How to cross-compile Clang/LLVM using Clang/LLVM

Introduction

This document contains information about building LLVM and Clang on a host machine, targeting another platform.

For more information on how to use Clang as a cross-compiler, please check https://clang.llvm.org/docs/CrossCompilation.html.

This document describes cross-building a compiler in a single stage, using an existing clang install as the host compiler.

Note

These instructions have been tested for targeting 32-bit ARM, AArch64, or 64-bit RISC-V from an x86_64 Linux host. But should be equally applicable to any other target.

Setting up a sysroot

You will need a sysroot that contains essential build dependencies compiled for the target architecture. In this case, we will be using CMake and Ninja on a Linux host and compiling against a Debian sysroot. Detailed instructions on producing sysroots are outside of the scope of this documentation, but the following instructions should work on any Linux distribution with these pre-requisites:

  • binfmt_misc configured to execute qemu-user for binaries of the target architecture. This is done by installing the qemu-user-static and binfmt-support packages on Debian-derived distributions.

  • Root access (setups involving proot or other tools to avoid this requirement may be possible, but aren’t described here).

  • The debootstrap tool. This is available in most distributions.

The following snippet will initialise sysroots for 32-bit Arm, AArch64, and 64-bit RISC-V (just pick the target(s) you are interested in):

sudo debootstrap --arch=armhf --variant=minbase --include=build-essential,symlinks stable sysroot-deb-armhf-stable
sudo debootstrap --arch=arm64 --variant=minbase --include=build-essential,symlinks stable sysroot-deb-arm64-stable
sudo debootstrap --arch=riscv64 --variant=minbase --include=build-essential,symlinks unstable sysroot-deb-riscv64-unstable

The created sysroot may contain absolute symlinks, which will resolve to a location within the host when accessed during compilation, so we must convert any absolute symlinks to relative ones:

sudo chroot sysroot-of-your-choice symlinks -cr .

Configuring CMake and building

For more information on how to configure CMake for LLVM/Clang, see Building LLVM with CMake. Following CMake’s recommended practice, we will create a toolchain file.

The following assumes you have a system install of clang and lld that will be used for cross compiling and that the listed commands are executed from within the root of a checkout of the llvm-project git repository.

First, set variables in your shell session that will be used throughout the build instructions:

SYSROOT=$HOME/sysroot-deb-arm64-stable
TARGET=aarch64-linux-gnu
CFLAGS=""

To customise details of the compilation target or choose a different architecture altogether, change the SYSROOT, TARGET, and CFLAGS variables to something matching your target. For example, for 64-bit RISC-V you might set SYSROOT=$HOME/sysroot-deb-riscv64-unstable, TARGET=riscv64-linux-gnu and CFLAGS="-march=rva20u64". Refer to documentation such as your target’s compiler documentation or processor manual for guidance on which CFLAGS settings may be appropriate. The specified TARGET should match the triple used within the sysroot (i.e. $SYSROOT/usr/lib/$TARGET should exist).

Then execute the following snippet to create a toolchain file:

cat - <<EOF > $TARGET-clang.cmake
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSROOT "$SYSROOT")
set(CMAKE_C_COMPILER_TARGET $TARGET)
set(CMAKE_CXX_COMPILER_TARGET $TARGET)
set(CMAKE_C_FLAGS_INIT "$CFLAGS")
set(CMAKE_CXX_FLAGS_INIT "$CFLAGS")
set(CMAKE_LINKER_TYPE LLD)
set(CMAKE_C_COMPILER clang)
set(CMAKE_CXX_COMPILER clang++)
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
EOF

Then configure and build by invoking cmake:

cmake -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PROJECTS="lld;clang" \
  -DCMAKE_TOOLCHAIN_FILE=$(pwd)/$TARGET-clang.cmake \
  -DLLVM_HOST_TRIPLE=$TARGET \
  -DCMAKE_INSTALL_PREFIX=$HOME/clang-$TARGET \
  -S llvm \
  -B build/$TARGET
cmake --build build/$TARGET

These options from the toolchain file and cmake invocation above are important:

  • CMAKE_SYSTEM_NAME: Perhaps surprisingly, explicitly setting this variable causes CMake to set CMAKE_CROSSCOMPIILING.

