1 =================================================================== 2 How To Cross-Compile Clang/LLVM using Clang/LLVM 3 =================================================================== 4 5 Introduction 6 ============ 7 8 This document contains information about building LLVM and 9 Clang on host machine, targeting another platform. 10 11 For more information on how to use Clang as a cross-compiler, 12 please check http://clang.llvm.org/docs/CrossCompilation.html. 13 14 TODO: Add MIPS and other platforms to this document. 15 16 Cross-Compiling from x86_64 to ARM 17 ================================== 18 19 In this use case, we'll be using CMake and Ninja, on a Debian-based Linux 20 system, cross-compiling from an x86_64 host (most Intel and AMD chips 21 nowadays) to a hard-float ARM target (most ARM targets nowadays). 22 23 The packages you'll need are: 24 25 * ``cmake`` 26 * ``ninja-build`` (from backports in Ubuntu) 27 * ``gcc-4.7-arm-linux-gnueabihf`` 28 * ``gcc-4.7-multilib-arm-linux-gnueabihf`` 29 * ``binutils-arm-linux-gnueabihf`` 30 * ``libgcc1-armhf-cross`` 31 * ``libsfgcc1-armhf-cross`` 32 * ``libstdc++6-armhf-cross`` 33 * ``libstdc++6-4.7-dev-armhf-cross`` 34 35 Configuring CMake 36 ----------------- 37 38 For more information on how to configure CMake for LLVM/Clang, 39 see :doc:`CMake`. 40 41 The CMake options you need to add are: 42 43 * ``-DCMAKE_CROSSCOMPILING=True`` 44 * ``-DCMAKE_INSTALL_PREFIX=<install-dir>`` 45 * ``-DLLVM_TABLEGEN=<path-to-host-bin>/llvm-tblgen`` 46 * ``-DCLANG_TABLEGEN=<path-to-host-bin>/clang-tblgen`` 47 * ``-DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf`` 48 * ``-DLLVM_TARGET_ARCH=ARM`` 49 * ``-DLLVM_TARGETS_TO_BUILD=ARM`` 50 51 If you're compiling with GCC, you can use architecture options for your target, 52 and the compiler driver will detect everything that it needs: 53 54 * ``-DCMAKE_CXX_FLAGS='-march=armv7-a -mcpu=cortex-a9 -mfloat-abi=hard'`` 55 56 However, if you're using Clang, the driver might not be up-to-date with your 57 specific Linux distribution, version or GCC layout, so you'll need to fudge. 58 59 In addition to the ones above, you'll also need: 60 61 * ``'-target arm-linux-gnueabihf'`` or whatever is the triple of your cross GCC. 62 * ``'--sysroot=/usr/arm-linux-gnueabihf'``, ``'--sysroot=/opt/gcc/arm-linux-gnueabihf'`` 63 or whatever is the location of your GCC's sysroot (where /lib, /bin etc are). 64 * Appropriate use of ``-I`` and ``-L``, depending on how the cross GCC is installed, 65 and where are the libraries and headers. 66 67 The TableGen options are required to compile it with the host compiler, 68 so you'll need to compile LLVM (or at least ``llvm-tblgen``) to your host 69 platform before you start. The CXX flags define the target, cpu (which in this case 70 defaults to ``fpu=VFP3`` with NEON), and forcing the hard-float ABI. If you're 71 using Clang as a cross-compiler, you will *also* have to set ``--sysroot`` 72 to make sure it picks the correct linker. 73 74 When using Clang, it's important that you choose the triple to be *identical* 75 to the GCC triple and the sysroot. This will make it easier for Clang to 76 find the correct tools and include headers. But that won't mean all headers and 77 libraries will be found. You'll still need to use ``-I`` and ``-L`` to locate 78 those extra ones, depending on your distribution. 79 80 Most of the time, what you want is to have a native compiler to the 81 platform itself, but not others. So there's rarely a point in compiling 82 all back-ends. For that reason, you should also set the 83 ``TARGETS_TO_BUILD`` to only build the back-end you're targeting to. 84 85 You must set the ``CMAKE_INSTALL_PREFIX``, otherwise a ``ninja install`` 86 will copy ARM binaries to your root filesystem, which is not what you 87 want. 88 89 Hacks 90 ----- 91 92 There are some bugs in current LLVM, which require some fiddling before 93 running CMake: 94 95 #. If you're using Clang as the cross-compiler, there is a problem in 96 the LLVM ARM back-end that is producing absolute relocations on 97 position-independent code (``R_ARM_THM_MOVW_ABS_NC``), so for now, you 98 should disable PIC: 99 100 .. code-block:: bash 101 102 -DLLVM_ENABLE_PIC=False 103 104 This is not a problem, since Clang/LLVM libraries are statically 105 linked anyway, it shouldn't affect much. 106 107 #. The ARM libraries won't be installed in your system. 