diff --git a/llvm/include/llvm/Support/CommandLine.h b/llvm/include/llvm/Support/CommandLine.h
--- a/llvm/include/llvm/Support/CommandLine.h
+++ b/llvm/include/llvm/Support/CommandLine.h
@@ -285,7 +285,9 @@
   StringRef ValueStr; // String describing what the value of this option is
   SmallVector<OptionCategory *, 1>
       Categories;                    // The Categories this option belongs to
-  SmallPtrSet<SubCommand *, 1> Subs; // The subcommands this option belongs to.
+
+  // Return the set of SubCommands that this Option was added to.
+  SmallPtrSet<SubCommand *, 4> getSubCommands() const;
 
   inline enum NumOccurrencesFlag getNumOccurrencesFlag() const {
     return (enum NumOccurrencesFlag)Occurrences;
@@ -304,6 +306,7 @@
   }
 
   inline unsigned getMiscFlags() const { return Misc; }
+  inline bool isFullyInitialized() const { return FullyInitialized; }
   inline unsigned getPosition() const { return Position; }
   inline unsigned getNumAdditionalVals() const { return AdditionalVals; }
 
@@ -317,12 +320,6 @@
     return getNumOccurrencesFlag() == cl::ConsumeAfter;
   }
 
-  bool isInAllSubCommands() const {
-    return any_of(Subs, [](const SubCommand *SC) {
-      return SC == &*AllSubCommands;
-    });
-  }
-
   //-------------------------------------------------------------------------===
   // Accessor functions set by OptionModifiers
   //
@@ -336,7 +333,6 @@
   void setMiscFlag(enum MiscFlags M) { Misc |= M; }
   void setPosition(unsigned pos) { Position = pos; }
   void addCategory(OptionCategory &C);
-  void addSubCommand(SubCommand &S) { Subs.insert(&S); }
 
 protected:
   explicit Option(enum NumOccurrencesFlag OccurrencesFlag,
@@ -354,7 +350,14 @@
 
   // addArgument - Register this argument with the commandline system.
   //
-  void addArgument();
+  void addArgument(SubCommand &SC);
+
+  // addArgument - Only called in done() method to add default
+  // TopLevelSubCommand.
+  void addArgument() {
+    if (!isFullyInitialized())
+      addArgument(*TopLevelSubCommand);
+  }
 
   /// Unregisters this option from the CommandLine system.
   ///
@@ -465,7 +468,7 @@
 
   sub(SubCommand &S) : Sub(S) {}
 
-  template <class Opt> void apply(Opt &O) const { O.addSubCommand(Sub); }
+  template <class Opt> void apply(Opt &O) const { O.addArgument(Sub); }
 };
 
 //===----------------------------------------------------------------------===//
@@ -1772,11 +1775,9 @@
       error("cl::alias must have argument name specified!");
     if (!AliasFor)
       error("cl::alias must have an cl::aliasopt(option) specified!");
-    if (!Subs.empty())
-      error("cl::alias must not have cl::sub(), aliased option's cl::sub() will be used!");
-    Subs = AliasFor->Subs;
     Categories = AliasFor->Categories;
-    addArgument();
+    for(SubCommand *SC: AliasFor->getSubCommands())
+      addArgument(*SC);
   }
 
 public:
diff --git a/llvm/lib/Support/CommandLine.cpp b/llvm/lib/Support/CommandLine.cpp
--- a/llvm/lib/Support/CommandLine.cpp
+++ b/llvm/lib/Support/CommandLine.cpp
@@ -142,7 +142,7 @@
   // This collects Options added with the cl::DefaultOption flag. Since they can
   // be overridden, they are not added to the appropriate SubCommands until
   // ParseCommandLineOptions actually runs.
-  SmallVector<Option*, 4> DefaultOptions;
+  SmallVector<std::pair<Option*, SubCommand*>, 4> DefaultOptions;
 
   // This collects the different option categories that have been registered.
   SmallPtrSet<OptionCategory *, 16> RegisteredOptionCategories;
@@ -182,15 +182,16 @@
   }
 
   void addLiteralOption(Option &Opt, StringRef Name) {
-    if (Opt.Subs.empty())
-      addLiteralOption(Opt, &*TopLevelSubCommand, Name);
-    else {
-      for (auto SC : Opt.Subs)
-        addLiteralOption(Opt, SC, Name);
-    }
+    for(SubCommand *SC: Opt.getSubCommands())
+      addLiteralOption(Opt, SC, Name);
   }
 
