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6.22. llvm/clang pass 2 22.1.3

The second llvm/clang pass installs the final compiler toolchain, then refreshes the LLVM runtimes with standalone cross builds and fixes Clang's default search paths for the book's custom filesystem layout.

Input assumption: llvm-project-22.1.3.src.tar.xz is already present in LBI_SOURCES, and sections 5.4 and 5.5 completed successfully.

Licenses

Apache-2.0 WITH LLVM-exception

Dependencies

musl (libc)
cmake
ninja
zlib-ng (libz)
llvm/clang pass 1

llvm is a modular compiler infrastructure project. we need it to provide the final compiler backend, linker integration, and toolchain libraries for the target system.

clang is a C and C++ frontend for LLVM. we need it to provide the final clang and clang++ compilers for the target system.

compiler-rt is the LLVM runtime support library project. we need it to provide Clang builtins and startup objects.

libunwind is a platform-independent stack unwinding library. we need it to provide unwind support used by C++ exception handling.

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

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

Extract Sources and Enter the LLVM Build Root

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

Configure llvm/clang Pass 2

Cross-build note: this pass follows LLVM's cross-compilation guidance for the compiler build itself. The top-level LLVM_ENABLE_RUNTIMES flow is intentionally not used here, because it tries to configure compiler-rt builtins with the just-built target compiler, which is not host-runnable in this setup.

lbi_cmake build-llvm-pass2 \
    -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_AR="$LBI_ROOT/system/tools/bin/$LBI_TARGET-ar" \
    -DCMAKE_RANLIB="$LBI_ROOT/system/tools/bin/$LBI_TARGET-ranlib" \
    -DCMAKE_NM="$LBI_ROOT/system/tools/bin/$LBI_TARGET-nm" \
    -DCMAKE_OBJCOPY="$LBI_ROOT/system/tools/bin/$LBI_TARGET-objcopy" \
    -DCMAKE_OBJDUMP="$LBI_ROOT/system/tools/bin/$LBI_TARGET-objdump" \
    -DCMAKE_STRIP="$LBI_ROOT/system/tools/bin/$LBI_TARGET-strip" \
    -DCMAKE_SYSROOT="$LBI_ROOT" \
    -DCMAKE_C_COMPILER_LAUNCHER="ccache" \
    -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
    -DCMAKE_FIND_ROOT_PATH="$LBI_ROOT;$LBI_ROOT/system" \
    -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER \
    -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY \
    -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY \
    -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY \
    -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
    -DCMAKE_C_COMPILER_TARGET="$LBI_TARGET" \
    -DCMAKE_CXX_COMPILER_TARGET="$LBI_TARGET" \
    -DCMAKE_ASM_COMPILER_TARGET="$LBI_TARGET" \
    -DCMAKE_C_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -isystem /system/headers $LWI_CFLAGS" \
    -DCMAKE_CXX_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -isystem /system/headers $LWI_CXXFLAGS" \
    -DCMAKE_SHARED_LINKER_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -B/system/libraries -B/system/libraries/clang/22/lib/linux -L/system/libraries $LBI_CUSTOM_LDFLAGS" \
    -DCMAKE_EXE_LINKER_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -B/system/libraries -B/system/libraries/clang/22/lib/linux -L/system/libraries $LBI_CUSTOM_LDFLAGS" \
    -DLLVM_NATIVE_TOOL_DIR="$LBI_ROOT/system/tools/bin" \
    -DLLVM_TABLEGEN="$LBI_ROOT/system/tools/bin/llvm-tblgen" \
    -DCLANG_TABLEGEN="$LBI_ROOT/system/tools/bin/clang-tblgen" \
    -DLLVM_ENABLE_PROJECTS="clang;lld" \
    -DLLVM_INSTALL_BINUTILS_SYMLINKS=ON \
    -DLLVM_HOST_TRIPLE="$LBI_TARGET" \
    -DLLVM_TARGETS_TO_BUILD="X86" \
    -DLLVM_DEFAULT_TARGET_TRIPLE="$LBI_TARGET" \
    -DDEFAULT_SYSROOT="/" \
    -DCLANG_DEFAULT_CXX_STDLIB=libc++ \
    -DCLANG_DEFAULT_LINKER=lld \
    -DCLANG_DEFAULT_RTLIB=compiler-rt \
    -DCLANG_DEFAULT_UNWINDLIB=libunwind \
    -DLLVM_ENABLE_ZLIB=ON \
    -DLLVM_ENABLE_ZSTD=OFF \
    -DLLVM_ENABLE_LIBXML2=OFF \
    -DLLVM_INCLUDE_TESTS=OFF \
    -DLLVM_BUILD_TESTS=OFF \
    -DCMAKE_BUILD_TYPE=Release

