diff --git a/llvm/lib/CodeGen/PeepholeOptimizer.cpp b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
--- a/llvm/lib/CodeGen/PeepholeOptimizer.cpp
+++ b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
@@ -458,7 +458,7 @@
 optimizeExtInstr(MachineInstr &MI, MachineBasicBlock &MBB,
                  SmallPtrSetImpl<MachineInstr*> &LocalMIs) {
   Register SrcReg, DstReg;
-  unsigned SubIdx;
+  unsigned SubIdx = 0;
   if (!TII->isCoalescableExtInstr(MI, SrcReg, DstReg, SubIdx))
     return false;