| =================================================================== |
| Cross-compilation using Clang |
| =================================================================== |
| |
| Introduction |
| ============ |
| |
| This document will guide you in choosing the right Clang options |
| for cross-compiling your code to a different architecture. It assumes you |
| already know how to compile the code in question for the host architecture, |
| and that you know how to choose additional include and library paths. |
| |
| However, this document is *not* a "how to" and won't help you setting your |
| build system or Makefiles, nor choosing the right CMake options, etc. |
| Also, it does not cover all the possible options, nor does it contain |
| specific examples for specific architectures. For a concrete example, the |
| `instructions for cross-compiling LLVM itself |
| <http://llvm.org/docs/HowToCrossCompileLLVM.html>`_ may be of interest. |
| |
| After reading this document, you should be familiar with the main issues |
| related to cross-compilation, and what main compiler options Clang provides |
| for performing cross-compilation. |
| |
| Cross compilation issues |
| ======================== |
| |
| In GCC world, every host/target combination has its own set of binaries, |
| headers, libraries, etc. So, it's usually simple to download a package |
| with all files in, unzip to a directory and point the build system to |
| that compiler, that will know about its location and find all it needs to |
| when compiling your code. |
| |
| On the other hand, Clang/LLVM is natively a cross-compiler, meaning that |
| one set of programs can compile to all targets by setting the ``-target`` |
| option. That makes it a lot easier for programers wishing to compile to |
| different platforms and architectures, and for compiler developers that |
| only have to maintain one build system, and for OS distributions, that |
| need only one set of main packages. |
| |
| But, as is true to any cross-compiler, and given the complexity of |
| different architectures, OS's and options, it's not always easy finding |
| the headers, libraries or binutils to generate target specific code. |
| So you'll need special options to help Clang understand what target |
| you're compiling to, where your tools are, etc. |
| |
| Another problem is that compilers come with standard libraries only (like |
| ``compiler-rt``, ``libcxx``, ``libgcc``, ``libm``, etc), so you'll have to |
| find and make available to the build system, every other library required |
| to build your software, that is specific to your target. It's not enough to |
| have your host's libraries installed. |
| |
| Finally, not all toolchains are the same, and consequently, not every Clang |
| option will work magically. Some options, like ``--sysroot`` (which |
| effectively changes the logical root for headers and libraries), assume |
| all your binaries and libraries are in the same directory, which may not |
| true when your cross-compiler was installed by the distribution's package |
| management. So, for each specific case, you may use more than one |
| option, and in most cases, you'll end up setting include paths (``-I``) and |
| library paths (``-L``) manually. |
| |
| To sum up, different toolchains can: |
| * be host/target specific or more flexible |
| * be in a single directory, or spread out across your system |
| * have different sets of libraries and headers by default |
| * need special options, which your build system won't be able to figure |
| out by itself |
| |
| General Cross-Compilation Options in Clang |
| ========================================== |
| |
| Target Triple |
| ------------- |
| |
| The basic option is to define the target architecture. For that, use |
| ``-target <triple>``. If you don't specify the target, CPU names won't |
| match (since Clang assumes the host triple), and the compilation will |
| go ahead, creating code for the host platform, which will break later |
| on when assembling or linking. |
| |
| The triple has the general format ``<arch><sub>-<vendor>-<sys>-<abi>``, where: |
| * ``arch`` = ``x86``, ``arm``, ``thumb``, ``mips``, etc. |
| * ``sub`` = for ex. on ARM: ``v5``, ``v6m``, ``v7a``, ``v7m``, etc. |
| * ``vendor`` = ``pc``, ``apple``, ``nvidia``, ``ibm``, etc. |
| * ``sys`` = ``none``, ``linux``, ``win32``, ``darwin``, ``cuda``, etc. |
| * ``abi`` = ``eabi``, ``gnu``, ``android``, ``macho``, ``elf``, etc. |
| |
| The sub-architecture options are available for their own architectures, |
| of course, so "x86v7a" doesn't make sense. The vendor needs to be |
| specified only if there's a relevant change, for instance between PC |
| and Apple. Most of the time it can be omitted (and Unknown) |
| will be assumed, which sets the defaults for the specified architecture. |
| The system name is generally the OS (linux, darwin), but could be special |
| like the bare-metal "none". |
| |
| When a parameter is not important, they can be omitted, or you can |
| choose ``unknown`` and the defaults will be used. If you choose a parameter |
