diff --git a/include/llvm/MC/MCSchedule.h b/include/llvm/MC/MCSchedule.h
index 673cdf6fb68..1edf204940c 100644
--- a/include/llvm/MC/MCSchedule.h
+++ b/include/llvm/MC/MCSchedule.h
@@ -174,6 +174,8 @@ public:
   unsigned MispredictPenalty;
   static const unsigned DefaultMispredictPenalty = 10;
 
+  bool CompleteModel;
+
 private:
   unsigned ProcID;
   const MCProcResourceDesc *ProcResourceTable;
@@ -194,6 +196,7 @@ public:
                   LoadLatency(DefaultLoadLatency),
                   HighLatency(DefaultHighLatency),
                   MispredictPenalty(DefaultMispredictPenalty),
+                  CompleteModel(true),
                   ProcID(0), ProcResourceTable(0), SchedClassTable(0),
                   NumProcResourceKinds(0), NumSchedClasses(0),
                   InstrItineraries(0) {
@@ -203,19 +206,23 @@ public:
 
   // Table-gen driven ctor.
   MCSchedModel(unsigned iw, int mbs, unsigned ll, unsigned hl,
-               unsigned mp, unsigned pi, const MCProcResourceDesc *pr,
+               unsigned mp, bool cm, unsigned pi, const MCProcResourceDesc *pr,
                const MCSchedClassDesc *sc, unsigned npr, unsigned nsc,
                const InstrItinerary *ii):
     IssueWidth(iw), MicroOpBufferSize(mbs), LoadLatency(ll), HighLatency(hl),
-    MispredictPenalty(mp), ProcID(pi), ProcResourceTable(pr),
-    SchedClassTable(sc), NumProcResourceKinds(npr), NumSchedClasses(nsc),
-    InstrItineraries(ii) {}
+    MispredictPenalty(mp), CompleteModel(cm), ProcID(pi),
+    ProcResourceTable(pr), SchedClassTable(sc), NumProcResourceKinds(npr),
+    NumSchedClasses(nsc), InstrItineraries(ii) {}
 
   unsigned getProcessorID() const { return ProcID; }
 
   /// Does this machine model include instruction-level scheduling.
   bool hasInstrSchedModel() const { return SchedClassTable; }
 
+  /// Return true if this machine model data for all instructions with a
+  /// scheduling class (itinerary class or SchedRW list).
+  bool isComplete() const { return CompleteModel; }
+
   unsigned getNumProcResourceKinds() const {
     return NumProcResourceKinds;
   }
diff --git a/include/llvm/Target/TargetSchedule.td b/include/llvm/Target/TargetSchedule.td
index 575cb83568a..e81a2fbade9 100644
--- a/include/llvm/Target/TargetSchedule.td
+++ b/include/llvm/Target/TargetSchedule.td
@@ -86,6 +86,15 @@ class SchedMachineModel {
   // Per-cycle resources tables.
   ProcessorItineraries Itineraries = NoItineraries;
 
+  // Subtargets that define a model for only a subset of instructions
+  // that have a scheduling class (itinerary class or SchedRW list)
+  // and may actually be generated for that subtarget must clear this
+  // bit. Otherwise, the scheduler considers an unmodelled opcode to
+  // be an error. This should only be set during initial bringup,
+  // or there will be no way to catch simple errors in the model
+  // resulting from changes to the instruction definitions.
+  bit CompleteModel = 1;
+
   bit NoModel = 0; // Special tag to indicate missing machine model.
 }
 
diff --git a/lib/CodeGen/TargetSchedule.cpp b/lib/CodeGen/TargetSchedule.cpp
index 64ee9d1c464..fd3f49657b0 100644
--- a/lib/CodeGen/TargetSchedule.cpp
+++ b/lib/CodeGen/TargetSchedule.cpp
@@ -210,7 +210,8 @@ unsigned TargetSchedModel::computeOperandLatency(
   // unit latency (defaultDefLatency may be too conservative).
 #ifndef NDEBUG
   if (SCDesc->isValid() && !DefMI->getOperand(DefOperIdx).isImplicit()
-      && !DefMI->getDesc().OpInfo[DefOperIdx].isOptionalDef()) {
+      && !DefMI->getDesc().OpInfo[DefOperIdx].isOptionalDef()
+      && SchedModel.isComplete()) {
     std::string Err;
     raw_string_ostream ss(Err);
     ss << "DefIdx " << DefIdx << " exceeds machine model writes for "
diff --git a/lib/Target/X86/X86SchedHaswell.td b/lib/Target/X86/X86SchedHaswell.td
index 62ba2bc1c9e..9748261262d 100644
--- a/lib/Target/X86/X86SchedHaswell.td
+++ b/lib/Target/X86/X86SchedHaswell.td
@@ -19,6 +19,10 @@ def HaswellModel : SchedMachineModel {
   let MicroOpBufferSize = 192; // Based on the reorder buffer.
   let LoadLatency = 4;
   let MispredictPenalty = 16;
+
+  // FIXME: SSE4 and AVX are unimplemented. This flag is set to allow
+  // the scheduler to assign a default model to unrecognized opcodes.
+  let CompleteModel = 0;
 }
 
 let SchedModel = HaswellModel in {
diff --git a/lib/Target/X86/X86SchedSandyBridge.td b/lib/Target/X86/X86SchedSandyBridge.td
index 52ead94714d..3011c6d9d64 100644
--- a/lib/Target/X86/X86SchedSandyBridge.td
+++ b/lib/Target/X86/X86SchedSandyBridge.td
@@ -20,6 +20,10 @@ def SandyBridgeModel : SchedMachineModel {
   let MicroOpBufferSize = 168; // Based on the reorder buffer.
   let LoadLatency = 4;
   let MispredictPenalty = 16;
+
+  // FIXME: SSE4 and AVX are unimplemented. This flag is set to allow
+  // the scheduler to assign a default model to unrecognized opcodes.
+  let CompleteModel = 0;
 }
 
 let SchedModel = SandyBridgeModel in {
diff --git a/utils/TableGen/SubtargetEmitter.cpp b/utils/TableGen/SubtargetEmitter.cpp
index 81bb6f8fd47..b9f9d060394 100644
--- a/utils/TableGen/SubtargetEmitter.cpp
+++ b/utils/TableGen/SubtargetEmitter.cpp
@@ -1198,6 +1198,11 @@ void SubtargetEmitter::EmitProcessorModels(raw_ostream &OS) {
     EmitProcessorProp(OS, PI->ModelDef, "LoadLatency", ',');
     EmitProcessorProp(OS, PI->ModelDef, "HighLatency", ',');
     EmitProcessorProp(OS, PI->ModelDef, "MispredictPenalty", ',');
+
+    OS << "  " << (bool)(PI->ModelDef ?
+                         PI->ModelDef->getValueAsBit("CompleteModel") : 0)
+       << ", // " << "CompleteModel\n";
+
     OS << "  " << PI->Index << ", // Processor ID\n";
     if (PI->hasInstrSchedModel())
       OS << "  " << PI->ModelName << "ProcResources" << ",\n"