diff --git a/llvm/docs/CommandGuide/tblgen.rst b/llvm/docs/CommandGuide/tblgen.rst
--- a/llvm/docs/CommandGuide/tblgen.rst
+++ b/llvm/docs/CommandGuide/tblgen.rst
@@ -99,6 +99,10 @@
 
  Show the version number of the program.
 
+.. option:: -warn-on-unused-entities
+
+ Generate warnings on unused classes and multiclasses.
+
 .. option:: -write-if-changed
 
   Write the output file only if it is new or has changed.
diff --git a/llvm/include/llvm/TableGen/Record.h b/llvm/include/llvm/TableGen/Record.h
--- a/llvm/include/llvm/TableGen/Record.h
+++ b/llvm/include/llvm/TableGen/Record.h
@@ -1598,6 +1598,7 @@
 
   bool IsAnonymous;
   bool IsClass;
+  bool IsUsed = false;
 
   void checkName();
 
@@ -1665,6 +1666,10 @@
 
   bool isClass() const { return IsClass; }
 
+  bool isUsed() const { return IsUsed; }
+
+  void setUsed() { IsUsed = true; }
+
   ArrayRef<Init *> getTemplateArgs() const {
     return TemplateArgs;
   }
diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
--- a/llvm/lib/TableGen/Main.cpp
+++ b/llvm/lib/TableGen/Main.cpp
@@ -68,6 +68,10 @@
     "no-warn-on-unused-template-args",
     cl::desc("Disable unused template argument warnings."));
 
+static cl::opt<bool>
+    WarnOnUnusedEntities("warn-on-unused-entities",
+                         cl::desc("Enable unused entity warnings."));
+
 static int reportError(const char *ProgName, Twine Msg) {
   errs() << ProgName << ": " << Msg;
   errs().flush();
@@ -127,6 +131,9 @@
     return 1;
   Records.stopTimer();
 
+  if (WarnOnUnusedEntities)
+    Parser.CheckUnusedEntities();
+
   // Write output to memory.
   Records.startBackendTimer("Backend overall");
   std::string OutString;
diff --git a/llvm/lib/TableGen/TGParser.h b/llvm/lib/TableGen/TGParser.h
--- a/llvm/lib/TableGen/TGParser.h
+++ b/llvm/lib/TableGen/TGParser.h
@@ -78,6 +78,7 @@
 struct MultiClass {
   Record Rec; // Placeholder for template args and Name.
   std::vector<RecordsEntry> Entries;
+  bool IsUsed = false;
 
   void dump() const;
 
@@ -219,7 +220,10 @@
     CurScope = CurScope->extractParent();
   }
 
+  void CheckUnusedEntities() const;
+
 private: // Semantic analysis methods.
+  void AddReference(Record *Rec, SMRange Loc);
   bool AddValue(Record *TheRec, SMLoc Loc, const RecordVal &RV);
   /// Set the value of a RecordVal within the given record. If `OverrideDefLoc`
   /// is set, the provided location overrides any existing location of the
diff --git a/llvm/lib/TableGen/TGParser.cpp b/llvm/lib/TableGen/TGParser.cpp
--- a/llvm/lib/TableGen/TGParser.cpp
+++ b/llvm/lib/TableGen/TGParser.cpp
@@ -212,6 +212,13 @@
   return nullptr;
 }
 
+void TGParser::AddReference(Record *Rec, SMRange Loc) {
+  Rec->setUsed();
+
+  if (TrackReferenceLocs)
+    Rec->appendReferenceLoc(Loc);
+}
+
 bool TGParser::AddValue(Record *CurRec, SMLoc Loc, const RecordVal &RV) {
   if (!CurRec)
     CurRec = &CurMultiClass->Rec;
@@ -683,8 +690,8 @@
                Lex.getCurStrVal() + "'");
     else
       TokError(Msg);
-  } else if (TrackReferenceLocs) {
-    Result->appendReferenceLoc(Lex.getLocRange());
+  } else {
+    AddReference(Result, Lex.getLocRange());
   }
 
