diff --git a/llvm/lib/CodeGen/RegAllocBase.cpp b/llvm/lib/CodeGen/RegAllocBase.cpp
--- a/llvm/lib/CodeGen/RegAllocBase.cpp
+++ b/llvm/lib/CodeGen/RegAllocBase.cpp
@@ -61,7 +61,14 @@
   VRM = &vrm;
   LIS = &lis;
   Matrix = &mat;
+#ifndef NDEBUG
+  BitVector OldReservedRegs(MRI->getReservedRegs());
+#endif
   MRI->freezeReservedRegs(vrm.getMachineFunction());
+#ifndef NDEBUG
+  assert(OldReservedRegs == MRI->getReservedRegs() &&
+         "Frozen reserved regs should not have changed!");
+#endif
   RegClassInfo.runOnMachineFunction(vrm.getMachineFunction());
 }