| that Clang doesn't know, like ``blerg``, it'll ignore and assume |
| ``unknown``, which is not always desired, so be careful. |
| |
| Finally, the ABI option is something that will pick default CPU/FPU, |
| define the specific behaviour of your code (PCS, extensions), |
| and also choose the correct library calls, etc. |
| |
| CPU, FPU, ABI |
| ------------- |
| |
| Once your target is specified, it's time to pick the hardware you'll |
| be compiling to. For every architecture, a default set of CPU/FPU/ABI |
| will be chosen, so you'll almost always have to change it via flags. |
| |
| Typical flags include: |
| * ``-mcpu=<cpu-name>``, like x86-64, swift, cortex-a15 |
| * ``-fpu=<fpu-name>``, like SSE3, NEON, controlling the FP unit available |
| * ``-mfloat-abi=<fabi>``, like soft, hard, controlling which registers |
| to use for floating-point |
| |
| The default is normally the common denominator, so that Clang doesn't |
| generate code that breaks. But that also means you won't get the best |
| code for your specific hardware, which may mean orders of magnitude |
| slower than you expect. |
| |
| For example, if your target is ``arm-none-eabi``, the default CPU will |
| be ``arm7tdmi`` using soft float, which is extremely slow on modern cores, |
| whereas if your triple is ``armv7a-none-eabi``, it'll be Cortex-A8 with |
| NEON, but still using soft-float, which is much better, but still not |
| great. |
| |
| Toolchain Options |
| ----------------- |
| |
| There are three main options to control access to your cross-compiler: |
| ``--sysroot``, ``-I``, and ``-L``. The two last ones are well known, |
| but they're particularly important for additional libraries |
| and headers that are specific to your target. |
| |
| There are two main ways to have a cross-compiler: |
| |
| #. When you have extracted your cross-compiler from a zip file into |
| a directory, you have to use ``--sysroot=<path>``. The path is the |
| root directory where you have unpacked your file, and Clang will |
| look for the directories ``bin``, ``lib``, ``include`` in there. |
| |
| In this case, your setup should be pretty much done (if no |
| additional headers or libraries are needed), as Clang will find |
| all binaries it needs (assembler, linker, etc) in there. |
| |
| #. When you have installed via a package manager (modern Linux |
| distributions have cross-compiler packages available), make |
| sure the target triple you set is *also* the prefix of your |
| cross-compiler toolchain. |
| |
| In this case, Clang will find the other binaries (assembler, |
| linker), but not always where the target headers and libraries |
| are. People add system-specific clues to Clang often, but as |
| things change, it's more likely that it won't find than the |
| other way around. |
| |
| So, here, you'll be a lot safer if you specify the include/library |
| directories manually (via ``-I`` and ``-L``). |
| |
| Target-Specific Libraries |
| ========================= |
| |
| All libraries that you compile as part of your build will be |
| cross-compiled to your target, and your build system will probably |
| find them in the right place. But all dependencies that are |
| normally checked against (like ``libxml`` or ``libz`` etc) will match |
| against the host platform, not the target. |
| |
| So, if the build system is not aware that you want to cross-compile |
| your code, it will get every dependency wrong, and your compilation |
| will fail during build time, not configure time. |
| |
| Also, finding the libraries for your target are not as easy |
| as for your host machine. There aren't many cross-libraries available |
| as packages to most OS's, so you'll have to either cross-compile them |
| from source, or download the package for your target platform, |
| extract the libraries and headers, put them in specific directories |
| and add ``-I`` and ``-L`` pointing to them. |
| |
| Also, some libraries have different dependencies on different targets, |
| so configuration tools to find dependencies in the host can get the |
| list wrong for the target platform. This means that the configuration |
| of your build can get things wrong when setting their own library |
| paths, and you'll have to augment it via additional flags (configure, |
| Make, CMake, etc). |
| |
| Multilibs |
| --------- |
| |
| When you want to cross-compile to more than one configuration, for |
| example hard-float-ARM and soft-float-ARM, you'll have to have multiple |
| copies of you libraries and (possibly) headers. |
| |
| Some Linux distributions have support for Multilib, which handle that |
| for you in an easier way, but if you're not careful and, for instance, |
| forget to specify ``-ccc-gcc-name armv7l-linux-gnueabihf-gcc`` (which |
| uses hard-float), Clang will pick the ``armv7l-linux-gnueabi-ld`` |
| (which uses soft-float) and linker errors will happen. |
| |
| The same is true if you're compiling for different ABIs, like ``gnueabi`` |
| and ``androideabi``, and might even link and run, but produce run-time |
| errors, which are much harder to track down and fix. |
| |