Explicit -lunwind required with clang++/libcxx

Beginners
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Hello,

I am attempting to create a bootstrapping build of LLVM with clang, compiler-rt, libcxx, libcxxabi, and libunwind. I am performing this build in an unorthodox sandbox environment, and intend to use the resulting toolchain in the same environment, so need to create as self-contained and relocatable a toolchain as possible.

I have succeeded in producing a toolchain using the 17.0.6 source using these compiler flags:

cmake \
  -S /path/to/llvm \
  -G Ninja \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_INSTALL_LIBDIR=lib \
  -DCLANG_ENABLE_BOOTSTRAP=ON \
  -DCLANG_DEFAULT_CXX_STDLIB=libc++ \
  -DCLANG_DEFAULT_RTLIB=compiler-rt \
  -DLIBCXX_USE_COMPILER_RT=YES \
  -DLIBCXXABI_USE_COMPILER_RT=YES \
  -DLIBCXXABI_USE_LLVM_UNWINDER=YES \
  -DBOOTSTRAP_CMAKE_BUILD_TYPE=Release \
  -DBOOTSTRAP_CLANG_DEFAULT_CXX_STDLIB=libc++ \
  -DBOOTSTRAP_CLANG_DEFAULT_RTLIB=compiler-rt \
  -DBOOTSTRAP_LIBCXX_USE_COMPILER_RT=YES \
  -DBOOTSTRAP_LIBCXXABI_USE_COMPILER_RT=YES \
  -DBOOTSTRAP_LIBCXXABI_USE_LLVM_UNWINDER=YES \
  -DBOOTSTRAP_LLVM_USE_LINKER=lld \
  -DLIBUNWIND_USE_COMPILER_RT=YES \
  -DBOOTSTRAP_LIBUNWIND_USE_COMPILER_RT=YES \
  -DLLVM_ENABLE_PROJECTS='clang;clang-tools-extra;lld;lldb' \
  -DLLVM_ENABLE_RUNTIMES='compiler-rt;libcxx;libcxxabi;libunwind' \
  -DLLVM_ENABLE_TERMINFO=OFF \
  -DLLVM_ENABLE_LIBXML2=OFF \
  -DLLVM_ENABLE_RTTI=ON \
  -DLLVM_ENABLE_EH=ON \
  -DLLVM_PARALLEL_LINK_JOBS=1 \
  -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON \
  -DDEFAULT_SYSROOT=/path/to/glibc/sysroot

I can successfully compile some basic programs with the resulting clang and clang++ with -fuse-ld=lld. However, with clang++, I must explicitly include -lunwind to compile a “hello world” program. Am I missing some configuration that would make this additional linker flag unnecessary?