Fix for TestNoreturnUnwind.py on i386

Summary:
The testcase TestNoreturnUnwind.py was failing
because the unwind from the vdso library was not
successful for clang compiler while it was passing
for gcc. It was passing for gcc since the unwind plan
used was the assembly plan and the ebp register was
set by the main function in case of gcc and was not
used by the functions in the call flow to the vdso, whereas
clang did not emit assembly prologue for main and so
 the assembly unwind was failing. Normally in case of
failure of assembly unwind, lldb switches to EH CFI frame
based unwinding, but this was not happening for
the first frame. This patch tries to fix this behaviour by
falling to EH CFI frame based unwinding in case of assembly
unwind failure even for the first frame.
The test is still marked as XFAIL since it relys on the fix
of another bug.

Reviewers: lldb-commits, jingham, zturner, tberghammer, jasonmolenda

Subscribers: jasonmolenda

Differential Revision: http://reviews.llvm.org/D15046

llvm-svn: 257465

2 files changed, 27 insertions, 4 deletions

diff --git a/lldb/packages/Python/lldbsuite/test/functionalities/unwind/noreturn/TestNoreturnUnwind.py b/lldb/packages/Python/lldbsuite/test/functionalities/unwind/noreturn/TestNoreturnUnwind.py
index a419500f7a1..c0a311f7ac9 100644
--- a/lldb/packages/Python/lldbsuite/test/functionalities/unwind/noreturn/TestNoreturnUnwind.py
+++ b/lldb/packages/Python/lldbsuite/test/functionalities/unwind/noreturn/TestNoreturnUnwind.py
@@ -14,7 +14,7 @@ import lldbsuite.test.lldbutil as lldbutil
 class NoreturnUnwind(TestBase):
     mydir = TestBase.compute_mydir(__file__)
 
-    @expectedFailurei386 #xfail to get buildbot green, failing config: i386 binary running on ubuntu 14.04 x86_64
+    @expectedFailurei386("llvm.org/pr25338")
     @skipIfWindows # clang-cl does not support gcc style attributes.
     def test (self):
         """Test that we can backtrace correctly with 'noreturn' functions on the stack"""
diff --git a/lldb/source/Plugins/Process/Utility/RegisterContextLLDB.cpp b/lldb/source/Plugins/Process/Utility/RegisterContextLLDB.cpp
index 278a1d5dabf..efda0edb70c 100644
--- a/lldb/source/Plugins/Process/Utility/RegisterContextLLDB.cpp
+++ b/lldb/source/Plugins/Process/Utility/RegisterContextLLDB.cpp
@@ -265,9 +265,32 @@ RegisterContextLLDB::InitializeZerothFrame()
 
     if (!ReadCFAValueForRow (row_register_kind, active_row, m_cfa))
     {
-        UnwindLogMsg ("could not read CFA register for this frame.");
-        m_frame_type = eNotAValidFrame;
-        return;
+        // Try the fall back unwind plan since the
+        // full unwind plan failed.
+        FuncUnwindersSP func_unwinders_sp;
+        UnwindPlanSP call_site_unwind_plan;
+        bool cfa_status = false;
+
+        if (m_sym_ctx_valid)
+        {
+            func_unwinders_sp = pc_module_sp->GetObjectFile()->GetUnwindTable().GetFuncUnwindersContainingAddress (m_current_pc, m_sym_ctx);
+        }
+
+        if(func_unwinders_sp.get() != nullptr)
+            call_site_unwind_plan = func_unwinders_sp->GetUnwindPlanAtCallSite(process->GetTarget(), m_current_offset_backed_up_one);
+
+        if (call_site_unwind_plan.get() != nullptr)
+        {
+            m_fallback_unwind_plan_sp = call_site_unwind_plan;
+            if(TryFallbackUnwindPlan())
+                cfa_status = true;
+        }
+        if (!cfa_status)
+        {
+            UnwindLogMsg ("could not read CFA value for first frame.");
+            m_frame_type = eNotAValidFrame;
+            return;
+        }
     }
 
     UnwindLogMsg ("initialized frame current pc is 0x%" PRIx64 " cfa is 0x%" PRIx64 " using %s UnwindPlan",