Build llvm/clang Pass 2

cmake --build build-llvm-pass2 $LWI_MAKE_FLAGS

Install llvm/clang Pass 2

DESTDIR="$LBI_ROOT" cmake --install build-llvm-pass2

Refresh LLVM C++ Runtimes

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

lbi_cmake build-runtimes-pass2 \
    -G Ninja \
    -DCMAKE_C_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang" \
    -DCMAKE_CXX_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang++" \
    -DLLVM_CMAKE_DIR="$LBI_SOURCES/llvm-project-22.1.3.src/llvm/build-llvm-pass2/lib/cmake/llvm" \
    -DCMAKE_SYSROOT="$LBI_ROOT" \
    -DCMAKE_C_COMPILER_LAUNCHER="ccache" \
    -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
    -DCMAKE_FIND_ROOT_PATH="$LBI_ROOT;$LBI_ROOT/system" \
    -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER \
    -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=$LBI_TARGET --sysroot=$LBI_ROOT -isystem /system/headers $LWI_CFLAGS" \
    -DCMAKE_CXX_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -isystem /system/headers $LWI_CXXFLAGS" \
    -DCMAKE_EXE_LINKER_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -B/system/libraries -B/system/libraries/clang/22/lib/linux -L/system/libraries $LBI_CUSTOM_LDFLAGS" \
    -DLLVM_ENABLE_RUNTIMES="libunwind;libcxxabi;libcxx" \
    -DLLVM_ENABLE_LIBXML2=OFF \
    -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF \
    -DLLVM_INCLUDE_TESTS=OFF \
    -DLLVM_INCLUDE_DOCS=OFF \
    -DLIBUNWIND_INSTALL_LIBRARY_DIR=/system/libraries \
    -DLIBUNWIND_INCLUDE_TESTS=OFF \
    -DLIBUNWIND_INCLUDE_DOCS=OFF \
    -DLIBCXXABI_INSTALL_LIBRARY_DIR=/system/libraries \
    -DLIBCXXABI_INCLUDE_TESTS=OFF \
    -DLIBCXX_INSTALL_LIBRARY_DIR=/system/libraries \
    -DLIBCXX_INCLUDE_TESTS=OFF \
    -DLIBCXX_INCLUDE_BENCHMARKS=OFF \
    -DLIBCXX_INCLUDE_DOCS=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

cmake --build build-runtimes-pass2 $LWI_MAKE_FLAGS
DESTDIR="$LBI_ROOT" cmake --install build-runtimes-pass2

Build compiler-rt Builtins and CRT Objects

Compiler-rt note: this follows upstream compiler-rt cross-build guidance for standalone builtins and CRT builds instead of the failing top-level LLVM runtimes wrapper.

cd "$LBI_SOURCES/llvm-project-22.1.3.src/compiler-rt"

