Index: clang/include/clang/Driver/Driver.h
===================================================================
--- clang/include/clang/Driver/Driver.h
+++ clang/include/clang/Driver/Driver.h
@@ -420,6 +420,10 @@
   // FIXME: This should be in CompilationInfo.
   std::string GetProgramPath(StringRef Name, const ToolChain &TC) const;
 
+  /// handleAutocompletions - Handle --autocomplete by searching and printing
+  /// possible flags, descriptions, and its arguments.
+  void handleAutocompletions(StringRef PassedFlags) const;
+
   /// HandleImmediateArgs - Handle any arguments which should be
   /// treated before building actions or binding tools.
   ///
Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -1157,6 +1157,56 @@
     OS << i << ',' << DiagnosticIDs::getCategoryNameFromID(i) << '\n';
 }
 
+void Driver::handleAutocompletions(StringRef PassedFlags) const {
+  // Print out all options that start with a given argument. This is used for
+  // shell autocompletion.
+  std::vector<std::string> SuggestedCompletions;
+
+  unsigned short DisableFlags =
+      options::NoDriverOption | options::Unsupported | options::Ignored;
+  // We want to show cc1-only options only when clang is invoked as "clang
+  // -cc1".
+  // When clang is invoked as "clang -cc1", we add "#" to the beginning of an
+  // --autocomplete
+  // option so that the clang driver can distinguish whether it is requested to
+  // show cc1-only options or not.
+  if (PassedFlags[0] == '#') {
+    DisableFlags &= ~options::NoDriverOption;
+    PassedFlags = PassedFlags.substr(1);
+  }
+
+  if (PassedFlags.find(',') == StringRef::npos) {
+    // If the flag is in the form of "--autocomplete=-foo",
+    // we were requested to print out all option names that start with "-foo".
+    // For example, "--autocomplete=-fsyn" is expanded to "-fsyntax-only".
+    SuggestedCompletions = Opts->findByPrefix(PassedFlags, DisableFlags);
+
+    // We have to query the -W flags manually as they're not in the OptTable.
+    // TODO: Find a good way to add them to OptTable instead and them remove
+    // this code.
+    for (StringRef S : DiagnosticIDs::getDiagnosticFlags())
+      if (S.startswith(PassedFlags))
+        SuggestedCompletions.push_back(S);
+  } else {
+    // If the flag is in the form of "--autocomplete=foo,bar", we were
+    // requested to print out all option values for "-foo" that start with
+    // "bar". For example,
+    // "--autocomplete=-stdlib=,l" is expanded to "libc++" and "libstdc++".
+    StringRef Option, Arg;
+    std::tie(Option, Arg) = PassedFlags.split(',');
+    SuggestedCompletions = Opts->suggestValueCompletions(Option, Arg);
+  }
+
+  // Sort the autocomplete candidates so that shells print them out in a
+  // deterministic order. We could sort in any way, but we chose
+  // case-insensitive sorting for consistency with the -help option
+  // which prints out options in the case-insensitive alphabetical order.
+  std::sort(SuggestedCompletions.begin(), SuggestedCompletions.end(),
+            [](StringRef A, StringRef B) { return A.compare_lower(B) < 0; });
+
+  llvm::outs() << llvm::join(SuggestedCompletions, "\n") << '\n';
+}
+
 bool Driver::HandleImmediateArgs(const Compilation &C) {
   // The order these options are handled in gcc is all over the place, but we
   // don't expect inconsistencies w.r.t. that to matter in practice.
@@ -1250,50 +1300,8 @@
   }
 
   if (Arg *A = C.getArgs().getLastArg(options::OPT_autocomplete)) {
-    // Print out all options that start with a given argument. This is used for
-    // shell autocompletion.
     StringRef PassedFlags = A->getValue();
-    std::vector<std::string> SuggestedCompletions;
-
-    unsigned short DisableFlags = options::NoDriverOption | options::Unsupported | options::Ignored;
-    // We want to show cc1-only options only when clang is invoked as "clang -cc1".
-    // When clang is invoked as "clang -cc1", we add "#" to the beginning of an --autocomplete
-    // option so that the clang driver can distinguish whether it is requested to show cc1-only options or not.
-    if (PassedFlags[0] == '#') {
-      DisableFlags &= ~options::NoDriverOption;
-      PassedFlags = PassedFlags.substr(1);
-    }
-
-    if (PassedFlags.find(',') == StringRef::npos) {
-      // If the flag is in the form of "--autocomplete=-foo",
-      // we were requested to print out all option names that start with "-foo".
-      // For example, "--autocomplete=-fsyn" is expanded to "-fsyntax-only".
-      SuggestedCompletions = Opts->findByPrefix(PassedFlags, DisableFlags);
-
-      // We have to query the -W flags manually as they're not in the OptTable.
-      // TODO: Find a good way to add them to OptTable instead and them remove
-      // this code.
-      for (StringRef S : DiagnosticIDs::getDiagnosticFlags())
-        if (S.startswith(PassedFlags))
-          SuggestedCompletions.push_back(S);
-    } else {
-      // If the flag is in the form of "--autocomplete=foo,bar", we were
-      // requested to print out all option values for "-foo" that start with
-      // "bar". For example,
-      // "--autocomplete=-stdlib=,l" is expanded to "libc++" and "libstdc++".
-      StringRef Option, Arg;
-      std::tie(Option, Arg) = PassedFlags.split(',');
-      SuggestedCompletions = Opts->suggestValueCompletions(Option, Arg);
-    }
-
-    // Sort the autocomplete candidates so that shells print them out in a
-    // deterministic order. We could sort in any way, but we chose
-    // case-insensitive sorting for consistency with the -help option
-    // which prints out options in the case-insensitive alphabetical order.
-    std::sort(SuggestedCompletions.begin(), SuggestedCompletions.end(),
-              [](StringRef A, StringRef B) { return A.compare_lower(B) < 0; });
-
-    llvm::outs() << llvm::join(SuggestedCompletions, "\n") << '\n';
+    handleAutocompletions(PassedFlags);
     return false;
   }