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Building from source

Host platforms

The LLVM Embedded Toolchain for Arm has been built and tested on Linux/Ubuntu 18.04.5 LTS.

Installing prerequisites

The build requires the following software to be installed, in addition to the [LLVM requirements|https://llvm.org/docs/GettingStarted.html#software]:

  • CMake 3.20 or above
  • Meson
  • Git
  • Ninja
  • Python

On a Ubuntu 18.04.5 LTS machine you can use the following commands to install the software mentioned above:

$ apt-get install clang # If the Clang version installed by the package manager is older than 6.0.0, download a recent version from https://releases.llvm.org or build from source
$ apt-get install python3
$ apt-get install git
$ apt-get install make
$ apt-get install ninja-build
$ apt-get install cmake # If the CMake version installed by the package manager is too old, download a recent version from https://cmake.org/download and add it to PATH
$ pip install meson

Building

The toolchain can be built directly with CMake.

mkdir build
cd build
cmake .. -GNinja -DFETCHCONTENT_QUIET=OFF
ninja llvm-toolchain

To make it easy to get started, the above command checks out and patches llvm-project & picolibc Git repos automatically. If you prefer you can check out and patch the repos manually and use those:

mkdir repos
git -C repos clone https://github.com/llvm/llvm-project.git
git -C repos/llvm-project apply ../../patches/llvm-project.patch
git -C repos clone https://github.com/picolibc/picolibc.git
git -C repos/picolibc apply ../../patches/picolibc.patch
mkdir build
cd build
cmake .. -GNinja -DFETCHCONTENT_SOURCE_DIR_LLVMPROJECT=../repos/llvm-project -DFETCHCONTENT_SOURCE_DIR_PICOLIBC=../repos/picolibc
ninja llvm-toolchain

Testing the toolchain

ninja check-llvm-toolchain

Packaging the toolchain

After building, create a zip or tar.gz file as appropriate for the platform:

ninja package-llvm-toolchain

Cross-compiling the toolchain for Windows

The LLVM Embedded Toolchain for Arm can be cross-compiled to run on Windows. The compilation itself still happens on Linux. In addition to the prerequisites mentioned in the Installing prerequisites section you will also need a Mingw-w64 toolchain based on GCC 7.1.0 or above installed. For example, to install it on Ubuntu Linux use the following command:

# apt-get install mingw-w64

The MinGW build includes GCC & MinGW libraries into the package.

The following three libraries are used:

Library Project Link
libstdc++-6.dll GCC https://gcc.gnu.org
libgcc_s_seh-1.dll GCC https://gcc.gnu.org
libwinpthread-1.dll Mingw-w64 http://mingw-w64.org

The libraries are distributed under their own licenses, this needs to be taken into consideration if you decide to redistribute the built toolchain.

To enable the MinGW build, set the LLVM_TOOLCHAIN_CROSS_BUILD_MINGW option:

cmake . -DLLVM_TOOLCHAIN_CROSS_BUILD_MINGW=ON
ninja package-llvm-toolchain

The same build directory can be used for both native and MinGW toolchains.

Known limitations

  • Depending on the state of the sources, build errors may occur when the latest revisions of the llvm-project & picolibc repos are used.

Divergences from upstream

picolibc:

  • Added a fix for building with -mthumb

LLVM:

  • Recognize $@ in a config file argument to mean the directory of the config file, allowing toolchain relative paths.