lbi_cmake build-compiler-rt-pass2 \
    -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_ASM_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang" \
    -DCMAKE_AR="$LBI_ROOT/system/tools/bin/$LBI_TARGET-ar" \
    -DCMAKE_NM="$LBI_ROOT/system/tools/bin/$LBI_TARGET-nm" \
    -DCMAKE_RANLIB="$LBI_ROOT/system/tools/bin/$LBI_TARGET-ranlib" \
    -DLLVM_CMAKE_DIR="$LBI_SOURCES/llvm-project-22.1.3.src/llvm/build-llvm-pass2/lib/cmake/llvm" \
    -DCMAKE_SYSROOT="$LBI_ROOT" \
    -DCMAKE_C_COMPILER_LAUNCHER="ccache" \
    -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
    -DCMAKE_C_COMPILER_TARGET="$LBI_TARGET" \
    -DCMAKE_CXX_COMPILER_TARGET="$LBI_TARGET" \
    -DCMAKE_ASM_COMPILER_TARGET="$LBI_TARGET" \
    -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
    -DCMAKE_C_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -isystem /system/headers $LWI_CFLAGS" \
    -DCMAKE_CXX_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -isystem /system/headers $LWI_CXXFLAGS" \
    -DCMAKE_ASM_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT $LWI_CFLAGS" \
    -DCMAKE_EXE_LINKER_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT -B/system/libraries -B/system/libraries/clang/22/lib/linux -L/system/libraries $LBI_CUSTOM_LDFLAGS" \
    -DCOMPILER_RT_INSTALL_PATH=/system/lib/clang/22 \
    -DCOMPILER_RT_BUILD_BUILTINS=ON \
    -DCOMPILER_RT_BUILD_CRT=ON \
    -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
    -DCOMPILER_RT_BUILD_MEMPROF=OFF \
    -DCOMPILER_RT_BUILD_ORC=OFF \
    -DCOMPILER_RT_BUILD_PROFILE=OFF \
    -DCOMPILER_RT_BUILD_CTX_PROFILE=OFF \
    -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
    -DCOMPILER_RT_BUILD_XRAY=OFF \
    -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
    -DCOMPILER_RT_INCLUDE_TESTS=OFF \
    -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF \
    -DCMAKE_BUILD_TYPE=Release

cmake --build build-compiler-rt-pass2 --target builtins $LWI_MAKE_FLAGS
cmake --build build-compiler-rt-pass2 --target crt $LWI_MAKE_FLAGS
DESTDIR="$LBI_ROOT" cmake --install build-compiler-rt-pass2

Make Clang Resource Files Visible in the Final Tree

mkdir -p "$LBI_ROOT/system/lib/clang/22/lib/$LBI_TARGET"

if [ -f "$LBI_ROOT/system/tools/lib/clang/22/lib/$LBI_TARGET/libclang_rt.builtins.a" ]; then
    ln -sf "/system/tools/lib/clang/22/lib/$LBI_TARGET/libclang_rt.builtins.a" \
        "$LBI_ROOT/system/lib/clang/22/lib/$LBI_TARGET/libclang_rt.builtins.a"
fi

Create Compiler and Linker Compatibility Symlinks

cd "$LBI_ROOT/system/binaries"

ln -sf ld.lld ld
ln -sf clang cc
ln -sf clang++ c++

ln -sf clang "$LBI_TARGET-clang"
ln -sf clang++ "$LBI_TARGET-clang++"
ln -sf clang "$LBI_TARGET-cc"
ln -sf clang++ "$LBI_TARGET-c++"
ln -sf ld.lld "$LBI_TARGET-ld"

Link Clang Startup Objects for musl Compatibility

CRTBEGIN_OBJ=$(find "$LBI_ROOT/system/libraries/clang" \
    -type f \( -name 'crtbeginS.o' -o -name 'clang_rt.crtbegin*.o' \) | head -n1)
CRTEND_OBJ=$(find "$LBI_ROOT/system/libraries/clang" \
    -type f \( -name 'crtendS.o' -o -name 'clang_rt.crtend*.o' \) | head -n1)

CRT_DIR=$(dirname "$CRTBEGIN_OBJ")

if [ -n "$CRTBEGIN_OBJ" ] && [ -n "$CRTEND_OBJ" ]; then
    ln -sf "$(basename "$CRTBEGIN_OBJ")" "$CRT_DIR/crtbeginS.o"
    ln -sf "$(basename "$CRTEND_OBJ")" "$CRT_DIR/crtendS.o"

    ln -sf "${CRTBEGIN_OBJ#$LBI_ROOT/system}" "$LBI_ROOT/system/libraries/crtbeginS.o"
    ln -sf "${CRTEND_OBJ#$LBI_ROOT/system}" "$LBI_ROOT/system/libraries/crtendS.o"
fi