   Lex.Lex();
@@ -706,6 +713,8 @@
   if (!Result)
     TokError("Couldn't find multiclass '" + Lex.getCurStrVal() + "'");
 
+  Result->IsUsed = true;
+
   Lex.Lex();
   return Result;
 }
@@ -1119,10 +1128,8 @@
 
   if (Init *I = Records.getGlobal(Name->getValue())) {
     // Add a reference to the global if it's a record.
-    if (TrackReferenceLocs) {
-      if (auto *Def = dyn_cast<DefInit>(I))
-        Def->getDef()->appendReferenceLoc(NameLoc);
-    }
+    if (auto *Def = dyn_cast<DefInit>(I))
+      AddReference(Def->getDef(), NameLoc);
     return I;
   }
 
@@ -2608,8 +2615,7 @@
       }
     }
 
-    if (TrackReferenceLocs)
-      Class->appendReferenceLoc(NameLoc);
+    AddReference(Class, NameLoc);
     return VarDefInit::get(Class, Args)->Fold();
   }
   case tgtok::l_brace: {           // Value ::= '{' ValueList '}'
@@ -4295,6 +4301,32 @@
   return false;
 }
 
+static bool isInCommonTableGenHeader(ArrayRef<SMLoc> Loc) {
+  // TODO: Marking common .td files explicitly would be more reliable.
+  auto BufferID = SrcMgr.FindBufferContainingLoc(Loc[0]);
+  assert(BufferID && "Invalid location!");
+  StringRef FileSpec =
+      SrcMgr.getBufferInfo(BufferID).Buffer->getBufferIdentifier();
+  return FileSpec.contains("/include/llvm/CodeGen/") ||
+         FileSpec.contains("/include/llvm/IR/") ||
+         FileSpec.contains("/include/llvm/TableGen/") ||
+         FileSpec.contains("/include/llvm/Target/");
+}
+
+void TGParser::CheckUnusedEntities() const {
+  for (const auto &I : MultiClasses) {
+    ArrayRef<SMLoc> Loc = I.second->Rec.getLoc();
+    if (!I.second->IsUsed && !isInCommonTableGenHeader(Loc))
+      PrintWarning(Loc, "multiclass '" + I.first + "' is unused");
+  }
+
+  for (const auto &I : Records.getClasses()) {
+    ArrayRef<SMLoc> Loc = I.second->getLoc();
+    if (!I.second->isUsed() && !isInCommonTableGenHeader(Loc))
+      PrintWarning(Loc, "class '" + I.first + "' is unused");
+  }
+}
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 LLVM_DUMP_METHOD void RecordsEntry::dump() const {
   if (Loop)
diff --git a/llvm/test/TableGen/warn-unused-class.td b/llvm/test/TableGen/warn-unused-class.td
new file mode 100644
--- /dev/null
+++ b/llvm/test/TableGen/warn-unused-class.td
@@ -0,0 +1,9 @@
+// RUN: llvm-tblgen --warn-on-unused-entities %s 2>&1 | \
+// RUN:     FileCheck --implicit-check-not=warning %s
+
+class Used {}
+class Unused {}
+
+def : Used;
+
+// CHECK: warning: class 'Unused' is unused
diff --git a/llvm/test/TableGen/warn-unused-multiclass.td b/llvm/test/TableGen/warn-unused-multiclass.td
new file mode 100644
--- /dev/null
+++ b/llvm/test/TableGen/warn-unused-multiclass.td
@@ -0,0 +1,9 @@
+// RUN: llvm-tblgen --warn-on-unused-entities %s 2>&1 | \
+// RUN:     FileCheck --implicit-check-not=warning %s
+
+multiclass Used { def foo; }
+multiclass Unused { def foo; }
+
+defm : Used;
+
+// CHECK: warning: multiclass 'Unused' is unused