1=================================================================== 2Cross-compilation using Clang 3=================================================================== 4 5Introduction 6============ 7 8This document will guide you in choosing the right Clang options 9for cross-compiling your code to a different architecture. It assumes you 10already know how to compile the code in question for the host architecture, 11and that you know how to choose additional include and library paths. 12 13However, this document is *not* a "how to" and won't help you setting your 14build system or Makefiles, nor choosing the right CMake options, etc. 15Also, it does not cover all the possible options, nor does it contain 16specific examples for specific architectures. For a concrete example, the 17`instructions for cross-compiling LLVM itself 18<https://llvm.org/docs/HowToCrossCompileLLVM.html>`_ may be of interest. 19 20After reading this document, you should be familiar with the main issues 21related to cross-compilation, and what main compiler options Clang provides 22for performing cross-compilation. 23 24Cross compilation issues 25======================== 26 27In GCC world, every host/target combination has its own set of binaries, 28headers, libraries, etc. So, it's usually simple to download a package 29with all files in, unzip to a directory and point the build system to 30that compiler, that will know about its location and find all it needs to 31when compiling your code. 32 33On the other hand, Clang/LLVM is natively a cross-compiler, meaning that 34one set of programs can compile to all targets by setting the ``-target`` 35option. That makes it a lot easier for programmers wishing to compile to 36different platforms and architectures, and for compiler developers that 37only have to maintain one build system, and for OS distributions, that 38need only one set of main packages. 39 40But, as is true to any cross-compiler, and given the complexity of 41different architectures, OS's and options, it's not always easy finding 42the headers, libraries or binutils to generate target specific code. 43So you'll need special options to help Clang understand what target 44you're compiling to, where your tools are, etc. 45 46Another problem is that compilers come with standard libraries only (like 47``compiler-rt``, ``libcxx``, ``libgcc``, ``libm``, etc), so you'll have to 48find and make available to the build system, every other library required 49to build your software, that is specific to your target. It's not enough to 50have your host's libraries installed. 51 52Finally, not all toolchains are the same, and consequently, not every Clang 53option will work magically. Some options, like ``--sysroot`` (which 54effectively changes the logical root for headers and libraries), assume 55all your binaries and libraries are in the same directory, which may not 56true when your cross-compiler was installed by the distribution's package 57management. So, for each specific case, you may use more than one 58option, and in most cases, you'll end up setting include paths (``-I``) and 59library paths (``-L``) manually. 60 61To sum up, different toolchains can: 62 * be host/target specific or more flexible 63 * be in a single directory, or spread out across your system 64 * have different sets of libraries and headers by default 65 * need special options, which your build system won't be able to figure 66 out by itself 67 68General Cross-Compilation Options in Clang 69========================================== 70 71Target Triple 72------------- 73 74The basic option is to define the target architecture. For that, use 75``-target <triple>``. If you don't specify the target, CPU names won't 76match (since Clang assumes the host triple), and the compilation will 77go ahead, creating code for the host platform, which will break later 78on when assembling or linking. 79 80The triple has the general format ``<arch><sub>-<vendor>-<sys>-<abi>``, where: 81 * ``arch`` = ``x86_64``, ``i386``, ``arm``, ``thumb``, ``mips``, etc. 82 * ``sub`` = for ex. on ARM: ``v5``, ``v6m``, ``v7a``, ``v7m``, etc. 83 * ``vendor`` = ``pc``, ``apple``, ``nvidia``, ``ibm``, etc. 84 * ``sys`` = ``none``, ``linux``, ``win32``, ``darwin``, ``cuda``, etc. 85 * ``abi`` = ``eabi``, ``gnu``, ``android``, ``macho``, ``elf``, etc. 86 87The sub-architecture options are available for their own architectures, 88of course, so "x86v7a" doesn't make sense. The vendor needs to be 89specified only if there's a relevant change, for instance between PC 90and Apple. Most of the time it can be omitted (and Unknown) 91will be assumed, which sets the defaults for the specified architecture. 92The system name is generally the OS (linux, darwin), but could be special 93like the bare-metal "none". 94 95When a parameter is not important, it can be omitted, or you can 96choose ``unknown`` and the defaults will be used. If you choose a parameter 97that Clang doesn't know, like ``blerg``, it'll ignore and assume 98``unknown``, which is not always desired, so be careful. 