Index: llvm/include/llvm/CodeGen/MachineInstr.h
===================================================================
--- llvm/include/llvm/CodeGen/MachineInstr.h
+++ llvm/include/llvm/CodeGen/MachineInstr.h
@@ -1152,8 +1152,14 @@
   void print(raw_ostream &OS, bool SkipOpers = false) const;
   void print(raw_ostream &OS, ModuleSlotTracker &MST,
              bool SkipOpers = false) const;
+  void print(raw_ostream &OS, ModuleSlotTracker &MST,
+             bool SkipOpers, const TargetInstrInfo * const) const;
   void dump() const;
 
+  // Useful at least to prevent UNKNOWN when outputting
+  // an MI that is a result of combining MIs.
+  void dump_with_given_TII(const TargetInstrInfo * const) const;
+
   //===--------------------------------------------------------------------===//
   // Accessors used to build up machine instructions.
 
Index: llvm/lib/CodeGen/MachineCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/MachineCombiner.cpp
+++ llvm/lib/CodeGen/MachineCombiner.cpp
@@ -134,7 +134,7 @@
   // are tracked in the InstrIdxForVirtReg map depth is looked up in InstrDepth
   for (auto *InstrPtr : InsInstrs) { // for each Use
     unsigned IDepth = 0;
-    DEBUG(dbgs() << "NEW INSTR "; InstrPtr->dump(); dbgs() << "\n";);
+    DEBUG(dbgs() << "NEW INSTR "; InstrPtr->dump_with_given_TII(TII); dbgs() << "\n";);
     for (const MachineOperand &MO : InstrPtr->operands()) {
       // Check for virtual register operand.
       if (!(MO.isReg() && TargetRegisterInfo::isVirtualRegister(MO.getReg())))
Index: llvm/lib/CodeGen/MachineInstr.cpp
===================================================================
--- llvm/lib/CodeGen/MachineInstr.cpp
+++ llvm/lib/CodeGen/MachineInstr.cpp
@@ -1701,7 +1701,8 @@
 }
 
 void MachineInstr::print(raw_ostream &OS, ModuleSlotTracker &MST,
-                         bool SkipOpers) const {
+                         bool SkipOpers,
+                         const TargetInstrInfo * const TII_in) const {
   // We can be a bit tidier if we know the MachineFunction.
   const MachineFunction *MF = nullptr;
   const TargetRegisterInfo *TRI = nullptr;
@@ -1742,6 +1743,9 @@
   if (StartOp != 0)
     OS << " = ";
 
+  if (!TII)
+    TII = TII_in;
+
   // Print the opcode name.
   if (TII)
     OS << TII->getName(getOpcode());
@@ -1986,6 +1990,24 @@
   OS << '\n';
 }
 