-  void addOption(Option *O, SubCommand *SC) {
+  void addOption(Option *O, SubCommand *SC, bool ProcessDefaultOptions = false) {
+    if (!ProcessDefaultOptions && O->isDefaultOption()) {
+      DefaultOptions.push_back(std::make_pair(O, SC));
+      return;
+    }
+
     bool HadErrors = false;
     if (O->hasArgStr()) {
       // If it's a DefaultOption, check to make sure it isn't already there.
@@ -232,22 +233,14 @@
       for (const auto &Sub : RegisteredSubCommands) {
         if (SC == Sub)
           continue;
-        addOption(O, Sub);
+        addOption(O, Sub, ProcessDefaultOptions);
       }
     }
   }
 
-  void addOption(Option *O, bool ProcessDefaultOption = false) {
-    if (!ProcessDefaultOption && O->isDefaultOption()) {
-      DefaultOptions.push_back(O);
-      return;
-    }
-
-    if (O->Subs.empty()) {
-      addOption(O, &*TopLevelSubCommand);
-    } else {
-      for (auto SC : O->Subs)
-        addOption(O, SC);
+  void addDefaultOptions() {
+    for (std::pair<Option *, SubCommand *> &DO : DefaultOptions) {
+      addOption(DO.first, DO.second, true);
     }
   }
 
@@ -285,17 +278,8 @@
   }
 
   void removeOption(Option *O) {
-    if (O->Subs.empty())
-      removeOption(O, &*TopLevelSubCommand);
-    else {
-      if (O->isInAllSubCommands()) {
-        for (auto SC : RegisteredSubCommands)
-          removeOption(O, SC);
-      } else {
-        for (auto SC : O->Subs)
-          removeOption(O, SC);
-      }
-    }
+    for (auto SC : RegisteredSubCommands)
+      removeOption(O, SC);
   }
 
   bool hasOptions(const SubCommand &Sub) const {
@@ -324,17 +308,8 @@
   }
 
   void updateArgStr(Option *O, StringRef NewName) {
-    if (O->Subs.empty())
-      updateArgStr(O, NewName, &*TopLevelSubCommand);
-    else {
-      if (O->isInAllSubCommands()) {
-        for (auto SC : RegisteredSubCommands)
-          updateArgStr(O, NewName, SC);
-      } else {
-        for (auto SC : O->Subs)
-          updateArgStr(O, NewName, SC);
-      }
-    }
+    for (auto SC : RegisteredSubCommands)
+      updateArgStr(O, NewName, SC);
   }
 
   void printOptionValues();
@@ -427,13 +402,27 @@
   GlobalParser->MoreHelp.push_back(Help);
 }
 
-void Option::addArgument() {
-  GlobalParser->addOption(this);
+void Option::addArgument(SubCommand &SC) {
+  GlobalParser->addOption(this, &SC);
   FullyInitialized = true;
 }
 
 void Option::removeArgument() { GlobalParser->removeOption(this); }
 
+SmallPtrSet<SubCommand *, 4> Option::getSubCommands() const {
+  // This can happen for enums and literal options.
+  if (ArgStr.empty())
+    return SmallPtrSet<SubCommand *, 4>{&*TopLevelSubCommand};
+
+  SmallPtrSet<SubCommand *, 4> Subs;
+  for (SubCommand *SC : GlobalParser->getRegisteredSubcommands()) {
+    auto I = SC->OptionsMap.find(ArgStr);
+    if (I != SC->OptionsMap.end())
+      Subs.insert(SC);
+  }
+  return Subs;
+}
+
 void Option::setArgStr(StringRef S) {
   if (FullyInitialized)
     GlobalParser->updateArgStr(this, S);
@@ -1261,9 +1250,7 @@
   auto &SinkOpts = ChosenSubCommand->SinkOpts;
   auto &OptionsMap = ChosenSubCommand->OptionsMap;
 
-  for (auto O: DefaultOptions) {
-    addOption(O, true);
-  }
+  addDefaultOptions();
 
   if (ConsumeAfterOpt) {
     assert(PositionalOpts.size() > 0 &&