99 100Finally, the ABI option is something that will pick default CPU/FPU, 101define the specific behaviour of your code (PCS, extensions), 102and also choose the correct library calls, etc. 103 104CPU, FPU, ABI 105------------- 106 107Once your target is specified, it's time to pick the hardware you'll 108be compiling to. For every architecture, a default set of CPU/FPU/ABI 109will be chosen, so you'll almost always have to change it via flags. 110 111Typical flags include: 112 * ``-mcpu=<cpu-name>``, like x86-64, swift, cortex-a15 113 * ``-mfpu=<fpu-name>``, like SSE3, NEON, controlling the FP unit available 114 * ``-mfloat-abi=<fabi>``, like soft, hard, controlling which registers 115 to use for floating-point 116 117The default is normally the common denominator, so that Clang doesn't 118generate code that breaks. But that also means you won't get the best 119code for your specific hardware, which may mean orders of magnitude 120slower than you expect. 121 122For example, if your target is ``arm-none-eabi``, the default CPU will 123be ``arm7tdmi`` using soft float, which is extremely slow on modern cores, 124whereas if your triple is ``armv7a-none-eabi``, it'll be Cortex-A8 with 125NEON, but still using soft-float, which is much better, but still not 126great. 127 128Toolchain Options 129----------------- 130 131There are three main options to control access to your cross-compiler: 132``--sysroot``, ``-I``, and ``-L``. The two last ones are well known, 133but they're particularly important for additional libraries 134and headers that are specific to your target. 135 136There are two main ways to have a cross-compiler: 137 138#. When you have extracted your cross-compiler from a zip file into 139 a directory, you have to use ``--sysroot=<path>``. The path is the 140 root directory where you have unpacked your file, and Clang will 141 look for the directories ``bin``, ``lib``, ``include`` in there. 142 143 In this case, your setup should be pretty much done (if no 144 additional headers or libraries are needed), as Clang will find 145 all binaries it needs (assembler, linker, etc) in there. 146 147#. When you have installed via a package manager (modern Linux 148 distributions have cross-compiler packages available), make 149 sure the target triple you set is *also* the prefix of your 150 cross-compiler toolchain. 151 152 In this case, Clang will find the other binaries (assembler, 153 linker), but not always where the target headers and libraries 154 are. People add system-specific clues to Clang often, but as 155 things change, it's more likely that it won't find than the 156 other way around. 157 158 So, here, you'll be a lot safer if you specify the include/library 159 directories manually (via ``-I`` and ``-L``). 160 161Target-Specific Libraries 162========================= 163 164All libraries that you compile as part of your build will be 165cross-compiled to your target, and your build system will probably 166find them in the right place. But all dependencies that are 167normally checked against (like ``libxml`` or ``libz`` etc) will match 168against the host platform, not the target. 169 170So, if the build system is not aware that you want to cross-compile 171your code, it will get every dependency wrong, and your compilation 172will fail during build time, not configure time. 173 174Also, finding the libraries for your target are not as easy 175as for your host machine. There aren't many cross-libraries available 176as packages to most OS's, so you'll have to either cross-compile them 177from source, or download the package for your target platform, 178extract the libraries and headers, put them in specific directories 179and add ``-I`` and ``-L`` pointing to them. 180 181Also, some libraries have different dependencies on different targets, 182so configuration tools to find dependencies in the host can get the 183list wrong for the target platform. This means that the configuration 184of your build can get things wrong when setting their own library 185paths, and you'll have to augment it via additional flags (configure, 186Make, CMake, etc). 187 188Multilibs 189--------- 190 191When you want to cross-compile to more than one configuration, for 192example hard-float-ARM and soft-float-ARM, you'll have to have multiple 193copies of your libraries and (possibly) headers. 194 195Some Linux distributions have support for Multilib, which handle that 196for you in an easier way, but if you're not careful and, for instance, 197forget to specify ``-ccc-gcc-name armv7l-linux-gnueabihf-gcc`` (which 198uses hard-float), Clang will pick the ``armv7l-linux-gnueabi-ld`` 199(which uses soft-float) and linker errors will happen. 200 201The same is true if you're compiling for different ABIs, like ``gnueabi`` 202and ``androideabi``, and might even link and run, but produce run-time 203errors, which are much harder to track down and fix. 204