+void MachineInstr::print(raw_ostream &OS, ModuleSlotTracker &MST,
+                         bool SkipOpers) const {
+  print(OS, MST, SkipOpers, 0 /* intentional null TargetInstrInfo*/);
+}
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+void MachineInstr::dump_with_given_TII(const TargetInstrInfo * const TII_in) const {
+  dbgs() << "  ";
+  const Module *M = nullptr;
+  if (const MachineBasicBlock *MBB = getParent())
+    if (const MachineFunction *MF = MBB->getParent())
+      M = MF->getFunction()->getParent();
+
+  ModuleSlotTracker MST(M);
+  print(dbgs(), MST, false /* SkipOpers */, TII_in);
+}
+#endif
+
 bool MachineInstr::addRegisterKilled(unsigned IncomingReg,
                                      const TargetRegisterInfo *RegInfo,
                                      bool AddIfNotFound) {
Index: llvm/test/CodeGen/AArch64/correct_debug_output_when_a_new_MachineInstr_is_generated_in_the_machine_combiner.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/correct_debug_output_when_a_new_MachineInstr_is_generated_in_the_machine_combiner.ll
@@ -0,0 +1,58 @@
+; Converted from a machine-reduced C++ test case.
+
+; The fix is actually generic, but this test is AArch64-specific.
+
+; RUN: llc -O=3 -mtriple=aarch64-linux-gnu -mcpu=cortex-a57 -debug-only=machine-combiner -debug -o - %s |& FileCheck %s
+
+; CHECK:     NEW INSTR{{.*}}MADDXrrr
+; CHECK-NOT: NEW INSTR{{.*}}UNKNOWN
+
+target triple = "aarch64-sarc-linux-gnu"
+
+%class.BtlConfig = type { %class.C }
+%class.C = type { %class.B }
+%class.B = type { %class.D* }
+%class.D = type { %class.basic_string.base, [4 x i8] }
+%class.basic_string.base = type <{ i64, i64, i32 }>
+%class.basic_string = type <{ i64, i64, i32, [4 x i8] }>
+@a = global %class.BtlConfig zeroinitializer, align 8
+@.str = private unnamed_addr constant [1 x i8] zeroinitializer, align 1
+declare i64 @_ZN1CI1D1AIS0_EE5m_fn1Ev(%class.C*) local_unnamed_addr
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1)
+define internal void @_GLOBAL__sub_I_main_adv.ii() section ".text.startup" {
+entry:
+  %tmp.i.i = alloca %class.D, align 8
+  %agg.tmp.i.i = alloca %class.D, align 8
+  %0 = bitcast %class.D* %tmp.i.i to i8*
+  %1 = bitcast %class.D* %agg.tmp.i.i to i8*
+  %call8.i.i = tail call i64 @_ZN1CI1D1AIS0_EE5m_fn1Ev(%class.C* getelementptr inbounds (%class.BtlConfig, %class.BtlConfig* @a, i64 0, i32 0))
+  %cmp9.i.i = icmp sgt i64 %call8.i.i, 0
+  br i1 %cmp9.i.i, label %for.body.lr.ph.i.i, label %__cxx_global_var_init.exit
+for.body.lr.ph.i.i:
+  %2 = bitcast %class.D* %agg.tmp.i.i to %class.basic_string*
+  br label %for.body.i.i
+for.body.i.i:
+  %conv11.i.i = phi i64 [ 0, %for.body.lr.ph.i.i ], [ %conv.i.i, %for.body.i.i ]
+  %i.010.i.i = phi i32 [ undef, %for.body.lr.ph.i.i ], [ %inc.i.i, %for.body.i.i ]
+  %3 = load %class.D*, %class.D** getelementptr inbounds (%class.BtlConfig, %class.BtlConfig* @a, i64 0, i32 0, i32 0, i32 0), align 8, !tbaa !1, !noalias !6
+  %arrayidx.i.i.i = getelementptr inbounds %class.D, %class.D* %3, i64 %conv11.i.i
+  %4 = bitcast %class.D* %arrayidx.i.i.i to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull %0, i8* %4, i64 24, i32 8, i1 false)
+  %inc.i.i = add i32 %i.010.i.i, 1
+  %conv.i.i = zext i32 %inc.i.i to i64
+  %call.i.i = call i64 @_ZN1CI1D1AIS0_EE5m_fn1Ev(%class.C* getelementptr inbounds (%class.BtlConfig, %class.BtlConfig* @a, i64 0, i32 0))
+  %cmp.i.i = icmp slt i64 %conv.i.i, %call.i.i
+  br i1 %cmp.i.i, label %for.body.i.i, label %__cxx_global_var_init.exit.loopexit
+__cxx_global_var_init.exit.loopexit:
+  br label %__cxx_global_var_init.exit
+__cxx_global_var_init.exit:
+  ret void
+}
+!1 = !{!2, !3, i64 0}
+!2 = !{!"foo", !3, i64 0}
+!3 = !{!"bar", !4, i64 0}
+!4 = !{!"baz", !5, i64 0}
+!5 = !{!"boo"}
+!6 = !{!7}
+!7 = distinct !{!7, !8, !"_ZN1CI1D1AIS0_EEixEl: %agg.result"}
+!8 = distinct !{!8, !"_ZN1CI1D1AIS0_EEixEl"}