opt.diff

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index deec33b..3000d90 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -469,6 +469,8 @@ def arch__errors__fatal : Flag<["-"], "arch_errors_fatal">;
 def arch : Separate<["-"], "arch">, Flags<[DriverOption]>;
 def arch__only : Separate<["-"], "arch_only">;
 def a : Joined<["-"], "a">;
+def autocomplete : Joined<["-","--"], "autocomplete=">, Flags<[DriverOption]>, Group<Action_Group>,
+  HelpText<"This is a flag for clang-autocompletion">;
 def bind__at__load : Flag<["-"], "bind_at_load">;
 def bundle__loader : Separate<["-"], "bundle_loader">;
 def bundle : Flag<["-"], "bundle">;
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index f36deff..a2ca679 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -76,6 +76,7 @@
 #endif
 
 using namespace clang::driver;
+using namespace clang::driver::options;
 using namespace clang;
 using namespace llvm::opt;
 
@@ -1216,6 +1217,15 @@ bool Driver::HandleImmediateArgs(const Compilation &C) {
     return false;
   }
 
+  if (Arg *A = C.getArgs().getLastArg(options::OPT_autocomplete)) {
+  // Call function in OptTable with argument Cur. This function is supposed to
+  // return all flags with start with Cur, in InputArglist.
+  for (std::string &S : Opts->matchFlags(A->getValue()))
+      llvm::outs() << S << " ";
+  llvm::outs() << "\n";
+    return false;
+  }
+
   if (C.getArgs().hasArg(options::OPT_print_libgcc_file_name)) {
     ToolChain::RuntimeLibType RLT = TC.GetRuntimeLibType(C.getArgs());
     switch (RLT) {
diff --git a/clang/test/Driver/autocomplete.c b/clang/test/Driver/autocomplete.c
new file mode 100644
index 0000000..fb7e409
--- /dev/null
+++ b/clang/test/Driver/autocomplete.c
@@ -0,0 +1,6 @@
+// RUN: %clang -autocomplete=-fsyn > %t
+// grep "-fsyntax-only" %t
+// RUN: %clang -autocomplete=-fno-sanitize-t > %t
+// grep "-fno-sanitize-thread-atomics -fno-sanitize-thread-func-entry-exit -fno-sanitize-thread-memory-access -fno-sanitize-trap=" %t
+// RUN: %clang -autocomplete=foo > %t
+// not grep "*" %t
diff --git a/llvm/include/llvm/Option/OptTable.h b/llvm/include/llvm/Option/OptTable.h
index 390e527..945e5fb 100644
--- a/llvm/include/llvm/Option/OptTable.h
+++ b/llvm/include/llvm/Option/OptTable.h
@@ -113,6 +113,8 @@ public:
     return getInfo(id).MetaVar;
   }
 
+  std::vector<std::string> matchFlags(StringRef Cur) const;
+
   /// \brief Parse a single argument; returning the new argument and
   /// updating Index.
   ///
@@ -153,6 +155,8 @@ public:
                          unsigned &MissingArgCount, unsigned FlagsToInclude = 0,
                          unsigned FlagsToExclude = 0) const;
 
+  InputArgList MatchFlags(StringRef Cur) const;
+
   /// \brief Render the help text for an option table.
   ///
   /// \param OS - The stream to write the help text to.
diff --git a/llvm/lib/Option/OptTable.cpp b/llvm/lib/Option/OptTable.cpp
index 7eafb00..6b1b2f3 100644
--- a/llvm/lib/Option/OptTable.cpp
+++ b/llvm/lib/Option/OptTable.cpp
@@ -186,6 +186,25 @@ static unsigned matchOption(const OptTable::Info *I, StringRef Str,
   return 0;
 }
 
+std::vector<std::string> OptTable::matchFlags(StringRef Cur) const {
+  std::vector<std::string> Ret;
+  const Info *Start = OptionInfos.begin() + FirstSearchableIndex;
+  const Info *End = OptionInfos.end();
+  for (; Start != End; ++Start) {
+    // for (OptTable::Info Opt : *this::Info) {
+    if (Start->Prefixes) {
+      for (int i = 0; Start->Prefixes[i]; i++) {
+        std::string s =
+            (std::string(Start->Prefixes[i]) + std::string(Start->Name));
+        StringRef CurFlag = s;
+        if (CurFlag.startswith(Cur))
+          Ret.push_back(s);
+      }
+    }
+  }
+  return Ret;
+}
+
 Arg *OptTable::ParseOneArg(const ArgList &Args, unsigned &Index,
                            unsigned FlagsToInclude,
                            unsigned FlagsToExclude) const {