[PATCH] Fix _Unwind_Backtrace for libc++abi built with libgcc.

[Logan Chien]

Hi danalbert, rengolin, jroelofs,

Implement an undocumented _US_FORCE_UNWIND state for force
unwinding.

http://reviews.llvm.org/D7031

Files:
  src/cxa_personality.cpp

Index: src/cxa_personality.cpp
===================================================================
--- src/cxa_personality.cpp
+++ src/cxa_personality.cpp
@@ -1101,9 +1101,17 @@
     _Unwind_SetGR(context, REG_UCB, 
reinterpret_cast<uint32_t>(unwind_exception));
 #endif
 
+    // Check the undocumented force unwinding behavior
+    const unsigned _US_FORCE_UNWIND = 8u;
+    bool is_force_unwinding = static_cast<unsigned>(state) & _US_FORCE_UNWIND;
+    state = static_cast<_Unwind_State>(static_cast<unsigned>(state) & 
~_US_FORCE_UNWIND);
+
     scan_results results;
     switch (state) {
     case _US_VIRTUAL_UNWIND_FRAME:
+        if (is_force_unwinding)
+            return continue_unwind(unwind_exception, context);
+
         // Phase 1 search:  All we're looking for in phase 1 is a handler that 
halts unwinding
         scan_eh_tab(results, _UA_SEARCH_PHASE, native_exception, 
unwind_exception, context);
         if (results.reason == _URC_HANDLER_FOUND)

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Index: src/cxa_personality.cpp
===================================================================
--- src/cxa_personality.cpp
+++ src/cxa_personality.cpp
@@ -1101,9 +1101,17 @@
     _Unwind_SetGR(context, REG_UCB, reinterpret_cast<uint32_t>(unwind_exception));
 #endif
 
+    // Check the undocumented force unwinding behavior
+    const unsigned _US_FORCE_UNWIND = 8u;
+    bool is_force_unwinding = static_cast<unsigned>(state) & _US_FORCE_UNWIND;
+    state = static_cast<_Unwind_State>(static_cast<unsigned>(state) & ~_US_FORCE_UNWIND);
+
     scan_results results;
     switch (state) {
     case _US_VIRTUAL_UNWIND_FRAME:
+        if (is_force_unwinding)
+            return continue_unwind(unwind_exception, context);
+
         // Phase 1 search:  All we're looking for in phase 1 is a handler that halts unwinding
         scan_eh_tab(results, _UA_SEARCH_PHASE, native_exception, unwind_exception, context);
         if (results.reason == _URC_HANDLER_FOUND)

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