5.5. LLVM runtimes (libunwind, libcxxabi, libcxx) 22.1.3

libunwind is a platform-independent stack unwinding library. we need it to provide the unwinder used by C++ exception handling in this book.

libcxxabi is the LLVM C++ ABI support library. we need it to provide C++ ABI and exception runtime support required by libcxx.

libcxx is the LLVM C++ standard library implementation. we need it to provide the C++ standard library used by Clang in later stages of this book.

Extract Sources and Create a Build Directory

cd "$LBI_SOURCES"
tar -xf llvm-project-22.1.3.src.tar.xz
cd llvm-project-22.1.3.src/runtimes

Configure LLVM Runtimes for musl

lbi_cmake build-runtimes \
    -G Ninja \
    -DCMAKE_C_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang" \
    -DCMAKE_CXX_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang++" \
    -DCMAKE_SYSROOT="$LBI_ROOT" \
    -DCMAKE_FIND_ROOT_PATH="$LBI_ROOT;$LBI_ROOT/system" \
    -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER \
    -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY \
    -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY \
    -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY \
    -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
    -DCMAKE_C_FLAGS="--target=$LBI_TARGET" \
    -DCMAKE_CXX_FLAGS="--target=$LBI_TARGET" \
    -DLLVM_ENABLE_RUNTIMES="libunwind;libcxxabi;libcxx" \
    -DLIBUNWIND_INSTALL_LIBRARY_DIR=/system/libraries \
    -DLIBCXXABI_INSTALL_LIBRARY_DIR=/system/libraries \
    -DLIBCXX_INSTALL_LIBRARY_DIR=/system/libraries \
    -DLLVM_ENABLE_ZLIB=OFF \
    -DLLVM_ENABLE_ZSTD=OFF \
    -DLLVM_ENABLE_LIBXML2=OFF \
    -DLIBCXX_HAS_MUSL_LIBC=ON \
    -DLIBCXX_HAS_ATOMIC_LIB=OFF \
    -DLIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL=OFF \
    -DLIBCXXABI_USE_LLVM_UNWINDER=ON \
    -DLIBCXX_USE_COMPILER_RT=ON \
    -DLIBCXXABI_USE_COMPILER_RT=ON \
    -DLIBUNWIND_USE_COMPILER_RT=ON \
    -DCMAKE_BUILD_TYPE=Release

Build LLVM Runtimes

ninja -C build-runtimes $LWI_MAKE_FLAGS

Install LLVM Runtimes into the Target Tree

DESTDIR="$LBI_ROOT" ninja -C build-runtimes install

Command Explanations

• -G Ninja: Uses the Ninja generator for fast incremental builds.
• lbi_cmake build-runtimes ...: Uses the book's CMake helper so install layout follows the environment policy (/binaries, /systembinaries, /libraries, /headers, /configuration, /variable, and documentation paths) instead of defaulting to /usr.
• -DCMAKE_C_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang": Uses the pass 1 target-prefixed Clang C compiler.
• -DCMAKE_CXX_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang++": Uses the pass 1 target-prefixed Clang C++ compiler.
• -DCMAKE_SYSROOT="$LBI_ROOT": Directs configure and compile checks at the target sysroot.
• -DCMAKE_FIND_ROOT_PATH="$LBI_ROOT;$LBI_ROOT/system": Restricts CMake's search roots to the target tree.
• -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER: Lets build-time tools (like cmake, ninja, and scripting runtimes) come from the host.
• -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY: Prevents linking target libraries from host paths such as /usr/lib.
• -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY: Prevents header discovery from host include paths.
• -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY: Forces package discovery into target roots instead of host package locations.
• -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY: Prevents CMake's compiler checks from requiring an executable link against libc++ before libc++ has been built.
• -DCMAKE_C_FLAGS="--target=$LBI_TARGET": Ensures C compilation targets the selected triple.
• -DCMAKE_CXX_FLAGS="--target=$LBI_TARGET": Ensures C++ compilation targets the selected triple.
• -DLLVM_ENABLE_RUNTIMES="libunwind;libcxxabi;libcxx": Builds exactly the unwinder, C++ ABI runtime, and C++ standard library runtimes.
• -DLIBUNWIND_INSTALL_LIBRARY_DIR=/system/libraries, -DLIBCXXABI_INSTALL_LIBRARY_DIR=/system/libraries, -DLIBCXX_INSTALL_LIBRARY_DIR=/system/libraries: Forces each runtime project to install its libraries into /system/libraries, because these projects can ignore generic CMAKE_INSTALL_LIBDIR defaults.
• -DLLVM_ENABLE_ZLIB=OFF, -DLLVM_ENABLE_ZSTD=OFF, -DLLVM_ENABLE_LIBXML2=OFF: Disables optional compression/XML dependencies in this pass so host find_package hits do not leak into the runtime bootstrap.
• -DLIBCXX_HAS_MUSL_LIBC=ON: Enables libcxx musl-specific behavior for the target libc environment.
• -DLIBCXX_HAS_ATOMIC_LIB=OFF: Prevents libcxx from linking -latomic as an external library in this pass.
• -DLIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL=OFF: Prevents libcxxabi from requiring glibc's __cxa_thread_atexit_impl symbol, which is not provided by musl.
• -DLIBCXXABI_USE_LLVM_UNWINDER=ON: Configures libcxxabi to use libunwind from this runtime set.
• -DLIBCXX_USE_COMPILER_RT=ON: Uses compiler-rt runtime libraries when building libcxx.
• -DLIBCXXABI_USE_COMPILER_RT=ON: Uses compiler-rt runtime libraries when building libcxxabi.
• -DLIBUNWIND_USE_COMPILER_RT=ON: Uses compiler-rt runtime libraries when building libunwind.
• -DCMAKE_BUILD_TYPE=Release: Uses release optimizations for runtime libraries.
• DESTDIR="$LBI_ROOT" ninja -C build-runtimes install: Installs into the target tree without writing to the host filesystem.

For additional runtime configuration details and accepted CMake options, see upstream documentation:

• https://llvm.org/docs/CMake.html