diff --git a/lib/CodeGen/SelectionDAG/InstrEmitter.cpp b/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
index 57e25fa..2ff66f8 100644
--- a/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
+++ b/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
@@ -30,6 +30,12 @@
 #include "llvm/Support/MathExtras.h"
 using namespace llvm;
 
+/// MinRCSize - Smallest register class we allow when constraining virtual
+/// registers.  If satisfying all register class constraints would require
+/// using a smaller register class, emit a COPY to a new virtual register
+/// instead.
+const unsigned MinRCSize = 4;
+
 /// CountResults - The results of target nodes have register or immediate
 /// operands first, then an optional chain, and optional glue operands (which do
 /// not go into the resulting MachineInstr).
@@ -284,7 +290,6 @@
   // a new virtual register and copy the value into it, but first attempt to
   // shrink VReg's register class within reason.  For example, if VReg == GR32
   // and II requires a GR32_NOSP, just constrain VReg to GR32_NOSP.
-  const unsigned MinRCSize = 4;
   if (II) {
     const TargetRegisterClass *DstRC = 0;
     if (IIOpNum < II->getNumOperands())
@@ -392,6 +397,30 @@
   }
 }
 
+unsigned InstrEmitter::ConstrainForSubReg(unsigned VReg, unsigned SubIdx,
+                                          EVT VT, DebugLoc DL) {
+  const TargetRegisterClass *VRC = MRI->getRegClass(VReg);
+  const TargetRegisterClass *RC = TRI->getSubClassWithSubReg(VRC, SubIdx);
+
+  // RC is a sub-class of VRC that supports SubIdx.  Try to constrain VReg
+  // within reason.
+  if (RC && RC != VRC)
+    RC = MRI->constrainRegClass(VReg, RC, MinRCSize);
+
+  // VReg has been adjusted.  It can be used with SubIdx operands now.
+  if (RC)
+    return VReg;
+
+  // VReg couldn't be reasonably constrained.  Emit a COPY to a new virtual
+  // register instead.
+  RC = TRI->getSubClassWithSubReg(TLI->getRegClassFor(VT), SubIdx);
+  assert(RC && "No legal register class for VT supports that SubIdx");
+  unsigned NewReg = MRI->createVirtualRegister(RC);
+  BuildMI(*MBB, InsertPos, DL, TII->get(TargetOpcode::COPY), NewReg)
+    .addReg(VReg);
+  return NewReg;
+}
+
 /// EmitSubregNode - Generate machine code for subreg nodes.
 ///
 void InstrEmitter::EmitSubregNode(SDNode *Node,
@@ -416,10 +445,12 @@
   }
 
   if (Opc == TargetOpcode::EXTRACT_SUBREG) {
-    // EXTRACT_SUBREG is lowered as %dst = COPY %src:sub
+    // EXTRACT_SUBREG is lowered as %dst = COPY %src:sub.  There are no
+    // constraints on the %dst register, COPY can target all legal register
+    // classes.
     unsigned SubIdx = cast<ConstantSDNode>(Node->getOperand(1))->getZExtValue();
+    const TargetRegisterClass *TRC = TLI->getRegClassFor(Node->getValueType(0));
 
-    // Figure out the register class to create for the destreg.
     unsigned VReg = getVR(Node->getOperand(0), VRBaseMap);
     MachineInstr *DefMI = MRI->getVRegDef(VReg);
     unsigned SrcReg, DstReg, DefSubIdx;
@@ -431,36 +462,24 @@
       // r1026 = extract_subreg r1025, 4
       // to a copy
       // r1026 = copy r1024
-      const TargetRegisterClass *TRC = MRI->getRegClass(SrcReg);
       VRBase = MRI->createVirtualRegister(TRC);
       BuildMI(*MBB, InsertPos, Node->getDebugLoc(),
               TII->get(TargetOpcode::COPY), VRBase).addReg(SrcReg);
     } else {
-      const TargetRegisterClass *TRC = MRI->getRegClass(VReg);
-      const TargetRegisterClass *SRC = TRC->getSubRegisterRegClass(SubIdx);
-      assert(SRC && "Invalid subregister index in EXTRACT_SUBREG");
+      // VReg may not support a SubIdx sub-register, and we may need to
+      // constrain its register class or issue a COPY to a compatible register
+      // class.
+      VReg = ConstrainForSubReg(VReg, SubIdx,
+                                Node->getOperand(0).getValueType(),
+                                Node->getDebugLoc());
 
-      // Figure out the register class to create for the destreg.
-      // Note that if we're going to directly use an existing register,
-      // it must be precisely the required class, and not a subclass
-      // thereof.
-      if (VRBase == 0 || SRC != MRI->getRegClass(VRBase)) {
-        // Create the reg
-        assert(SRC && "Couldn't find source register class");
-        VRBase = MRI->createVirtualRegister(SRC);
-      }
+      // Create the destreg if it is missing.
+      if (VRBase == 0)
+        VRBase = MRI->createVirtualRegister(TRC);
 
       // Create the extract_subreg machine instruction.
-      MachineInstr *MI = BuildMI(*MF, Node->getDebugLoc(),
-                                 TII->get(TargetOpcode::COPY), VRBase);
-
-      // Add source, and subreg index
-      AddOperand(MI, Node->getOperand(0), 0, 0, VRBaseMap, /*IsDebug=*/false,
-                 IsClone, IsCloned);
-      assert(TargetRegisterInfo::isVirtualRegister(MI->getOperand(1).getReg())&&
-             "Cannot yet extract from physregs");
-      MI->getOperand(1).setSubReg(SubIdx);
-      MBB->insert(InsertPos, MI);
+      BuildMI(*MBB, InsertPos, Node->getDebugLoc(),
+              TII->get(TargetOpcode::COPY), VRBase).addReg(VReg, 0, SubIdx);
     }
   } else if (Opc == TargetOpcode::INSERT_SUBREG ||
              Opc == TargetOpcode::SUBREG_TO_REG) {
diff --git a/lib/CodeGen/SelectionDAG/InstrEmitter.h b/lib/CodeGen/SelectionDAG/InstrEmitter.h
index 19fc044..c081f38 100644
--- a/lib/CodeGen/SelectionDAG/InstrEmitter.h
+++ b/lib/CodeGen/SelectionDAG/InstrEmitter.h
@@ -77,6 +77,12 @@
                   DenseMap<SDValue, unsigned> &VRBaseMap,
                   bool IsDebug, bool IsClone, bool IsCloned);
 
+  /// ConstrainForSubReg - Try to constrain VReg to a register class that
+  /// supports SubIdx sub-registers.  Emit a copy if that isn't possible.
+  /// Return the virtual register to use.
+  unsigned ConstrainForSubReg(unsigned VReg, unsigned SubIdx,
+                              EVT VT, DebugLoc DL);
+
   /// EmitSubregNode - Generate machine code for subreg nodes.
   ///
   void EmitSubregNode(SDNode *Node, DenseMap<SDValue, unsigned> &VRBaseMap,