108 But the CMake prepare step, which checks for 109 dependencies, will check the *host* libraries, not the *target* 110 ones. Below there's a list of some dependencies, but your project could 111 have more, or this document could be outdated. You'll see the errors 112 while linking as an indication of that. 113 114 Debian based distros have a way to add ``multiarch``, which adds 115 a new architecture and allows you to install packages for those 116 systems. See https://wiki.debian.org/Multiarch/HOWTO for more info. 117 118 But not all distros will have that, and possibly not an easy way to 119 install them in any anyway, so you'll have to build/download 120 them separately. 121 122 A quick way of getting the libraries is to download them from 123 a distribution repository, like Debian (http://packages.debian.org/jessie/), 124 and download the missing libraries. Note that the ``libXXX`` 125 will have the shared objects (``.so``) and the ``libXXX-dev`` will 126 give you the headers and the static (``.a``) library. Just in 127 case, download both. 128 129 The ones you need for ARM are: ``libtinfo``, ``zlib1g``, 130 ``libxml2`` and ``liblzma``. In the Debian repository you'll 131 find downloads for all architectures. 132 133 After you download and unpack all ``.deb`` packages, copy all 134 ``.so`` and ``.a`` to a directory, make the appropriate 135 symbolic links (if necessary), and add the relevant ``-L`` 136 and ``-I`` paths to ``-DCMAKE_CXX_FLAGS`` above. 137 138 139 Running CMake and Building 140 -------------------------- 141 142 Finally, if you're using your platform compiler, run: 143 144 .. code-block:: bash 145 146 $ cmake -G Ninja <source-dir> <options above> 147 148 If you're using Clang as the cross-compiler, run: 149 150 .. code-block:: bash 151 152 $ CC='clang' CXX='clang++' cmake -G Ninja <source-dir> <options above> 153 154 If you have ``clang``/``clang++`` on the path, it should just work, and special 155 Ninja files will be created in the build directory. I strongly suggest 156 you to run ``cmake`` on a separate build directory, *not* inside the 157 source tree. 158 159 To build, simply type: 160 161 .. code-block:: bash 162 163 $ ninja 164 165 It should automatically find out how many cores you have, what are 166 the rules that needs building and will build the whole thing. 167 168 You can't run ``ninja check-all`` on this tree because the created 169 binaries are targeted to ARM, not x86_64. 170 171 Installing and Using 172 -------------------- 173 174 After the LLVM/Clang has built successfully, you should install it 175 via: 176 177 .. code-block:: bash 178 179 $ ninja install 180 181 which will create a sysroot on the install-dir. You can then tar 182 that directory into a binary with the full triple name (for easy 183 identification), like: 184 185 .. code-block:: bash 186 187 $ ln -sf <install-dir> arm-linux-gnueabihf-clang 188 $ tar zchf arm-linux-gnueabihf-clang.tar.gz arm-linux-gnueabihf-clang 189 190 If you copy that tarball to your target board, you'll be able to use 191 it for running the test-suite, for example. Follow the guidelines at 192 http://llvm.org/docs/lnt/quickstart.html, unpack the tarball in the 193 test directory, and use options: 194 195 .. code-block:: bash 196 197 $ ./sandbox/bin/python sandbox/bin/lnt runtest nt \ 198 --sandbox sandbox \ 199 --test-suite `pwd`/test-suite \ 200 --cc `pwd`/arm-linux-gnueabihf-clang/bin/clang \ 201 --cxx `pwd`/arm-linux-gnueabihf-clang/bin/clang++ 202 203 Remember to add the ``-jN`` options to ``lnt`` to the number of CPUs 204 on your board. Also, the path to your clang has to be absolute, so 205 you'll need the `pwd` trick above. 206
