Index: lib/Transforms/Utils/LowerSwitch.cpp
===================================================================
--- lib/Transforms/Utils/LowerSwitch.cpp
+++ lib/Transforms/Utils/LowerSwitch.cpp
@@ -121,6 +121,13 @@
   for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
     BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
 
+    // If the block has no more predecessors, it should be a dead Default block which
+    // will be erased later or dead block resulted from previous optimization passes.
+    // In either case, skip it and don't waste time processing it.
+    if (Cur != &F.getEntryBlock() && pred_begin(Cur) == pred_end(Cur)) {
+      continue;
+    }
+
     if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
       Changed = true;
       processSwitchInst(SI, DeleteList);