--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/tristate.h	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,82 @@
+/* "True" vs "False" vs "Unknown".
+   Copyright (C) 2019-2020 Free Software Foundation, Inc.
+   Contributed by David Malcolm <dmalcolm@redhat.com>.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3, or (at your option)
+any later version.
+
+GCC is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_TRISTATE_H
+#define GCC_TRISTATE_H
+
+/* "True" vs "False" vs "Unknown".  */
+
+class tristate {
+ public:
+  enum value {
+    TS_UNKNOWN,
+    TS_TRUE,
+    TS_FALSE
+  };
+
+  tristate (enum value val) : m_value (val) {}
+  tristate (bool val) : m_value (val ? TS_TRUE : TS_FALSE) {}
+  static tristate unknown () { return tristate (TS_UNKNOWN); }
+
+  const char *as_string () const;
+
+  bool is_known () const { return m_value != TS_UNKNOWN; }
+  bool is_true () const { return m_value == TS_TRUE; }
+  bool is_false () const { return m_value == TS_FALSE; }
+
+  tristate not_ () const;
+  tristate or_ (tristate other) const;
+  tristate and_ (tristate other) const;
+
+  bool operator== (const tristate &other) const
+  {
+    return m_value == other.m_value;
+  }
+
+  bool operator!= (const tristate &other) const
+  {
+    return m_value != other.m_value;
+  }
+
+ private:
+  enum value m_value;
+};
+
+/* Overloaded boolean operators on tristates.  */
+
+inline tristate
+operator ! (tristate t)
+{
+  return t.not_ ();
+}
+
+inline tristate
+operator || (tristate a, tristate b)
+{
+  return a.or_ (b);
+}
+
+inline tristate
+operator && (tristate a, tristate b)
+{
+  return a.and_ (b);
+}
+
+#endif /* GCC_TRISTATE_H */