Index: lib/CodeGen/UnreachableBlockElim.cpp
===================================================================
--- lib/CodeGen/UnreachableBlockElim.cpp
+++ lib/CodeGen/UnreachableBlockElim.cpp
@@ -25,6 +25,7 @@
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
@@ -195,18 +196,30 @@
         }
 
       if (phi->getNumOperands() == 3) {
-        unsigned Input = phi->getOperand(1).getReg();
-        unsigned Output = phi->getOperand(0).getReg();
-
-        phi++->eraseFromParent();
+        const MachineOperand &Input = phi->getOperand(1);
+        const MachineOperand &Output = phi->getOperand(0);
+        unsigned InputReg = Input.getReg();
+        unsigned OutputReg = Output.getReg();
+        assert(Output.getSubReg() == 0 && "Cannot have output subregister");
         ModifiedPHI = true;
 
-        if (Input != Output) {
+        if (InputReg != OutputReg) {
           MachineRegisterInfo &MRI = F.getRegInfo();
-          MRI.constrainRegClass(Input, MRI.getRegClass(Output));
-          MRI.replaceRegWith(Output, Input);
+          unsigned InputSub = Input.getSubReg();
+          if (InputSub == 0) {
+            MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg));
+            MRI.replaceRegWith(OutputReg, InputReg);
+          } else {
+            // The input register to the PHI has a subregister:
+            // insert a COPY instead of simply replacing the output
+            // with the input.
+            const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
+            BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
+                    TII->get(TargetOpcode::COPY), OutputReg)
+                .addReg(InputReg, getRegState(Input), InputSub);
+          }
+          phi++->eraseFromParent();
         }
-
         continue;
       }
 
Index: test/CodeGen/Hexagon/unreachable-mbb-phi-subreg.mir
===================================================================
--- /dev/null
+++ test/CodeGen/Hexagon/unreachable-mbb-phi-subreg.mir
@@ -0,0 +1,25 @@
+# RUN: llc -march=hexagon -run-pass unreachable-mbb-elimination %s -o - | FileCheck %s
+
+---
+name: fred
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: %d0
+    successors: %bb.2
+
+    %0 : doubleregs = COPY %d0
+    J2_jump %bb.2, implicit-def %pc
+
+  bb.1:
+    successors: %bb.2
+    A2_nop
+
+  bb.2:
+    ; Make sure that the subregister from the PHI operand is preserved.
+    ; CHECK: %[[REG:[0-9]+]] = COPY %0.isub_lo
+    ; CHECK: %r0 = COPY %[[REG]]
+    %1 : intregs = PHI %0.isub_lo, %bb.0, %0.isub_hi, %bb.1
+    %r0 = COPY %1
+...
+