diff --git a/lib/CodeGen/VirtRegRewriter.cpp b/lib/CodeGen/VirtRegRewriter.cpp
index e1e249a..6da6a9b 100644
--- a/lib/CodeGen/VirtRegRewriter.cpp
+++ b/lib/CodeGen/VirtRegRewriter.cpp
@@ -810,6 +810,21 @@
         ReusedOp NewOp = Op;
         Reuses.erase(Reuses.begin()+ro);
 
+        // MI may be using only a sub-register of PhysRegUsed.
+        unsigned RealPhysRegUsed = MI->getOperand(NewOp.Operand).getReg();
+        unsigned SubIdx = 0;
+        assert(TargetRegisterInfo::isPhysicalRegister(RealPhysRegUsed) &&
+               "A reuse cannot be a virtual register");
+        if (PRRU != RealPhysRegUsed) {
+          // What was the sub-register index?
+          unsigned SubReg;
+          for (SubIdx = 1; (SubReg = TRI->getSubReg(PRRU, SubIdx)); SubIdx++)
+            if (SubReg == RealPhysRegUsed)
+              break;
+          assert(SubReg == RealPhysRegUsed &&
+                 "Operand physreg is not a sub-register of PhysRegUsed");
+        }
+
         // Ok, we're going to try to reload the assigned physreg into the
         // slot that we were supposed to in the first place.  However, that
         // register could hold a reuse.  Check to see if it conflicts or
@@ -842,7 +857,6 @@
         Spills.ClobberPhysReg(NewPhysReg);
         Spills.ClobberPhysReg(NewOp.PhysRegReused);
 
-        unsigned SubIdx = MI->getOperand(NewOp.Operand).getSubReg();
         unsigned RReg = SubIdx ? TRI->getSubReg(NewPhysReg, SubIdx) : NewPhysReg;
         MI->getOperand(NewOp.Operand).setReg(RReg);
         MI->getOperand(NewOp.Operand).setSubReg(0);