in compiling
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all back-ends. For that reason, you should also set the
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``TARGETS_TO_BUILD`` to only build the back-end you're targeting to.
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You must set the ``CMAKE_INSTALL_PREFIX``, otherwise a ``ninja install``
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will copy ARM binaries to your root filesystem, which is not what you
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want.
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Hacks
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-----
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There are some bugs in current LLVM, which require some fiddling before
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running CMake:
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#. If you're using Clang as the cross-compiler, there is a problem in
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the LLVM ARM back-end that is producing absolute relocations on
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position-independent code (``R_ARM_THM_MOVW_ABS_NC``), so for now, you
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should disable PIC:
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.. code-block:: bash
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-DLLVM_ENABLE_PIC=False
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This is not a problem, since Clang/LLVM libraries are statically
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linked anyway, it shouldn't affect much.
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#. The ARM libraries won't be installed in your system.
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But the CMake prepare step, which checks for
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dependencies, will check the *host* libraries, not the *target*
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ones. Below there's a list of some dependencies, but your project could
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have more, or this document could be outdated. You'll see the errors
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while linking as an indication of that.
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Debian based distros have a way to add ``multiarch``, which adds
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a new architecture and allows you to install packages for those
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systems. See https://wiki.debian.org/Multiarch/HOWTO for more info.
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But not all distros will have that, and possibly not an easy way to
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install them in any anyway, so you'll have to build/download
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them separately.
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A quick way of getting the libraries is to download them from
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a distribution repository, like Debian (http://packages.debian.org/jessie/),
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and download the missing libraries. Note that the ``libXXX``
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will have the shared objects (``.so``) and the ``libXXX-dev`` will
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give you the headers and the static (``.a``) library. Just in
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case, download both.
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The ones you need for ARM are: ``libtinfo``, ``zlib1g``,
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``libxml2`` and ``liblzma``. In the Debian repository you'll
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find downloads for all architectures.
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After you download and unpack all ``.deb`` packages, copy all
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``.so`` and ``.a`` to a directory, make the appropriate
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symbolic links (if necessary), and add the relevant ``-L``
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and ``-I`` paths to ``-DCMAKE_CXX_FLAGS`` above.
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Running CMake and Building
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--------------------------
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Finally, if you're using your platform compiler, run:
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.. code-block:: bash
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$ cmake -G Ninja <source-dir> <options above>
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If you're using Clang as the cross-compiler, run:
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.. code-block:: bash
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$ CC='clang' CXX='clang++' cmake -G Ninja <source-dir> <options above>
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If you have ``clang``/``clang++`` on the path, it should just work, and special
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Ninja files will be created in the build directory. I strongly suggest
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you to run ``cmake`` on a separate build directory, *not* inside the
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source tree.
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To build, simply type:
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.. code-block:: bash
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$ ninja
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It should automatically find out how many cores you have, what are
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the rules that needs building and will build the whole thing.
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You can't run ``ninja check-all`` on this tree because the created
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binaries are targeted to ARM, not x86_64.
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Installing and Using
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--------------------
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After the LLVM/Clang has built successfully, you should install it
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via:
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.. code-block:: bash
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$ ninja install
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which will create a sysroot on the install-dir. You can then tar
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that directory into a binary with the full triple name (for easy
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identification), like:
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.. code-block:: bash
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$ ln -sf <install-dir> arm-linux-gnueabihf-clang
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$ tar zchf arm-linux-gnueabihf-clang.tar.gz arm-linux-gnueabihf-clang
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If you copy that tarball to your target board, you'll be able to use
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it for running the test-suite, for example. Follow the guidelines at
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https://llvm.org/docs/lnt/quickstart.html, unpack the tarball in the
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test directory, and use options:
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.. code-block:: bash
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$ ./sandbox/bin/python sandbox/bin/lnt runtest nt \
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--sandbox sandbox \
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--test-suite `pwd`/test-suite \
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--cc `pwd`/arm-linux-gnueabihf-clang/bin/clang \
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--cxx `pwd`/arm-linux-gnueabihf-clang/bin/clang++
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Remember to add the ``-jN`` options to ``lnt`` to the number of CPUs
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on your board. Also, the path to your clang has to be absolute, so
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you'll need the `pwd` trick above.
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