diff --git a/compiler/optimizing/instruction_simplifier.cc b/compiler/optimizing/instruction_simplifier.cc
index 0adb931..fcb3471 100644
--- a/compiler/optimizing/instruction_simplifier.cc
+++ b/compiler/optimizing/instruction_simplifier.cc
@@ -65,6 +65,7 @@
   void VisitUShr(HUShr* instruction) OVERRIDE;
   void VisitXor(HXor* instruction) OVERRIDE;
   void VisitInstanceOf(HInstanceOf* instruction) OVERRIDE;
+  bool IsDominatedByInputNullCheck(HInstruction* instr);
 
   OptimizingCompilerStats* stats_;
   bool simplification_occurred_ = false;
@@ -174,9 +175,20 @@
   }
 }
 
+bool InstructionSimplifierVisitor::IsDominatedByInputNullCheck(HInstruction* instr) {
+  HInstruction* input = instr->InputAt(0);
+  for (HUseIterator<HInstruction*> it(input->GetUses()); !it.Done(); it.Advance()) {
+    HInstruction* use = it.Current()->GetUser();
+    if (use->IsNullCheck() && use->StrictlyDominates(instr)) {
+      return true;
+    }
+  }
+  return false;
+}
+
 void InstructionSimplifierVisitor::VisitCheckCast(HCheckCast* check_cast) {
   HLoadClass* load_class = check_cast->InputAt(1)->AsLoadClass();
-  if (!check_cast->InputAt(0)->CanBeNull()) {
+  if (!check_cast->InputAt(0)->CanBeNull() || IsDominatedByInputNullCheck(check_cast)) {
     check_cast->ClearMustDoNullCheck();
   }
 
@@ -198,7 +210,7 @@
 }
 
 void InstructionSimplifierVisitor::VisitInstanceOf(HInstanceOf* instruction) {
-  if (!instruction->InputAt(0)->CanBeNull()) {
+  if (!instruction->InputAt(0)->CanBeNull() || IsDominatedByInputNullCheck(instruction)) {
     instruction->ClearMustDoNullCheck();
   }
 }
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 2850368..680fb47 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -3920,7 +3920,9 @@
       }
       for (size_t i = 0, e = moves_.Size(); i < e; ++i) {
         DCHECK(!destination.OverlapsWith(moves_.Get(i).GetDestination()))
-            << "Overlapped destination for two moves in a parallel move.";
+            << "Overlapped destination for two moves in a parallel move: "
+            << moves_.Get(i).GetSource() << " ==> " << moves_.Get(i).GetDestination() << " and "
+            << source << " ==> " << destination;
       }
     }
     moves_.Add(MoveOperands(source, destination, type, instruction));
diff --git a/compiler/optimizing/ssa_builder.cc b/compiler/optimizing/ssa_builder.cc
index 59a2852..aac5211 100644
--- a/compiler/optimizing/ssa_builder.cc
+++ b/compiler/optimizing/ssa_builder.cc
@@ -504,7 +504,7 @@
     // typed and the value in a dex register will not be used for both floating point and
     // non-floating point operations. So the only reason an instruction would want a floating
     // point equivalent is for an unused phi that will be removed by the dead phi elimination phase.
-    DCHECK(user->IsPhi());
+    DCHECK(user->IsPhi()) << "is actually " << user->DebugName() << " (" << user->GetId() << ")";
     return value;
   }
 }