LLVM libunwind is an implementation of the interface defined by the HP libunwind project. It was contributed Apple as a way to enable clang++ to port to platforms that do not have a system unwinder. It is intended to be a small and fast implementation of the ABI, leaving off some features of HP's libunwind that never materialized (e.g. remote unwinding).
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Unversioned shared library for LLVM libunwind