diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index e60adbf9e4e1..17ccdb2bd5cc 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -497,6 +497,10 @@ bool SelectionDAGISel::runOnMachineFunction(MachineFunction &mf) {
       if (J == E) break;
       To = J->second;
     }
+    // Make sure the new register has a sufficiently constrained register class.
+    if (TargetRegisterInfo::isVirtualRegister(From) &&
+        TargetRegisterInfo::isVirtualRegister(To))
+      MRI.constrainRegClass(To, MRI.getRegClass(From));
     // Replace it.
     MRI.replaceRegWith(From, To);
   }
diff --git a/lib/Target/ARM/ARMFastISel.cpp b/lib/Target/ARM/ARMFastISel.cpp
index 02ebe665151d..2845db336f62 100644
--- a/lib/Target/ARM/ARMFastISel.cpp
+++ b/lib/Target/ARM/ARMFastISel.cpp
@@ -176,6 +176,8 @@ class ARMFastISel : public FastISel {
 
     // Utility routines.
   private:
+    unsigned constrainOperandRegClass(const MCInstrDesc &II, unsigned OpNum,
+                                      unsigned Op);
     bool isTypeLegal(Type *Ty, MVT &VT);
     bool isLoadTypeLegal(Type *Ty, MVT &VT);
     bool ARMEmitCmp(const Value *Src1Value, const Value *Src2Value,
@@ -291,6 +293,23 @@ ARMFastISel::AddOptionalDefs(const MachineInstrBuilder &MIB) {
   return MIB;
 }
 
+unsigned ARMFastISel::constrainOperandRegClass(const MCInstrDesc &II,
+                                               unsigned Op, unsigned OpNum) {
+  if (TargetRegisterInfo::isVirtualRegister(Op)) {
+    const TargetRegisterClass *RegClass =
+        TII.getRegClass(II, OpNum, &TRI, *FuncInfo.MF);
+    if (!MRI.constrainRegClass(Op, RegClass)) {
+      // If it's not legal to COPY between the register classes, something
+      // has gone very wrong before we got here.
+      unsigned NewOp = createResultReg(RegClass);
+      AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL,
+                              TII.get(TargetOpcode::COPY), NewOp).addReg(Op));
+      return NewOp;
+    }
+  }
+  return Op;
+}
+
 unsigned ARMFastISel::FastEmitInst_(unsigned MachineInstOpcode,
                                     const TargetRegisterClass* RC) {
   unsigned ResultReg = createResultReg(RC);
@@ -306,6 +325,9 @@ unsigned ARMFastISel::FastEmitInst_r(unsigned MachineInstOpcode,
   unsigned ResultReg = createResultReg(RC);
   const MCInstrDesc &II = TII.get(MachineInstOpcode);
 
+  // Make sure the input operand is sufficiently constrained to be legal
+  // for this instruction.
+  Op0 = constrainOperandRegClass(II, Op0, 1);
   if (II.getNumDefs() >= 1) {
     AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, II, ResultReg)
                    .addReg(Op0, Op0IsKill * RegState::Kill));
@@ -326,6 +348,11 @@ unsigned ARMFastISel::FastEmitInst_rr(unsigned MachineInstOpcode,
   unsigned ResultReg = createResultReg(RC);
   const MCInstrDesc &II = TII.get(MachineInstOpcode);
 
+  // Make sure the input operands are sufficiently constrained to be legal
+  // for this instruction.
+  Op0 = constrainOperandRegClass(II, Op0, 1);
+  Op1 = constrainOperandRegClass(II, Op1, 2);
+
   if (II.getNumDefs() >= 1) {
     AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, II, ResultReg)
                    .addReg(Op0, Op0IsKill * RegState::Kill)
@@ -349,6 +376,12 @@ unsigned ARMFastISel::FastEmitInst_rrr(unsigned MachineInstOpcode,
   unsigned ResultReg = createResultReg(RC);
   const MCInstrDesc &II = TII.get(MachineInstOpcode);
 
+  // Make sure the input operands are sufficiently constrained to be legal
+  // for this instruction.
+  Op0 = constrainOperandRegClass(II, Op0, 1);
+  Op1 = constrainOperandRegClass(II, Op1, 2);
+  Op2 = constrainOperandRegClass(II, Op1, 3);
+
   if (II.getNumDefs() >= 1) {
     AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, II, ResultReg)
                    .addReg(Op0, Op0IsKill * RegState::Kill)
@@ -373,6 +406,9 @@ unsigned ARMFastISel::FastEmitInst_ri(unsigned MachineInstOpcode,
   unsigned ResultReg = createResultReg(RC);
   const MCInstrDesc &II = TII.get(MachineInstOpcode);
 
+  // Make sure the input operand is sufficiently constrained to be legal
+  // for this instruction.
+  Op0 = constrainOperandRegClass(II, Op0, 1);
   if (II.getNumDefs() >= 1) {
     AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, II, ResultReg)
                    .addReg(Op0, Op0IsKill * RegState::Kill)
@@ -395,6 +431,9 @@ unsigned ARMFastISel::FastEmitInst_rf(unsigned MachineInstOpcode,
   unsigned ResultReg = createResultReg(RC);
   const MCInstrDesc &II = TII.get(MachineInstOpcode);
 
+  // Make sure the input operand is sufficiently constrained to be legal
+  // for this instruction.
+  Op0 = constrainOperandRegClass(II, Op0, 1);
   if (II.getNumDefs() >= 1) {
     AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, II, ResultReg)
                    .addReg(Op0, Op0IsKill * RegState::Kill)
@@ -418,6 +457,10 @@ unsigned ARMFastISel::FastEmitInst_rri(unsigned MachineInstOpcode,
   unsigned ResultReg = createResultReg(RC);
   const MCInstrDesc &II = TII.get(MachineInstOpcode);
 
+  // Make sure the input operands are sufficiently constrained to be legal
+  // for this instruction.
+  Op0 = constrainOperandRegClass(II, Op0, 1);
+  Op1 = constrainOperandRegClass(II, Op1, 2);
   if (II.getNumDefs() >= 1) {
     AddOptionalDefs(BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, II, ResultReg)
                    .addReg(Op0, Op0IsKill * RegState::Kill)
diff --git a/test/CodeGen/ARM/fast-isel.ll b/test/CodeGen/ARM/fast-isel.ll
index cd3fc1caa883..5981cab7dcb1 100644
--- a/test/CodeGen/ARM/fast-isel.ll
+++ b/test/CodeGen/ARM/fast-isel.ll
@@ -1,6 +1,6 @@
 ; RUN: llc < %s -O0 -fast-isel-abort -relocation-model=dynamic-no-pic -mtriple=armv7-apple-ios -verify-machineinstrs | FileCheck %s --check-prefix=ARM
 ; RUN: llc < %s -O0 -fast-isel-abort -relocation-model=dynamic-no-pic -mtriple=armv7-linux-gnueabi -verify-machineinstrs | FileCheck %s --check-prefix=ARM
-; RUN: llc < %s -O0 -fast-isel-abort -relocation-model=dynamic-no-pic -mtriple=thumbv7-apple-ios | FileCheck %s --check-prefix=THUMB
+; RUN: llc < %s -O0 -fast-isel-abort -relocation-model=dynamic-no-pic -mtriple=thumbv7-apple-ios -verify-machineinstrs | FileCheck %s --check-prefix=THUMB
 
 ; Very basic fast-isel functionality.
 define i32 @test0(i32 %a, i32 %b) nounwind {