Add Clang Driver Wrapper Defaults

mv "$LBI_ROOT/system/binaries/clang" "$LBI_ROOT/system/binaries/clang.real"
mv "$LBI_ROOT/system/binaries/clang++" "$LBI_ROOT/system/binaries/clang++.real"

cat > "$LBI_ROOT/system/binaries/clang" <<'EOF'
#!/bin/sh
exec /system/binaries/clang.real \
    -isystem /system/headers \
    -B/system/libraries \
    -Wno-unused-command-line-argument \
    -B/system/libraries/clang/22/lib/linux \
    -L/system/libraries \
    "$@"
EOF

cat > "$LBI_ROOT/system/binaries/clang++" <<'EOF'
#!/bin/sh
exec /system/binaries/clang++.real \
    -nostdinc++ \
    -I/system/headers/c++/v1 \
    -isystem /system/libraries/clang/22/include \
    -isystem /system/headers \
    -B/system/libraries \
    -B/system/libraries/clang/22/lib/linux \
    -L/system/libraries \
    -Wl,-rpath,/system/libraries \
    "$@" \
    -Wno-unused-command-line-argument \
    -lc++ \
    -lc++abi \
    -lunwind
EOF

chmod 755 "$LBI_ROOT/system/binaries/clang" "$LBI_ROOT/system/binaries/clang++"

cd "$LBI_ROOT/system/binaries"
ln -sf clang cc
ln -sf clang++ c++
ln -sf clang "$LBI_TARGET-clang"
ln -sf clang++ "$LBI_TARGET-clang++"
ln -sf clang "$LBI_TARGET-cc"
ln -sf clang++ "$LBI_TARGET-c++"

