diff --git a/lib/ossbuild-common.bash b/lib/ossbuild-common.bash
new file mode 100644
index 0000000..cea30d1
--- /dev/null
+++ b/lib/ossbuild-common.bash
@@ -0,0 +1,6 @@
+# Helper functions for project build scripts.
+
+# Check is current system is running GNU/Linux based QNAP QTS OS.
+is-qnap-qts() {
+    [ -r '/etc/os-release' ] && grep -q '^NAME="QTS"$' '/etc/os-release'
+}
diff --git a/projects/llvm/README.org b/projects/llvm/README.org
new file mode 100644
index 0000000..d15afd9
--- /dev/null
+++ b/projects/llvm/README.org
@@ -0,0 +1,21 @@
+#+title: LLVM Compiler Infrastructure
+
+See https://llvm.org.
+
+Tools:
+- clang(++)
+- clang-extra-tools
+  - clangd
+  - clang-format
+  - clang-tidy
+  - ...
+- lld
+- lldb
+- ...
+
+Runtimes
+- libc++
+- libc++abi
+- libunwind
+
+To build the runtimes, the build script uses the [[https://libcxx.llvm.org/BuildingLibcxx.html#bootstrapping-build][Bootstrapping build]] method.
diff --git a/projects/llvm/ossbuild-llvm.bash b/projects/llvm/ossbuild-llvm.bash
new file mode 100755
index 0000000..406c53b
--- /dev/null
+++ b/projects/llvm/ossbuild-llvm.bash
@@ -0,0 +1,284 @@
+#!/usr/bin/env bash
+
+LIB_DIR="$(dirname "$(realpath "${BASH_SOURCE[0]}")")/../../lib"
+source "${LIB_DIR}/ossbuild-common.bash"
+
+llvm-source-workon() {
+    echo "[INF] ${FUNCNAME[0]} ..."
+    local version="${1}"
+
+    local repository="https://github.com/llvm/llvm-project"
+    local archive="llvm-project-${version}.src.tar.xz"
+    pushd "${HOME}/work/oss" &> /dev/null
+    [ -e "${archive}" ] || {
+        wget "${repository}/releases/download/llvmorg-${version}/${archive}"
+    }
+
+    local src_dir
+    src_dir="${HOME}/work/oss/$(basename "${archive}" .tar.xz)"
+
+    [ -d "${src_dir}" ] || tar xvf "${archive}"
+    popd &> /dev/null
+    pushd "${src_dir}" &> /dev/null
+}
+
+llvm-cmake-generate() {
+    echo "[INF] ${FUNCNAME[0]} ..."
+    local build_type="${1:-Release}"
+    local projects="${2:-}"
+    local prefix="${3:-}"
+
+    local source_dir='llvm'
+    [ -d "${source_dir}" ] || {
+        echo "[ERR] could not find source dir: '${source_dir}'"
+        return 1
+    }
+
+    local -a cmake_extra_args
+    local build_dir="build-${build_type,,}"
+    if [ -n "${projects}" ]; then
+        build_dir="${build_dir}.${projects//;/.}"
+    else
+        # get runtime target from GCC
+        local gcc_runtime_target
+        gcc_runtime_target="$(gcc -xc /dev/null -v -E 2>&1 | grep '^Target:' | cut -d' ' -f2)"
+        #projects='clang;clang-tools-extra;lld;lldb;compiler-rt;libunwind;libcxx;libcxxabi'
+        projects='clang;clang-tools-extra;compiler-rt;lld;lldb'
+        cmake_extra_args+=(
+            -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind"
+
+            # must match triple of GCC used for first build step (here:
+            # /opt/lib/gcc/x86_64-openwrt-linux-gnu/8.4.0)
+            -DLLVM_RUNTIME_TARGETS="${gcc_runtime_target}"
+            # required )in case of a non-standard triple) in order to allow the
+            # final clang to find the GCC resource dir which contains files
+            # required by the linker (like crti.o, libgcc, libc_nonshared.a in
+            # e.g. /opt/lib/gcc/x86_64-openwrt-linux-gnu/8.4.0)
+            #
+            # Note: This also sets LLVM_DEFAULT_TARGET_TRIPLE.
+            -DLLVM_HOST_TRIPLE="${gcc_runtime_target}"
+
+            # Avoid error during cmake toolchain generation:
+            #
+            # ERROR: Compiler doesn't support generation of unwind tables if
+            # exception support is disabled. Building libunwind DSO with runtime
+            # dependency on C++ ABI library is not supported.
