1 =================================================================== 2 Cross-compilation using Clang 3 =================================================================== 4 5 Introduction 6 ============ 7 8 This document will guide you in choosing the right Clang options 9 for cross-compiling your code to a different architecture. It assumes you 10 already know how to compile the code in question for the host architecture, 11 and that you know how to choose additional include and library paths. 12 13 However, this document is *not* a "how to" and won't help you setting your 14 build system or Makefiles, nor choosing the right CMake options, etc. 15 Also, it does not cover all the possible options, nor does it contain 16 specific examples for specific architectures. For a concrete example, the 17 `instructions for cross-compiling LLVM itself 18 <http://llvm.org/docs/HowToCrossCompileLLVM.html>`_ may be of interest. 19 20 After reading this document, you should be familiar with the main issues 21 related to cross-compilation, and what main compiler options Clang provides 22 for performing cross-compilation. 23 24 Cross compilation issues 25 ======================== 26 27 In GCC world, every host/target combination has its own set of binaries, 28 headers, libraries, etc. So, it's usually simple to download a package 29 with all files in, unzip to a directory and point the build system to 30 that compiler, that will know about its location and find all it needs to 31 when compiling your code. 32 33 On the other hand, Clang/LLVM is natively a cross-compiler, meaning that 34 one set of programs can compile to all targets by setting the ``-target`` 35 option. That makes it a lot easier for programers wishing to compile to 36 different platforms and architectures, and for compiler developers that 37 only have to maintain one build system, and for OS distributions, that 38 need only one set of main packages. 39 40 But, as is true to any cross-compiler, and given the complexity of 41 different architectures, OS's and options, it's not always easy finding 42 the headers, libraries or binutils to generate target specific code. 43 So you'll need special options to help Clang understand what target 44 you're compiling to, where your tools are, etc. 45 46 Another problem is that compilers come with standard libraries only (like 47 ``compiler-rt``, ``libcxx``, ``libgcc``, ``libm``, etc), so you'll have to 48 find and make available to the build system, every other library required 49 to build your software, that is specific to your target. It's not enough to 50 have your host's libraries installed. 51 52 Finally, not all toolchains are the same, and consequently, not every Clang 53 option will work magically. Some options, like ``--sysroot`` (which 54 effectively changes the logical root for headers and libraries), assume 55 all your binaries and libraries are in the same directory, which may not 56 true when your cross-compiler was installed by the distribution's package 57 management. So, for each specific case, you may use more than one 58 option, and in most cases, you'll end up setting include paths (``-I``) and 59 library paths (``-L``) manually. 60 61 To 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 68 General Cross-Compilation Options in Clang 69 ========================================== 70 71 Target Triple 72 ------------- 73 74 The 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 76 match (since Clang assumes the host triple), and the compilation will 77 go ahead, creating code for the host platform, which will break later 78 on when assembling or linking. 79 80 The triple has the general format ``<arch><sub>-<vendor>-<sys>-<abi>``, where: 81 * ``arch`` = ``x86``, ``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 87 The sub-architecture options are available for their own architectures, 88 of course, so "x86v7a" doesn't make sense. The vendor needs to be 89 specified only if there's a relevant change, for instance between PC 90 and Apple. Most of the time it can be omitted (and Unknown) 91 will be assumed, which sets the defaults for the specified architecture. 92 The system name is generally the OS (linux, darwin), but could be special 93 like the bare-metal "none". 94 95 When a parameter is not important, they can be omitted, or you can 96 choose ``unknown`` and the defaults will be used. If you choose a parameter 97 that Clang doesn't know, like ``blerg``, it'll ignore and assume 98 ``unknown``, which is not always desired, so be careful. 99 100 Finally, the ABI option is something that will pick default CPU/FPU, 101 define the specific behaviour of your code (PCS, extensions), 102 and also choose the correct library calls, etc. 103 104 CPU, FPU, ABI 105 ------------- 106 107 Once your target is specified, it's time to pick the hardware you'll 108 be compiling to. For every architecture, a default set of CPU/FPU/ABI 109 will be chosen, so you'll almost always have to change it via flags. 110 111 Typical flags include: 112 * ``-mcpu=<cpu-name>``, like x86-64, swift, cortex-a15 113 * ``-fpu=<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 116 117 The default is normally the common denominator, so that Clang doesn't 118 generate code that breaks. But that also means you won't get the best 119 code for your specific hardware, which may mean orders of magnitude 120 slower than you expect. 121 122 For example, if your target is ``arm-none-eabi``, the default CPU will 123 be ``arm7tdmi`` using soft float, which is extremely slow on modern cores, 124 whereas if your triple is ``armv7a-none-eabi``, it'll be Cortex-A8 with 125 NEON, but still using soft-float, which is much better, but still not 126 great. 127 128 Toolchain Options 129 ----------------- 130 131 There are three main options to control access to your cross-compiler: 132 ``--sysroot``, ``-I``, and ``-L``. The two last ones are well known, 133 but they're particularly important for additional libraries 134 and headers that are specific to your target. 135 136 There 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``). 160 161 Target-Specific Libraries 162 ========================= 163 164 All libraries that you compile as part of your build will be 165 cross-compiled to your target, and your build system will probably 166 find them in the right place. But all dependencies that are 167 normally checked against (like ``libxml`` or ``libz`` etc) will match 168 against the host platform, not the target. 169 170 So, if the build system is not aware that you want to cross-compile 171 your code, it will get every dependency wrong, and your compilation 172 will fail during build time, not configure time. 173 174 Also, finding the libraries for your target are not as easy 175 as for your host machine. There aren't many cross-libraries available 176 as packages to most OS's, so you'll have to either cross-compile them 177 from source, or download the package for your target platform, 178 extract the libraries and headers, put them in specific directories 179 and add ``-I`` and ``-L`` pointing to them. 180 181 Also, some libraries have different dependencies on different targets, 182 so configuration tools to find dependencies in the host can get the 183 list wrong for the target platform. This means that the configuration 184 of your build can get things wrong when setting their own library 185 paths, and you'll have to augment it via additional flags (configure, 186 Make, CMake, etc). 187 188 Multilibs 189 --------- 190 191 When you want to cross-compile to more than one configuration, for 192 example hard-float-ARM and soft-float-ARM, you'll have to have multiple 193 copies of your libraries and (possibly) headers. 194 195 Some Linux distributions have support for Multilib, which handle that 196 for you in an easier way, but if you're not careful and, for instance, 197 forget to specify ``-ccc-gcc-name armv7l-linux-gnueabihf-gcc`` (which 198 uses hard-float), Clang will pick the ``armv7l-linux-gnueabi-ld`` 199 (which uses soft-float) and linker errors will happen. 200 201 The same is true if you're compiling for different ABIs, like ``gnueabi`` 202 and ``androideabi``, and might even link and run, but produce run-time 203 errors, which are much harder to track down and fix. 204 205
