Add a new idea of a "fallback" UnwindPlan to the RegisterContextLLDB

class.  If we try to unwind a stack frame to find a caller stack
frame, and we fail to get a valid-looking frame, AND if the UnwindPlan
we used is an assembly-inspection based UnwindPlan, then we should 
throw away the assembly-inspection UnwindPlan and try unwinding with 
the architectural default UnwindPlan.  

This code path won't be taken if eh_frame unwind instructions are available -
lldb will always prefer those once it's off the zeroth frame.

The problem I'm trying to fix here is the class of unwind failures that
happen when we have hand-written assembly on the stack, with no eh_frame,
and lldb's assembly parser fails to understand the assembly.  People usually
write their hand-written assembly to follow the frame-pointer-preserving
conventions of the platform so the architectural default UnwindPlan will 
often work.  We won't have the spill location for most of the non-volatile
registers if we fall back to this, but it's better than stopping the unwind
prematurely.

This is a bit of a tricky change that I believe is correct, but if we get
unwinds that go of into the weeds / unwind bogus frames at the end of the
stack, I'll need to revisit it.

<rdar://problem/16099440> 

llvm-svn: 201839

1 files changed, 36 insertions, 0 deletions

diff --git a/lldb/source/Plugins/Process/Utility/UnwindLLDB.cpp b/lldb/source/Plugins/Process/Utility/UnwindLLDB.cpp
index a3a7002ea09..5db08e5c26d 100644
--- a/lldb/source/Plugins/Process/Utility/UnwindLLDB.cpp
+++ b/lldb/source/Plugins/Process/Utility/UnwindLLDB.cpp
@@ -158,6 +158,12 @@ UnwindLLDB::AddOneMoreFrame (ABI *abi)
 
     if (reg_ctx_sp.get() == NULL)
     {
+        // If the RegisterContextLLDB has a fallback UnwindPlan, it will switch to that and return
+        // true.  Subsequent calls to TryFallbackUnwindPlan() will return false.
+        if (m_frames[cur_idx - 1]->reg_ctx_lldb_sp->TryFallbackUnwindPlan())
+        {
+            return AddOneMoreFrame (abi);
+        }
         if (log)
             log->Printf ("%*sFrame %d did not get a RegisterContext, stopping.",
                          cur_idx < 100 ? cur_idx : 100, "", cur_idx);
@@ -166,6 +172,12 @@ UnwindLLDB::AddOneMoreFrame (ABI *abi)
 
     if (!reg_ctx_sp->IsValid())
     {
+        // If the RegisterContextLLDB has a fallback UnwindPlan, it will switch to that and return
+        // true.  Subsequent calls to TryFallbackUnwindPlan() will return false.
+        if (m_frames[cur_idx - 1]->reg_ctx_lldb_sp->TryFallbackUnwindPlan())
+        {
+            return AddOneMoreFrame (abi);
+        }
         if (log)
         {
             log->Printf("%*sFrame %d invalid RegisterContext for this frame, stopping stack walk", 
@@ -175,6 +187,12 @@ UnwindLLDB::AddOneMoreFrame (ABI *abi)
     }
     if (!reg_ctx_sp->GetCFA (cursor_sp->cfa))
     {
+        // If the RegisterContextLLDB has a fallback UnwindPlan, it will switch to that and return
+        // true.  Subsequent calls to TryFallbackUnwindPlan() will return false.
+        if (m_frames[cur_idx - 1]->reg_ctx_lldb_sp->TryFallbackUnwindPlan())
+        {
+            return AddOneMoreFrame (abi);
+        }
         if (log)
         {
             log->Printf("%*sFrame %d did not get CFA for this frame, stopping stack walk",
@@ -189,6 +207,12 @@ UnwindLLDB::AddOneMoreFrame (ABI *abi)
         // these.
         if (reg_ctx_sp->IsTrapHandlerFrame() == false)
         {
+            // If the RegisterContextLLDB has a fallback UnwindPlan, it will switch to that and return
+            // true.  Subsequent calls to TryFallbackUnwindPlan() will return false.
+            if (m_frames[cur_idx - 1]->reg_ctx_lldb_sp->TryFallbackUnwindPlan())
+            {
+                return AddOneMoreFrame (abi);
+            }
             if (log)
             {
                 log->Printf("%*sFrame %d did not get a valid CFA for this frame, stopping stack walk",
@@ -199,6 +223,12 @@ UnwindLLDB::AddOneMoreFrame (ABI *abi)
     }
     if (!reg_ctx_sp->ReadPC (cursor_sp->start_pc))
     {
+        // If the RegisterContextLLDB has a fallback UnwindPlan, it will switch to that and return
+        // true.  Subsequent calls to TryFallbackUnwindPlan() will return false.
+        if (m_frames[cur_idx - 1]->reg_ctx_lldb_sp->TryFallbackUnwindPlan())
+        {
+            return AddOneMoreFrame (abi);
+        }
         if (log)
         {
             log->Printf("%*sFrame %d did not get PC for this frame, stopping stack walk",
@@ -208,6 +238,12 @@ UnwindLLDB::AddOneMoreFrame (ABI *abi)
     }
     if (abi && !abi->CodeAddressIsValid (cursor_sp->start_pc))
     {
+        // If the RegisterContextLLDB has a fallback UnwindPlan, it will switch to that and return
+        // true.  Subsequent calls to TryFallbackUnwindPlan() will return false.
+        if (m_frames[cur_idx - 1]->reg_ctx_lldb_sp->TryFallbackUnwindPlan())
+        {
+            return AddOneMoreFrame (abi);
+        }
         if (log)
         {
             log->Printf("%*sFrame %d did not get a valid PC, stopping stack walk",