NOTE: This article is not complete yet. Having working and production ready llvm/clang cross-compiler involves much more work than just what is explained here (such that having llvm/clang use correct target associated sysroot, static linker, C/C++ libraries, etc.) See clang Universal driver. For working and production ready clang/llvm cross-compiler use dedicated tools (such as EmbToolkit) generating one for you.
Generally speaking, a cross-compiler is a compiler that runs on platform A (the host), but generates executables for platform B (the target). These two platforms may (but do not need to) differ in CPU, operating system, and/or executable format.
Clang (and llvm) are host cross compilers. They by default have the capability to cross compile, but also produce host binaries. See Usage
- A host system with a working compiler (can be GCC, Clang, etc).
- A bash shell or comparable environment. If you are not using a bash shell, you might have to modify some of the command lines below. If you have just installed the basic Cygwin package, you have to run the setup.exe again and install the following packages:
- GNU Make or CMake
- GNU Binutils (a fairly recent version of Binutils, try 2.21 or above if you get assembly compilation errors)
- GIT or SVN (if building from sources)
Checking out sources
Clang/LLVM sources can be checked out either with GIT or SVN.
For GIT, in bash:
mkdir crossllvm cd crossllvm git clone http://llvm.org/git/llvm.git cd llvm/tools git clone http://llvm.org/git/clang.git cd ../projects git clone http://llvm.org/git/compiler-rt.git
For SVN, in bash:
mkdir crossllvm cd crossllvm svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm cd llvm/tools svn co http://llvm.org/svn/llvm-project/cfe/trunk clang cd ../projects svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt
Building from sources (Possibly Unstable)
After having checked out the sources (see above), in bash, from the "crossllvm" directory:
mkdir build cd build ../llvm/configure --enable-optimized make make install
Or with CMake:
mkdir build cd build cmake -DCMAKE_BUILD_TYPE:STRING=Release make make install
Now you have got LLVM and Clang built!
A faster and more up-to-date way is posted here. It uses ninja for much faster building.
A Clang cross-compiler is generated by this buildscript. Dissect it to learn more. It uses some gcc and binutils to provide a fully working toolchain.
Open a terminal
sudo apt-get install clang
Note, you might have to disable host functionality. See Useful Flags
$ git clone http://llvm.org/git/libcxx.git $ export TRIPLE=-apple- $ cd libcxx/lib $ ./buildit
Built libc++ is not installed by default, you can use make install or copy files manually.
After building you will have a compiler able to output multiple output formats regardless of your current platform.
For example, for cross compiling to ARM, you can use
-march=armv7-a -mfloat-abi=soft -ccc-host-triple arm-elf
Since 3.1, it can be shortened to
-target armv7--eabi -mcpu=cortex-a9
An example for compiling to a generic X86 ELF target would be:
Some usefull flags for OS development.
Indicated that the file should be compiled for a freestanding enviroment (like a kernel), not a hosted (userspace), environment.
Disable special handling and optimizations of builtin functions like strlen and malloc.
Disables standard library
Makes sure the standard library headers are not included.
Makes sure the standard C++ library headers are not included. This makes sense if you build a custom version of libc++ and want to avoid including system one.
Using system roots
Due to Clang's dependency on ld, you may get the error message "/usr/bin/ld: this linker was not configured to use sysroots". If you receive this error, you'll probably have to compile binutils with sysroot support. There is information how to do this GCC Cross-Compiler#Binutils - note the "--with-sysroot" flag.
TODO: describe non-svn build from released tarballs. TODO: EmbToolkit project recently added support to clang/llvm based cross compiler, ddd note about how it works.