diff --git a/llvm/lib/CodeGen/RegisterScavenging.cpp b/llvm/lib/CodeGen/RegisterScavenging.cpp
--- a/llvm/lib/CodeGen/RegisterScavenging.cpp
+++ b/llvm/lib/CodeGen/RegisterScavenging.cpp
@@ -369,6 +369,10 @@
   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
   LiveRegUnits Used(TRI);
 
+  assert(From->getParent() == To->getParent() &&
+         "Target instruction is in other than current basic block, use "
+         "enterBasicBlockEnd first");
+
   for (MachineBasicBlock::iterator I = From;; --I) {
     const MachineInstr &MI = *I;
 
@@ -423,6 +427,8 @@
       if (I == MBB.begin())
         break;
     }
+    assert(I != MBB.begin() && "Did not find target instruction while "
+                               "iterating backwards");
   }
 
   return std::make_pair(Survivor, Pos);