[SystemZ][z/OS] Modify cxxabi to be compatible with existing z/OS runtime

This patch is to enable exception handling on the z/OS platform that is compatible with the existing z/OS runtime. 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.

Differential Revision: https://reviews.llvm.org/D99913

GitOrigin-RevId: 85593e7bb5a8bfde80aaf4eb4e15f982aebe2282
diff --git a/src/cxa_exception.cpp b/src/cxa_exception.cpp
index b17c79e..9cb2bf8 100644
--- a/src/cxa_exception.cpp
+++ b/src/cxa_exception.cpp
@@ -444,6 +444,14 @@
+#if defined(__MVS__)
+    // Remove the exception object from the linked list of exceptions that the z/OS unwinder
+    // maintains before adding it to the libc++abi list of caught exceptions.
+    // The libc++abi will manage the lifetime of the exception from this point forward.
+    _UnwindZOS_PopException();
     if (native_exception)
         // Increment the handler count, removing the flag about being rethrown
diff --git a/src/cxa_personality.cpp b/src/cxa_personality.cpp
index f58d4de..b28c58d 100644
--- a/src/cxa_personality.cpp
+++ b/src/cxa_personality.cpp
@@ -622,7 +622,7 @@
         results.reason = _URC_FATAL_PHASE1_ERROR;
-    // Start scan by getting exception table address
+    // Start scan by getting exception table address.
     const uint8_t *lsda = (const uint8_t *)_Unwind_GetLanguageSpecificData(context);
     if (lsda == 0)
@@ -912,6 +912,8 @@
 _LIBCXXABI_FUNC_VIS _Unwind_Reason_Code
+#elif defined(__MVS__)
@@ -1121,7 +1123,7 @@
         // Either we didn't do a phase 1 search (due to forced unwinding), or
-        //  phase 1 reported no catching-handlers.
+        // phase 1 reported no catching-handlers.
         // Search for a (non-catching) cleanup
         if (is_force_unwinding)