opt.diff

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index d812bd8..4f1ea08 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -469,6 +469,7 @@ 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=">;
 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..bec4bf4 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -1216,6 +1216,13 @@ bool Driver::HandleImmediateArgs(const Compilation &C) {
     return false;
   }
 
+  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.
+    llvm::outs() << llvm::join(Opts->findByPrefix(A->getValue()), " ") << '\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..dab6455
--- /dev/null
+++ b/clang/test/Driver/autocomplete.c
@@ -0,0 +1,3 @@
+// RUN: %clang --autocomplete=-fsyn | grep '\-fsyntax-only'
+// RUN: %clang --autocomplete=-s | grep '\-std=.*-stdlib='
+// RUN: %clang --autocomplete=foo | grep -v .
diff --git a/clang/utils/bash-autocomplete.sh b/clang/utils/bash-autocomplete.sh
new file mode 100644
index 0000000..f01089f
--- /dev/null
+++ b/clang/utils/bash-autocomplete.sh
@@ -0,0 +1,14 @@
+_clang()
+{
+    local cur prev words cword flags
+    _init_completion -n : || return
+
+    flags=$( clang --autocomplete="$cur" )
+    if [[ "$flags" == "" || "$cur" == "" ]]; then
+        _filedir
+    else
+        COMPREPLY=( $( compgen -W "$flags" -- "$cur") )
+    fi
+
+} &&
+    complete -F _clang clang
diff --git a/llvm/include/llvm/Option/OptTable.h b/llvm/include/llvm/Option/OptTable.h
index 390e527..ebf78db 100644
--- a/llvm/include/llvm/Option/OptTable.h
+++ b/llvm/include/llvm/Option/OptTable.h
@@ -113,6 +113,14 @@ public:
     return getInfo(id).MetaVar;
   }
 
+  /// \brief Find flags from OptTable which starts with Cur.
+  /// This is used for shell autocompletion.
+  ///
+  /// \param [in] Cur - String of user input.
+  ///
+  /// \return The vector of flags which starts with Cur.
+  std::vector<std::string> findByPrefix(StringRef Cur) const;
+
   /// \brief Parse a single argument; returning the new argument and
   /// updating Index.
   ///
diff --git a/llvm/lib/Option/OptTable.cpp b/llvm/lib/Option/OptTable.cpp
index 7eafb00..b09f8d3 100644
--- a/llvm/lib/Option/OptTable.cpp
+++ b/llvm/lib/Option/OptTable.cpp
@@ -186,6 +186,20 @@ static unsigned matchOption(const OptTable::Info *I, StringRef Str,
   return 0;
 }
 
+std::vector<std::string> OptTable::findByPrefix(StringRef Cur) const {
+  std::vector<std::string> Ret;
+  for (const Info &In : OptionInfos.slice(FirstSearchableIndex)) {
+    if (!In.Prefixes)
+      continue;
+    for (int I = 0; In.Prefixes[I]; I++) {
+      std::string S = (std::string(In.Prefixes[I]) + std::string(In.Name));
+      if (StringRef(S).startswith(Cur))
+        Ret.push_back(S);
+    }
+  }
+  return Ret;
+}
+
 Arg *OptTable::ParseOneArg(const ArgList &Args, unsigned &Index,
                            unsigned FlagsToInclude,
                            unsigned FlagsToExclude) const {