+            -DRUNTIMES_${gcc_runtime_target}_LIBUNWIND_ENABLE_SHARED="Off"
+        )
+    fi
+    local log="${build_dir/build-/cmake-}.log"
+
+    if [ -z "${prefix}" ]; then
+        if is-qnap-qts; then
+            prefix="$(readlink /opt)/local"
+        else
+            prefix='/usr/local'
+        fi
+    fi
+
+    if [ -d "${build_dir}" ]; then
+        if [ -v FORCE_REGENERATE ]; then
+            if [ "${FORCE_REGENERATE}" != "no-rm" ]; then
+               rm -rf "${build_dir}"
+            fi
+        else
+            echo "[INF] keep existing build dir: '${build_dir}'"
+            return 0
+        fi
+    fi
+
+    local -a cmd=(
+        cmake
+        -S "${source_dir}"
+        -B "${build_dir}"
+        -G Ninja
+        -DCMAKE_BUILD_TYPE="${build_type}"
+        -DCMAKE_INSTALL_PREFIX="${prefix}"
+        -DLLVM_ENABLE_PROJECTS="${projects}"
+        #-DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,\$ORIGIN/../lib -L${prefix}/lib"
+        -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,${prefix}/lib -L${prefix}/lib -L/opt/lib"
+        "${cmake_extra_args[@]}"
+    )
+
+    if command -v ccache &> /dev/null; then
+        cmd+=(
+            -DLLVM_CCACHE_BUILD="ON"
+            -DLLVM_CCACHE_PARAMS="CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_BASEDIR='${PWD}/${build_dir}'"
+        )
+    fi
+
+    if is-qnap-qts; then
+        # get gcc_install_prefix
+        local gcc_install_prefix
+        gcc_install_prefix="$(gcc -x c -E /dev/null -v 2>&1 | rg 'Configured with' | sed -e 's/.* --prefix=\([^ ]\+\).*/\1/')"
+
+        cmd+=(
+            # enable clang to locate the GCC toolchain if it is in a
+            # non-standard location (like ~/opt~ on QNAP Entware)
+            -DGCC_INSTALL_PREFIX="${gcc_install_prefix}"
+
+            # CMAKE_EXE_LINKER_FLAGS and LLVM_PARALLEL_LINK_JOBS flags reduce linker
+            # memory usage (without them, llvm build fails on qnas)
+            -DCMAKE_EXE_LINKER_FLAGS="-Wl,--reduce-memory-overheads -Wl,--hash-size=1021"
+            -DLLVM_PARALLEL_LINK_JOBS=1
+
+            # avoid cmake warning: 'runtime library [libz.so.1] in /lib may be
+            # hidden by files in ...'
+            -DZLIB_ROOT="${prefix}"
+        )
+
+        # From https://lists.llvm.org/pipermail/cfe-users/2016-November/001076.html:
+        #
+        # ... gcc's configure file comes with handy flags like:
+        # - --with-local-prefix= which allows to override the default system
+        #   header include /usr/local/include
+        # - --with-native-system-header-dir= to override the default system
+        #   header include /include
+        # - --enable-linker-build-id to enable build ids by default (in clang
+        #   the equivalent is -DENABLE_LINKER_BUILD_ID)
+        # - --with-linker-hash-style=gnu to force the gnu hash style when
+        #   emitting the symbol table.
+        #
+        # However, LLVM/clang doesn't seem to provide an equivalent for
+        # --with-local-prefix, --with-native-system-header-dir and
+        # --with-linker-hash-style. So instead, some sources need to be
+        # patched.
+        echo
+        local clang_toolchain_src
+        clang_toolchain_src='clang/lib/Driver/ToolChains/Linux.cpp'
+        [ -e "${clang_toolchain_src}.orig" ] || {
+            echo "[INF] patching ${clang_toolchain_src} ..."
+            sed -i.orig \
+                -e 's,/usr/,/opt/,g' "${clang_toolchain_src}"
+        }
+        echo "[INF] using patch:"
+        diff -u "${clang_toolchain_src}"{.orig,} || true
+        echo "[INF] end of patch (${clang_toolchain_src})"
+
+        echo
+        clang_toolchain_src='clang/lib/Driver/ToolChains/Gnu.cpp'
+        [ -e "${clang_toolchain_src}.orig" ] || {
+            echo "[INF] patching ${clang_toolchain_src} ..."
