Building Clang with eBPF Target for ARC HS Hosts

Preface

If you want to build eBPF programs right on the target, then you need to properly build Clang for ARC. There is no need for all possible targets, so we are going to build Clang with support of eBPF target only.

Notes for CentOS 7

It’s necessary to install the latest available development tools for CentOS 7 to make it possible to build clang. Use centos-release-scl repository to install the latest tools and enable them:

sudo yum install centos-release-scl
sudo yum install devtoolset-9
scl enable devtoolset-9 bash

Also llvm build system requires new CMake. Install cmake3 packages and use it instead of cmake:

sudo yum install cmake3

Notes for Ubuntu 18.04

llvm build system requires new CMake. To install it on Ubuntu 18.04 you can use cmake package provided by CMake’s team:

https://apt.kitware.com

Building Clang Toolchain for Host

Prepare sources:

git clone https://github.com/llvm/llvm-project.git
mkdir llvm-project/build
cd llvm-project/build

Prepare build system using CMake:

cmake -DCMAKE_INSTALL_PREFIX=/tools/clang-bpf \
      -DLLVM_ENABLE_PROJECTS=clang            \
      -DLLVM_TARGETS_TO_BUILD=BPF             \
      -DLLVM_INCLUDE_BENCHMARKS=OFF           \
      -DLLVM_ENABLE_PIC=ON                    \
      -DLLVM_ENABLE_WARNINGS=OFF              \
      -DLLVM_ENABLE_ZLIB=OFF                  \
      -DLLVM_INCLUDE_EXAMPLES=OFF             \
      -DLLVM_INCLUDE_TESTS=OFF                \
      -DCMAKE_BUILD_TYPE=Release              \
      -G "Unix Makefiles"                     \
      ../llvm

Build and install to /tools/clang-bpf:

make -j $(nproc)
make install

Building Native Clang Toolchain for ARC

Prepare sources:

git clone https://github.com/llvm/llvm-project.git
mkdir llvm-project/build
cd llvm-project/build

Clang’s build system (CMake) uses a toolchain file for configuring cross-compiler. Suppose that the toolchain for ARC HS 4x is placed in /tools/arc-linux-gnu (the directory which contains bin). Save this configuration to ~/arc.cmake:

SET(CMAKE_SYSTEM_NAME Linux)
SET(CMAKE_HOST_SYSTEM_PROCESSOR arc)
SET(CMAKE_HOST_SYSTEM_PROCESSOR Linux)
SET(CMAKE_HOST_SYSTEM_PROCESSOR gnu)
SET(CMAKE_SYSTEM_VERSION 1)

SET(CMAKE_C_COMPILER /tools/arc-linux-gnu/bin/arc-linux-gnu-gcc)
SET(CMAKE_CXX_COMPILER /tools/arc-linux-gnu/bin/arc-linux-gnu-g++)
SET(CMAKE_FIND_ROOT_PATH /tools/arc-linux-gnu/sysroot)

# Search for programs in the build host directories
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)

# ... for libraries and headers in the target directories
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
SET(HAVE_POSIX_REGEX 0)
SET(HAVE_STEADY_CLOCK 0)

Prepare build system using CMake:

cmake -DCMAKE_TOOLCHAIN_FILE=~/arc.cmake          \
      -DCMAKE_INSTALL_PREFIX=/tools/clang-bpf-arc \
      -DLLVM_ENABLE_PROJECTS=clang                \
      -DLLVM_TARGETS_TO_BUILD=BPF                 \
      -DCMAKE_BUILD_TYPE=MinSizeRel               \
      -DLLVM_INCLUDE_BENCHMARKS=OFF               \
      -DLLVM_ENABLE_PIC=ON                        \
      -DBUILD_SHARED_LIBS=ON                      \
      -DLLVM_ENABLE_WARNINGS=OFF                  \
      -DLLVM_ENABLE_ZLIB=OFF                      \
      -DLLVM_INCLUDE_EXAMPLES=OFF                 \
      -DLLVM_INCLUDE_TESTS=OFF                    \
      -DCMAKE_EXE_LINKER_FLAGS="-latomic"         \
      -DCMAKE_MODULE_LINKER_FLAGS="-latomic"      \
      -DCMAKE_SHARED_LINKER_FLAGS="-latomic"      \
      -G "Unix Makefiles"                         \
      ../llvm

Explanation for some options:

-DCMAKE_BUILD_TYPE=MinSizeRel - Turn on optimizations for size.
-DLLVM_INCLUDE_BENCHMARKS=OFF - Turn off benchmarks to avoid fails while building.
-DBUILD_SHARED_LIBS=ON - Don’t build Clang as one large blob (more than 100 MB) because linker cannot resolve relocations for such large binaries.
-DCMAKE_***_LINKER_FLAGS="-latomic" - Somehow -latomic is not passed to the linker while building. Thus we need to pass it manually.

Build and install to /tools/clang-bpf-arc:

make -j $(nproc)
make install

Then you can copy this directory to your target (e.g., to the overlay for Buildroot).