Command Explanations

LBI_HOST_NINJA=$(command -v ninja || command -v samu || true): Finds a host-runnable Ninja-compatible build tool before CMake has a chance to cache a target-side executable.
if [ -z "$LBI_HOST_NINJA" ]; then ... exit 1; fi: Stops early with a clear error if the host does not have ninja or samu.
lbi_cmake build-llvm-pass2: Uses the book's CMake helper for install layout while placing generated files in the build-llvm-pass2 directory.
-DCMAKE_C_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang" and -DCMAKE_CXX_COMPILER=...: Build pass 2 with the pass 1 cross compiler instead of the host compiler.
-DCMAKE_AR, -DCMAKE_RANLIB, -DCMAKE_NM, -DCMAKE_OBJCOPY, -DCMAKE_OBJDUMP, and -DCMAKE_STRIP: Use the target-prefixed LLVM binutils-compatible tools.
-DCMAKE_SYSROOT="$LBI_ROOT" and the CMAKE_FIND_ROOT_PATH* settings: Keep configure checks and dependency lookups inside the target tree while still allowing host build tools such as cmake and ninja.
-DCMAKE_MAKE_PROGRAM="$LBI_HOST_NINJA": Pins the Ninja generator to a host-runnable build executor so CMake does not cache a target-side /usr/bin/ninja from LBI_ROOT.
-DCMAKE_C_COMPILER_LAUNCHER="ccache" and -DCMAKE_CXX_COMPILER_LAUNCHER="ccache": Use ccache for repeated compiler invocations when it is available.
-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY: Prevent configure probes from requiring runnable target executables.
-DCMAKE_C_COMPILER_TARGET, -DCMAKE_CXX_COMPILER_TARGET, and -DCMAKE_ASM_COMPILER_TARGET: Tell CMake and Clang which target triple to use for C, C++, and assembly.
-DCMAKE_C_FLAGS=... -isystem /system/headers and -DCMAKE_CXX_FLAGS=... -isystem /system/headers: Make the build use the book's custom libc header location instead of assuming /usr/include.
-DCMAKE_EXE_LINKER_FLAGS=... -B/system/libraries -B/system/libraries/clang/22/lib/linux -L/system/libraries and the matching shared-linker flags: Make link steps find musl startup files, Clang CRT objects, and libc in the book's custom library layout.
-DLLVM_NATIVE_TOOL_DIR, -DLLVM_TABLEGEN, and -DCLANG_TABLEGEN: Reuse the pass 1 native helper tools instead of rebuilding host tools in this pass.
-DLLVM_ENABLE_PROJECTS="clang;lld": Build the final frontend and linker in the main compiler pass.
-DLLVM_INSTALL_BINUTILS_SYMLINKS=ON: Installs LLVM tool aliases for common binutils command names.
-DLLVM_HOST_TRIPLE="$LBI_TARGET", -DLLVM_TARGETS_TO_BUILD="X86", and -DLLVM_DEFAULT_TARGET_TRIPLE="$LBI_TARGET": Keep this pass focused on the target architecture and make the installed compiler default to the book's target triple.
-DDEFAULT_SYSROOT="/": Make the installed Clang use the target root layout at runtime instead of baking in the temporary build host path.
-DCLANG_DEFAULT_CXX_STDLIB=libc++, -DCLANG_DEFAULT_LINKER=lld, -DCLANG_DEFAULT_RTLIB=compiler-rt, and -DCLANG_DEFAULT_UNWINDLIB=libunwind: Set the intended default runtime family for later builds.
-DLLVM_ENABLE_ZLIB=ON: Enables zlib support using the target zlib-compatible library.
-DLLVM_ENABLE_ZSTD=OFF and -DLLVM_ENABLE_LIBXML2=OFF: Disable optional dependencies that are not part of this pass.
-DLLVM_INCLUDE_TESTS=OFF and -DLLVM_BUILD_TESTS=OFF: Skip LLVM tests in the target compiler build.
-DCMAKE_BUILD_TYPE=Release: Uses release optimization for the final target compiler.
cmake --build build-llvm-pass2 $LWI_MAKE_FLAGS: Builds the configured compiler tree with the shared parallel build setting.
DESTDIR="$LBI_ROOT" cmake --install build-llvm-pass2: Installs the final compiler into the target root instead of the host system.
LLVM_ENABLE_RUNTIMES is omitted from the main pass on purpose: the top-level runtime bootstrap tries to configure compiler-rt builtins with the just-built target compiler, which fails in this cross setup because that compiler is not host-runnable.
The standalone build-runtimes-pass2 step refreshes libunwind, libcxxabi, and libcxx with the final compiler and the final library/header layout.
-DLLVM_CMAKE_DIR="$LBI_SOURCES/llvm-project-22.1.3.src/llvm/build-llvm-pass2/lib/cmake/llvm": Points standalone runtime and compiler-rt builds at the just-configured LLVM package files.
The runtime CMAKE_FIND_ROOT_PATH*, CMAKE_TRY_COMPILE_TARGET_TYPE, compile flags, and linker flags repeat the same cross-build protections used for the main pass.
-DLLVM_ENABLE_RUNTIMES="libunwind;libcxxabi;libcxx": Builds only the C++ runtime stack in the standalone runtime pass.
-DLLVM_INCLUDE_DOCS=OFF, LIBUNWIND_INCLUDE_*, LIBCXXABI_INCLUDE_TESTS=OFF, and LIBCXX_INCLUDE_*: Skip tests, benchmarks, and documentation for the runtime rebuild.
