Enable usage of eh_frame based unwind plan as a fallback

[tberghammer]

Summary

Enable usage of eh_frame based unwind plan as a fallback

Previously if the instruction emulation based unwind plan failed then
we fall back to the arch default unwind plan. Change it to fall back
to the eh_frame based one even on non call sites if we have eh_frame
as that one tend to be more reliable.

[jasonmolenda]

Great idea.

lldb/trunk/source/Plugins/Process/Utility/RegisterContextLLDB.cpp

RegisterContextLLDB::GetFullUnwindPlanForFrame ()
     // Typically the NonCallSite UnwindPlan is the unwind created by inspecting the assembly language instructions
     if (behaves_like_zeroth_frame && process)
     {
         unwind_plan_sp = func_unwinders_sp->GetUnwindPlanAtNonCallSite (process->GetTarget(), m_thread, m_current_offset_backed_up_one);
         if (unwind_plan_sp && unwind_plan_sp->PlanValidAtAddress (m_current_pc))
         {
             if (unwind_plan_sp->GetSourcedFromCompiler() == eLazyBoolNo)
             {
-                // We probably have an UnwindPlan created by inspecting assembly instructions, and we probably
-                // don't have any eh_frame instructions available.
-                // The assembly profilers work really well with compiler-generated functions but hand-written
-                // assembly can be problematic.  We'll set the architecture default UnwindPlan as our fallback
-                // UnwindPlan in case this doesn't work out when we try to unwind.
+                // We probably have an UnwindPlan created by inspecting assembly instructions. The
+                // assembly profilers work really well with compiler-generated functions but hand-
+                // written assembly can be problematic. We set the eh_frame based unwind plan as our
+                // fallback unwind plan if instruction emulation doesn't work out even for non call
+                // sites if it is available and use the architecture default unwind plan if it is
+                // not available. The eh_frame unwind plan is more reliable even on non call sites
+                // then the architecture default plan and for hand written assembly code it is often
+                // written in a way that it valid at all location what helps in the most common
+                // cases when the instruction emulation fails.
+                UnwindPlanSP eh_frame_unwind_plan = func_unwinders_sp->GetEHFrameUnwindPlan (process->GetTarget(), m_current_offset_backed_up_one);
+                if (eh_frame_unwind_plan &&
+                    eh_frame_unwind_plan.get() != unwind_plan_sp.get() &&
+                    eh_frame_unwind_plan->GetSourceName() != unwind_plan_sp->GetSourceName())
+                {
+                    m_fallback_unwind_plan_sp = eh_frame_unwind_plan;
+                }
+                else
+                {
                     m_fallback_unwind_plan_sp = arch_default_unwind_plan_sp;
                 }
+            }
             UnwindLogMsgVerbose ("frame uses %s for full UnwindPlan", unwind_plan_sp->GetSourceName().GetCString());
             return unwind_plan_sp;
         }
     }
 
     // Typically this is unwind info from an eh_frame section intended for exception handling; only valid at call sites
     if (process)
     {
RegisterContextLLDB::GetFullUnwindPlanForFrame ()
     // We'd prefer to use an UnwindPlan intended for call sites when we're at a call site but if we've
     // struck out on that, fall back to using the non-call-site assembly inspection UnwindPlan if possible.
     if (process)
     {
         unwind_plan_sp = func_unwinders_sp->GetUnwindPlanAtNonCallSite (process->GetTarget(), m_thread, m_current_offset_backed_up_one);
     }
     if (unwind_plan_sp && unwind_plan_sp->GetSourcedFromCompiler() == eLazyBoolNo)
     {
-        // We probably have an UnwindPlan created by inspecting assembly instructions, and we probably
-        // don't have any eh_frame instructions available.
-        // The assembly profilers work really well with compiler-generated functions but hand-written
-        // assembly can be problematic.  We'll set the architecture default UnwindPlan as our fallback
-        // UnwindPlan in case this doesn't work out when we try to unwind.
+        // We probably have an UnwindPlan created by inspecting assembly instructions. The assembly
+        // profilers work really well with compiler-generated functions but hand- written assembly
+        // can be problematic. We set the eh_frame based unwind plan as our fallback unwind plan if
+        // instruction emulation doesn't work out even for non call sites if it is available and use
+        // the architecture default unwind plan if it is not available. The eh_frame unwind plan is
+        // more reliable even on non call sites then the architecture default plan and for hand
+        // written assembly code it is often written in a way that it valid at all location what
+        // helps in the most common cases when the instruction emulation fails.
+        UnwindPlanSP eh_frame_unwind_plan = func_unwinders_sp->GetEHFrameUnwindPlan (process->GetTarget(), m_current_offset_backed_up_one);
+        if (eh_frame_unwind_plan &&
+            eh_frame_unwind_plan.get() != unwind_plan_sp.get() &&
+            eh_frame_unwind_plan->GetSourceName() != unwind_plan_sp->GetSourceName())
+        {
+            m_fallback_unwind_plan_sp = eh_frame_unwind_plan;
+        }
+        else
+        {
             m_fallback_unwind_plan_sp = arch_default_unwind_plan_sp;
         }
+    }
 
     if (IsUnwindPlanValidForCurrentPC(unwind_plan_sp, valid_offset))
     {
         UnwindLogMsgVerbose ("frame uses %s for full UnwindPlan", unwind_plan_sp->GetSourceName().GetCString());
         return unwind_plan_sp;
     }
 
     // If we're on the first instruction of a function, and we have an architectural default UnwindPlan