  • CMAKE_{C,CXX}_COMPILER_TARGET: This will be used to set the --target argument to clang. The triple should match the triple used within the sysroot (i.e. $SYSROOT/usr/lib/$TARGET should exist).

  • CMAKE_FIND_ROOT_PATH_MODE_*: These control the search behaviour for finding libraries, includes or binaries. Setting these prevents files for the host being used in the build.

  • LLVM_HOST_TRIPLE: Specifies the target triple of the system the built LLVM will run on, which also implicitly sets other defaults such as LLVM_DEFAULT_TARGET_TRIPLE. For example, if you are using an x86_64 host to compile for RISC-V, this will be a RISC-V triple.

  • CMAKE_SYSROOT: The path to the sysroot containing libraries and headers for the target.

  • CMAKE_INSTALL_PREFIX: Setting this avoids installing binaries compiled for the target system into system directories for the host system. It is not required unless you are going to use the install target.

See LLVM’s build documentation for more guidance on CMake variables (e.g. LLVM_TARGETS_TO_BUILD may be useful if your cross-compiled binaries only need to support compiling for one target).

Working around a ninja dependency issue

If you followed the instructions above to create a sysroot, you may run into a longstanding problem related to path canonicalization in ninja. GCC canonicalizes system headers in dependency files, so when ninja reads them it does not need to do so. Clang does not do this, and unfortunately ninja does not implement the canonicalization logic at all, meaning for some system headers with symlinks in the paths, it can incorrectly compute a non-existing path and consider it as always modified.

If you are suffering from this issue, you will find any attempt at an incremental build (including the suggested command to build the install target in the next section) results in recompiling everything. ninja -C build/$TARGET -t deps shows files in $SYSROOT/include/* that do not exist (as the $SYSROOT/include folder does not exist) and you can further confirm these files are causing ninja to determine a rebuild is necessary with ninja -C build/$TARGET -d deps.

A workaround is to create a symlink so that the incorrect $SYSROOT/include/* dependencies resolve to files within $SYSROOT/usr/include/*. This works in practice for the simple cross-compilation use case described here, but is not a general solution.

sudo ln -s usr/include $SYSROOT/include

Testing the just-built compiler

Confirm the clang binary was built for the expected target architecture:

$ file -L ./build/aarch64-linux-gnu/bin/clang
./build/aarch64-linux-gnu/bin/clang: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, for GNU/Linux 3.7.0, BuildID[sha1]=516b8b366a790fcd3563bee4aec0cdfcb90bb1c7, not stripped

If you have qemu-user installed you can test the produced target binary either by invoking qemu-{target}-static directly:

$ qemu-aarch64-static -L $SYSROOT ./build/aarch64-linux-gnu/bin/clang --version
clang version 21.0.0git (https://github.com/llvm/llvm-project cedfdc6e889c5c614a953ed1f44bcb45a405f8da)
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/asb/llvm-project/build/aarch64-linux-gnu/bin

Or, if binfmt_misc is configured (as was necessary for debootstrap):

$ export QEMU_LD_PREFIX=$SYSROOT; ./build/aarch64-linux-gnu/bin/clang --version
clang version 21.0.0git (https://github.com/llvm/llvm-project cedfdc6e889c5c614a953ed1f44bcb45a405f8da)
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/asb/llvm-project/build/aarch64-linux-gnu/bin

Installing and using

Note

Use of the install target requires that you have set CMAKE_INSTALL_PREFIX otherwise it will attempt to install in directories under / on your host.

If you want to transfer a copy of the built compiler to another machine, you can first install it to a location on the host via:

cmake --build build/$TARGET --target=install

This will install the LLVM/Clang headers, binaries, libraries, and other files to paths within CMAKE_INSTALL_PREFIX. Then tar that directory for transfer to a device that runs the target architecture natively:

tar -czvf clang-$TARGET.tar.gz -C $HOME clang-$TARGET

The generated toolchain is portable, but requires compatible versions of any shared libraries it links against. This means using a sysroot that is as similar to your target operating system as possible is desirable. Other CMake variables may be helpful, for instance LLVM_STATIC_LINK_CXX_STDLIB.