-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF: Keep runtime outputs out of target-triple subdirectories where possible so they land in the normal library layout.
-DLIBUNWIND_INSTALL_LIBRARY_DIR=/system/libraries, -DLIBCXXABI_INSTALL_LIBRARY_DIR=/system/libraries, and -DLIBCXX_INSTALL_LIBRARY_DIR=/system/libraries: Force the runtime libraries into the book's custom /system/libraries path.
-DLIBCXX_HAS_MUSL_LIBC=ON: Enables libcxx behavior for the musl target.
-DLIBCXX_HAS_ATOMIC_LIB=OFF: Prevents an external -latomic dependency in this pass.
-DLIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL=OFF: Avoids relying on the glibc-specific __cxa_thread_atexit_impl symbol.
-DLIBCXXABI_USE_LLVM_UNWINDER=ON: Links libcxxabi against libunwind from this runtime set.
-DLIBCXX_USE_COMPILER_RT=ON, -DLIBCXXABI_USE_COMPILER_RT=ON, and -DLIBUNWIND_USE_COMPILER_RT=ON: Use compiler-rt runtime libraries instead of libgcc-style runtime assumptions.
cmake --build build-runtimes-pass2 $LWI_MAKE_FLAGS and DESTDIR="$LBI_ROOT" cmake --install build-runtimes-pass2: Build and install the refreshed C++ runtime stack into the target root.
lbi_cmake build-compiler-rt-pass2: Configures compiler-rt as a standalone cross build.
-DCMAKE_ASM_COMPILER="$LBI_ROOT/system/tools/bin/$LBI_TARGET-clang" and -DCMAKE_ASM_COMPILER_TARGET="$LBI_TARGET": Build assembly runtime objects with the target Clang.
-DCMAKE_ASM_FLAGS="--target=$LBI_TARGET --sysroot=$LBI_ROOT $LWI_CFLAGS": Applies the target triple and sysroot to assembly compilation.
-DCOMPILER_RT_INSTALL_PATH=/system/lib/clang/22: Install compiler-rt into Clang's resource-directory layout so the driver can find builtins by default.
-DCOMPILER_RT_BUILD_BUILTINS=ON and -DCOMPILER_RT_BUILD_CRT=ON: Build the Clang builtins library and CRT startup objects needed by later links.
-DCOMPILER_RT_BUILD_LIBFUZZER=OFF, -DCOMPILER_RT_BUILD_MEMPROF=OFF, -DCOMPILER_RT_BUILD_ORC=OFF, -DCOMPILER_RT_BUILD_PROFILE=OFF, -DCOMPILER_RT_BUILD_CTX_PROFILE=OFF, -DCOMPILER_RT_BUILD_SANITIZERS=OFF, and -DCOMPILER_RT_BUILD_XRAY=OFF: Skip runtime components that are not needed for this target bootstrap.
-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON: Limits compiler-rt output to the configured target.
-DCOMPILER_RT_INCLUDE_TESTS=OFF: Skips compiler-rt tests.
cmake --build build-compiler-rt-pass2 --target builtins $LWI_MAKE_FLAGS and --target crt: Build only the compiler-rt targets needed by the target toolchain.
DESTDIR="$LBI_ROOT" cmake --install build-compiler-rt-pass2: Installs compiler-rt outputs into the target root.
mkdir -p "$LBI_ROOT/system/lib/clang/22/lib/$LBI_TARGET": Creates the Clang resource-library directory expected by the driver.
The resource-file symlink step: Covers cases where compiler-rt still lands under /system/tools/lib/clang/... while the Clang driver looks under /system/lib/clang/....
cd "$LBI_ROOT/system/binaries": Runs compiler/linker symlink creation from the target binary directory so the relative links stay simple.
ln -sf ld.lld ld, ln -sf clang cc, and ln -sf clang++ c++: Provide conventional compiler and linker command names.
The $LBI_TARGET-* symlinks: Preserve cross-style tool names for later build systems that look for target-prefixed compilers.
find "$LBI_ROOT/system/libraries/clang" ... | head -n1: Locates the installed compiler-rt CRT begin/end objects regardless of their exact upstream-generated names.
CRT_DIR=$(dirname "$CRTBEGIN_OBJ"): Records the resource directory containing those CRT objects.
The crtbeginS.o and crtendS.o links: Make the Clang CRT objects visible both in the resource directory and in /system/libraries, where musl-oriented links and later builds expect them.
mv "$LBI_ROOT/system/binaries/clang" ... clang.real and the matching clang++ move: Preserve the real compiler binaries before installing wrapper scripts at the public command names.
cat > "$LBI_ROOT/system/binaries/clang" <<'EOF' and the matching clang++ block: Writes wrapper scripts with fixed default include, startup-object, and library search paths.
The Clang wrapper scripts: Add the header path and startup-object/library search paths needed by the book's non-FHS layout, so later package builds do not have to export manual CPPFLAGS, CFLAGS, or LDFLAGS just to find musl headers and libc.
chmod 755 "$LBI_ROOT/system/binaries/clang" "$LBI_ROOT/system/binaries/clang++": Makes the wrapper scripts executable.
The final compiler symlink block: Recreates cc, c++, and target-prefixed aliases so they point at the wrapper scripts rather than bypassing them.

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