Index: include/llvm/CodeGen/GlobalISel/Utils.h
===================================================================
--- include/llvm/CodeGen/GlobalISel/Utils.h
+++ include/llvm/CodeGen/GlobalISel/Utils.h
@@ -21,6 +21,7 @@
 
 class MachineFunction;
 class MachineInstr;
+class MachineOperand;
 class MachineOptimizationRemarkEmitter;
 class MachineOptimizationRemarkMissed;
 class MachineRegisterInfo;
@@ -57,7 +58,7 @@
                                   const TargetInstrInfo &TII,
                                   const RegisterBankInfo &RBI,
                                   MachineInstr &InsertPt, const MCInstrDesc &II,
-                                  unsigned Reg, unsigned OpIdx);
+                                  const MachineOperand &RegMO, unsigned OpIdx);
 
 /// Mutate the newly-selected instruction \p I to constrain its (possibly
 /// generic) virtual register operands to the instruction's register class.
Index: lib/CodeGen/GlobalISel/Utils.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Utils.cpp
+++ lib/CodeGen/GlobalISel/Utils.cpp
@@ -42,23 +42,25 @@
   return Reg;
 }
 
-
 unsigned llvm::constrainOperandRegClass(
     const MachineFunction &MF, const TargetRegisterInfo &TRI,
     MachineRegisterInfo &MRI, const TargetInstrInfo &TII,
     const RegisterBankInfo &RBI, MachineInstr &InsertPt, const MCInstrDesc &II,
-    unsigned Reg, unsigned OpIdx) {
+    const MachineOperand &RegMO, unsigned OpIdx) {
+  unsigned Reg = RegMO.getReg();
   // Assume physical registers are properly constrained.
   assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
          "PhysReg not implemented");
 
   const TargetRegisterClass *RegClass = TII.getRegClass(II, OpIdx, &TRI, MF);
   // Some of the target independent instructions, like COPY, may not impose any
-  // register class constraints on some of their operands:
+  // register class constraints on some of their operands: If it's a use, we can
+  // skip constraining as the instruction defining the register would constrain
+  // it.
   if (!RegClass) {
-    assert(!isTargetSpecificOpcode(II.getOpcode()) &&
+    assert((!isTargetSpecificOpcode(II.getOpcode()) || RegMO.isUse()) &&
            "Only target independent instructions are allowed to have operands "
-           "with no register class constraints");
+           "with no register class constraints unless they are uses");
     // FIXME: Just bailing out like this here could be not enough, unless we
     // expect the users of this function to do the right thing for PHIs and
     // COPY:
@@ -108,7 +110,7 @@
     // insert COPYs if that's impossible.
     // constrainOperandRegClass does that for us.
     MO.setReg(constrainOperandRegClass(MF, TRI, MRI, TII, RBI, I, I.getDesc(),
-                                       Reg, OpI));
+                                       MO, OpI));
 
     // Tie uses to defs as indicated in MCInstrDesc if this hasn't already been
     // done.
Index: lib/Target/AArch64/AArch64CallLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64CallLowering.cpp
+++ lib/Target/AArch64/AArch64CallLowering.cpp
@@ -369,8 +369,7 @@
   if (Callee.isReg())
     MIB->getOperand(0).setReg(constrainOperandRegClass(
         MF, *TRI, MRI, *MF.getSubtarget().getInstrInfo(),
-        *MF.getSubtarget().getRegBankInfo(), *MIB, MIB->getDesc(),
-        Callee.getReg(), 0));
+        *MF.getSubtarget().getRegBankInfo(), *MIB, MIB->getDesc(), Callee, 0));
 
   // Finally we can copy the returned value back into its virtual-register. In
   // symmetry with the arugments, the physical register must be an
Index: lib/Target/ARM/ARMCallLowering.cpp
===================================================================
--- lib/Target/ARM/ARMCallLowering.cpp
+++ lib/Target/ARM/ARMCallLowering.cpp
@@ -521,7 +521,7 @@
     if (CalleeReg && !TRI->isPhysicalRegister(CalleeReg))
       MIB->getOperand(0).setReg(constrainOperandRegClass(
           MF, *TRI, MRI, *STI.getInstrInfo(), *STI.getRegBankInfo(),
-          *MIB.getInstr(), MIB->getDesc(), CalleeReg, 0));
+          *MIB.getInstr(), MIB->getDesc(), Callee, 0));
   }
 
   SmallVector<ArgInfo, 8> ArgInfos;
Index: lib/Target/X86/X86CallLowering.cpp
===================================================================
--- lib/Target/X86/X86CallLowering.cpp
+++ lib/Target/X86/X86CallLowering.cpp
@@ -438,8 +438,7 @@
   if (Callee.isReg())
     MIB->getOperand(0).setReg(constrainOperandRegClass(
         MF, *TRI, MRI, *MF.getSubtarget().getInstrInfo(),
-        *MF.getSubtarget().getRegBankInfo(), *MIB, MIB->getDesc(),
-        Callee.getReg(), 0));
+        *MF.getSubtarget().getRegBankInfo(), *MIB, MIB->getDesc(), Callee, 0));
 
   // Finally we can copy the returned value back into its virtual-register. In
   // symmetry with the arguments, the physical register must be an