Fix unwinding past main on OSX

When reaching the bottom of the real stack, OSX does not change the $pc
to 0x0, like llvm libunwind.  It returns 0 from unw_step, and leaves the
$pc unchanged, so we will track cur and prev pc and guard for this as well
Fixes Localloc codegen test, and also verified a simple throw from main
works as expected.

1 files changed, 22 insertions, 0 deletions

diff --git a/src/pal/src/exception/seh-unwind.cpp b/src/pal/src/exception/seh-unwind.cpp
index 6a547979b1..8d9a06b144 100644
--- a/src/pal/src/exception/seh-unwind.cpp
+++ b/src/pal/src/exception/seh-unwind.cpp
@@ -125,6 +125,9 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP
     int st;
     unw_context_t unwContext;
     unw_cursor_t cursor;
+#if defined(__APPLE__)
+    DWORD64 curPc;
+#endif
 
 #if UNWIND_CONTEXT_IS_UCONTEXT_T
     WinContextToUnwindContext(context, &unwContext);
@@ -147,6 +150,18 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP
     WinContextToUnwindCursor(context, &cursor);
 #endif
 
+#if defined(__APPLE__)
+    // OSX appears to do two different things when unwinding
+    // 1: If it reaches where it cannot unwind anymore, say a 
+    // managed frame.  It wil return 0, but also update the $pc
+    // 2: If it unwinds all the way to _start it will return
+    // 0 from the step, but $pc will stay the same.
+    // The behaviour of libunwind from nongnu.org is to null the PC
+    // So we bank the original PC here, so we can compare it after
+    // the step
+    curPc = context->Rip;
+#endif
+
     st = unw_step(&cursor);
     if (st < 0)
     {
@@ -154,7 +169,14 @@ BOOL PAL_VirtualUnwind(CONTEXT *context, KNONVOLATILE_CONTEXT_POINTERS *contextP
     }
 
     // Update the passed in windows context to reflect the unwind
+    //
     UnwindContextToWinContext(&cursor, context);
+#if defined(__APPLE__)
+    if (st == 0 && context->Rip == curPc)
+    {
+        context->Rip = 0;
+    }
+#endif
 
     if (contextPointers != NULL)
     {