LLVM Unwinder - libunwind
libunwind is an implementation of the interface defined by the HP libunwind project. It was contributed by 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).
The unwinder has two levels of API. The high level APIs are the _Unwind_* functions which implement functionality required by __cxa_* exception functions. The low level APIs are the unw_* functions which are an interface defined by the old HP libunwind project.