+            sed -i.orig \
+                -e 's,\(basePath = "/\)usr\(/lib/gcc/"\),\1opt\2,g' \
+                "${clang_toolchain_src}"
+        }
+        echo "[INF] using patch:"
+        diff -u "${clang_toolchain_src}"{.orig,} || true
+        echo "[INF] end of patch (${clang_toolchain_src})"
+    fi
+
+    echo "[INF] generating build system for: ${projects}" | tee "${log}"
+    echo "${cmd[@]}" | tee -a "${log}"
+    "${cmd[@]}" 2>&1 | tee -a "${log}"
+    local -i retval=${?}
+
+    echo
+    local logprfx="INF"
+    if [ ${retval} -ne 0 ] || grep -q "Configuring incomplete" "${log}" ; then
+        logprfx="ERR"
+        echo "[${logprfx}] failed to generate build system for: ${projects}"
+        [ ${retval} -ne 0 ] || retval=10
+    else
+        echo "[${logprfx}] successfully generated build system for: ${projects}"
+    fi
+
+    echo "[${logprfx}] build dir: '${build_dir}'"
+    echo "[${logprfx}] cmake log: '${log}'"
+
+    return ${retval}
+}
+
+# shellcheck disable=SC2120
+llvm-cmake-build() {
+    echo "[INF] ${FUNCNAME[0]} ..."
+    local build_type="${1:-Release}"
+    local build_args="${2:-}"
+    local build_dir="${3:-build-${build_type,,}}"
+    local cmd=(
+        cmake --build "${build_dir}"
+        --
+    )
+
+    if [ -n "${build_args}" ]; then
+        read -a extra_args <<< "${build_args}"
+        cmd+=(
+            "${extra_args[@]}"
+        )
+    elif is-qnap-qts; then
+        cmd+=(
+            # compiling on QTS sometimes fails (probably due to memory
+            # limitations), so limit the parallel jobs to 2
+            -j2
+            # don't stop at failures
+            -k0
+        )
+    fi
+
+    # enable verbose logging when V is non-empty
+    [ -z "${V:-}" ] || cmd+=(-v)
+
+    echo "${cmd[@]}"
+    "${cmd[@]}"
+}
+
+llvm-cmake-install() {
+    echo "[INF] ${FUNCNAME[0]} ..."
+    local build_type="${1:-Release}"
+    local build_dir="${2:-build-${build_type,,}}"
+    local cmd=(
+        sudo cmake --build "${build_dir}"
+        --
+        install
+    )
+    echo "${cmd[@]}"
+    "${cmd[@]}"
+}
+
+llvm-cmake-generate-fallback() {
+    echo "[INF] ${FUNCNAME[0]} ..."
+    local build_type="${1:-Release}"
+    local build_dir="${2:-build-${build_type,,}}"
+
+    # Fallback 1: Build a smaller subset of LLVM projects
+    local llvm_projects=(
+        clang
+        clang-tools-extra
+        lld
+    )
+    local IFS=';'
+    llvm-cmake-generate "${build_type}" "${llvm_projects[*]}"
+
+    # Fallback 2: Generate build systems for individual projects
+    IFS=';' read -a LLVM_PROJECTS \
+       <<< $(grep 'set(LLVM_ALL_PROJECTS ' llvm/CMakeLists.txt | cut -d\" -f2)
+    local project
+    for project in "${llvm_projects[@]}"; do
+        llvm-cmake-generate "${build_type}" "${project}"
+    done
+}
+
+ossbuild-llvm() {
+    local version="${1:-15.0.5}"
+    local build_type="${2:-Release}" # or 'RelWithDebInfo', but beware of the huge binaries
+
+    llvm-source-workon "${version}"
+    llvm-cmake-generate "${build_type}"
+
+    # Ideally, llcm-cmake-build should build the most LLVM projects. However, if
+    # it doesn't, it might make sense to build projects individually (see
+    # `llvm-cmake-generate-fallback` above).
+    llvm-cmake-build "${build_type}"
+    # Install the toolchain
+    llvm-cmake-install "${build_type}"
+
+    return $?
+}
+
+
+# stop here when sourced
+return 0 2>/dev/null || true
+set -euo pipefail
+[ "${DEBUG:-0}" -le 0 ] || set -x
+
+ossbuild-llvm "${@}"