diff --git a/llvm/lib/CodeGen/PeepholeOptimizer.cpp b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
index a762ed7c51d4..b16fa79cd662 100644
--- a/llvm/lib/CodeGen/PeepholeOptimizer.cpp
+++ b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
@@ -156,6 +156,14 @@ optimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
   if (!DstRC)
     return false;
 
+  // The ext instr may be operating on a sub-register of SrcReg as well.
+  // PPC::EXTSW is a 32 -> 64-bit sign extension, but it reads a 64-bit
+  // register.
+  // If UseSrcSubIdx is Set, SubIdx also applies to SrcReg, and only uses of
+  // SrcReg:SubIdx should be replaced.
+  bool UseSrcSubIdx = TM->getRegisterInfo()->
+    getSubClassWithSubReg(MRI->getRegClass(SrcReg), SubIdx) != 0;
+
   // The source has other uses. See if we can replace the other uses with use of
   // the result of the extension.
   SmallPtrSet<MachineBasicBlock*, 4> ReachedBBs;
@@ -184,6 +192,10 @@ optimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
       continue;
     }
 
+    // Only accept uses of SrcReg:SubIdx.
+    if (UseSrcSubIdx && UseMO.getSubReg() != SubIdx)
+      continue;
+
     // It's an error to translate this:
     //
     //    %reg1025 = <sext> %reg1024
@@ -259,10 +271,14 @@ optimizeExtInstr(MachineInstr *MI, MachineBasicBlock *MBB,
       }
 
       unsigned NewVR = MRI->createVirtualRegister(RC);
-      BuildMI(*UseMBB, UseMI, UseMI->getDebugLoc(),
-              TII->get(TargetOpcode::COPY), NewVR)
+      MachineInstr *Copy = BuildMI(*UseMBB, UseMI, UseMI->getDebugLoc(),
+                                   TII->get(TargetOpcode::COPY), NewVR)
         .addReg(DstReg, 0, SubIdx);
-
+      // SubIdx applies to both SrcReg and DstReg when UseSrcSubIdx is set.
+      if (UseSrcSubIdx) {
+        Copy->getOperand(0).setSubReg(SubIdx);
+        Copy->getOperand(0).setIsUndef();
+      }
       UseMO->setReg(NewVR);
       ++NumReuse;
       Changed = true;
diff --git a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
index 28b3bc1596e1..47f09dca77d3 100644
--- a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
+++ b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
@@ -79,6 +79,22 @@ ScheduleHazardRecognizer *PPCInstrInfo::CreateTargetPostRAHazardRecognizer(
 
   return new PPCScoreboardHazardRecognizer(II, DAG);
 }
+
+// Detect 32 -> 64-bit extensions where we may reuse the low sub-register.
+bool PPCInstrInfo::isCoalescableExtInstr(const MachineInstr &MI,
+                                         unsigned &SrcReg, unsigned &DstReg,
+                                         unsigned &SubIdx) const {
+  switch (MI.getOpcode()) {
+  default: return false;
+  case PPC::EXTSW:
+  case PPC::EXTSW_32_64:
+    SrcReg = MI.getOperand(1).getReg();
+    DstReg = MI.getOperand(0).getReg();
+    SubIdx = PPC::sub_32;
+    return true;
+  }
+}
+
 unsigned PPCInstrInfo::isLoadFromStackSlot(const MachineInstr *MI,
                                            int &FrameIndex) const {
   switch (MI->getOpcode()) {
diff --git a/llvm/lib/Target/PowerPC/PPCInstrInfo.h b/llvm/lib/Target/PowerPC/PPCInstrInfo.h
index 7d49aa129e36..374213ea435b 100644
--- a/llvm/lib/Target/PowerPC/PPCInstrInfo.h
+++ b/llvm/lib/Target/PowerPC/PPCInstrInfo.h
@@ -92,6 +92,9 @@ public:
   CreateTargetPostRAHazardRecognizer(const InstrItineraryData *II,
                                      const ScheduleDAG *DAG) const;
 
+  bool isCoalescableExtInstr(const MachineInstr &MI,
+                             unsigned &SrcReg, unsigned &DstReg,
+                             unsigned &SubIdx) const;
   unsigned isLoadFromStackSlot(const MachineInstr *MI,
                                int &FrameIndex) const;
   unsigned isStoreToStackSlot(const MachineInstr *MI,
diff --git a/llvm/test/CodeGen/PowerPC/coalesce-ext.ll b/llvm/test/CodeGen/PowerPC/coalesce-ext.ll
new file mode 100644
index 000000000000..08a2cc773fd5
--- /dev/null
+++ b/llvm/test/CodeGen/PowerPC/coalesce-ext.ll
@@ -0,0 +1,17 @@
+; RUN: llc -march=ppc64 < %s | FileCheck %s
+; Check that the peephole optimizer knows about sext and zext instructions.
+; CHECK: test1sext
+define i32 @test1sext(i64 %A, i64 %B, i32* %P, i64 *%P2) nounwind {
+  %C = add i64 %A, %B
+  ; CHECK: add [[SUM:r[0-9]+]], r3, r4
+  %D = trunc i64 %C to i32
+  %E = shl i64 %C, 32
+  %F = ashr i64 %E, 32
+  ; CHECK: extsw [[EXT:r[0-9]+]], [[SUM]]
+  store volatile i64 %F, i64 *%P2
+  ; CHECK: std [[EXT]]
+  store volatile i32 %D, i32* %P
+  ; Reuse low bits of extended register, don't extend live range of SUM.
+  ; CHECK: stw [[EXT]]
+  ret i32 %D
+}