Index: include/llvm/CodeGen/VirtRegMap.h
===================================================================
--- include/llvm/CodeGen/VirtRegMap.h
+++ include/llvm/CodeGen/VirtRegMap.h
@@ -102,14 +102,7 @@
 
     /// @brief creates a mapping for the specified virtual register to
     /// the specified physical register
-    void assignVirt2Phys(unsigned virtReg, unsigned physReg) {
-      assert(TargetRegisterInfo::isVirtualRegister(virtReg) &&
-             TargetRegisterInfo::isPhysicalRegister(physReg));
-      assert(Virt2PhysMap[virtReg] == NO_PHYS_REG &&
-             "attempt to assign physical register to already mapped "
-             "virtual register");
-      Virt2PhysMap[virtReg] = physReg;
-    }
+    void assignVirt2Phys(unsigned virtReg, MCPhysReg physReg);
 
     /// @brief clears the specified virtual register's, physical
     /// register mapping
Index: lib/CodeGen/MachineVerifier.cpp
===================================================================
--- lib/CodeGen/MachineVerifier.cpp
+++ lib/CodeGen/MachineVerifier.cpp
@@ -945,7 +945,6 @@
     VerifyStackMapConstant(VarStart + StatepointOpers::NumDeoptOperandsOffset);
 
     // TODO: verify we have properly encoded deopt arguments
-   
   };
 }
 
@@ -1947,9 +1946,11 @@
       SlotIndex PEnd = LiveInts->getMBBEndIdx(*PI);
       const VNInfo *PVNI = LR.getVNInfoBefore(PEnd);
 
-      // All predecessors must have a live-out value if this is not a
-      // subregister liverange.
-      if (!PVNI && LaneMask.none()) {
+      // All predecessors must have a live-out value. However for a phi
+      // instruction with subregister intervals
+      // only one of the subregisters (not necessarily the current one) needs to
+      // be defined.
+      if (!PVNI && (LaneMask.none() || !IsPHI) ) {
         report("Register not marked live out of predecessor", *PI);
         report_context(LR, Reg, LaneMask);
         report_context(*VNI);
Index: lib/CodeGen/RegAllocPBQP.cpp
===================================================================
--- lib/CodeGen/RegAllocPBQP.cpp
+++ lib/CodeGen/RegAllocPBQP.cpp
@@ -738,7 +738,15 @@
 
     if (PReg == 0) {
       const TargetRegisterClass &RC = *MRI.getRegClass(LI.reg);
-      PReg = RC.getRawAllocationOrder(MF).front();
+      const ArrayRef<MCPhysReg> RawPRegOrder = RC.getRawAllocationOrder(MF);
+      for (unsigned CandidateReg : RawPRegOrder) {
+        if (!VRM.getRegInfo().isReserved(CandidateReg)) {
+          PReg = CandidateReg;
+          break;
+        }
+      }
+      assert(PReg &&
+             "No un-reserved physical registers in this register class");
     }
 
     VRM.assignVirt2Phys(LI.reg, PReg);
Index: lib/CodeGen/VirtRegMap.cpp
===================================================================
--- lib/CodeGen/VirtRegMap.cpp
+++ lib/CodeGen/VirtRegMap.cpp
@@ -72,6 +72,17 @@
   Virt2SplitMap.resize(NumRegs);
 }
 
+void VirtRegMap::assignVirt2Phys(unsigned virtReg, MCPhysReg physReg) {
+  assert(TargetRegisterInfo::isVirtualRegister(virtReg) &&
+         TargetRegisterInfo::isPhysicalRegister(physReg));
+  assert(Virt2PhysMap[virtReg] == NO_PHYS_REG &&
+         "attempt to assign physical register to already mapped "
+         "virtual register");
+  assert(!getRegInfo().isReserved(physReg) &&
+         "Attempt to map virtReg to a reserved physReg");
+  Virt2PhysMap[virtReg] = physReg;
+}
+
 unsigned VirtRegMap::createSpillSlot(const TargetRegisterClass *RC) {
   unsigned Size = TRI->getSpillSize(*RC);
   unsigned Align = TRI->getSpillAlignment(*RC);