This patch is to enable exception handling on the z/OS platform that is compatible with the existing z/OS runtime. It uses an external z/OS specific unwind header to match the unwinder present on z/OS. No functionality of libcxxabi has been changed for other platforms. With this patch the hope is we can add z/OS as a platform to perform testing on any C++ ABI changes.
There is a primary difference for the z/OS implementation. On z/OS the thrown object is added to a linked list of caught and uncaught exceptions. The unwinder uses the top one as the current exception it is trying to find the landing pad for. We have to pop the top exception after we get it’s landing pad for our unwinder to correctly get any subsequent rethrows or nested exception calls.