diff --git a/gcc/c-family/c-cppbuiltin.c b/gcc/c-family/c-cppbuiltin.c
index 826b3b9..c34b76b 100644
--- a/gcc/c-family/c-cppbuiltin.c
+++ b/gcc/c-family/c-cppbuiltin.c
@@ -576,10 +576,13 @@ c_cpp_builtins_optimize_pragma (cpp_reader *pfile, tree prev_tree,
   else if (prev->x_optimize_size && !cur->x_optimize_size)
     cpp_undef (pfile, "__OPTIMIZE_SIZE__");
 
-  if (!prev->x_optimize && cur->x_optimize)
+  if (!prev->x_optimize && cur->x_optimize) {
     cpp_define (pfile, "__OPTIMIZE__");
-  else if (prev->x_optimize && !cur->x_optimize)
+    cpp_define (pfile, "_FORTIFY_SOURCE=(__OPTIMIZE__ > 0 ? 2 : 0)");
+  } else if (prev->x_optimize && !cur->x_optimize) {
     cpp_undef (pfile, "__OPTIMIZE__");
+    cpp_undef (pfile, "_FORTIFY_SOURCE");
+  }
 
   prev_fast_math = fast_math_flags_struct_set_p (prev);
   cur_fast_math  = fast_math_flags_struct_set_p (cur);
@@ -1336,7 +1339,10 @@ c_cpp_builtins (cpp_reader *pfile)
   builtin_define_with_value ("__USER_LABEL_PREFIX__", user_label_prefix, 0);
 
   /* Fortify Source enabled by default w/optimization.  */
-  cpp_define (pfile, "_FORTIFY_SOURCE=((defined __OPTIMIZE__ && __OPTIMIZE__ > 0) ? 2 : 0)");
+  if (cpp_defined (pfile,
+		   (const unsigned char *) "__OPTIMIZE__",
+		   (int) strlen("__OPTIMIZE__")))
+    cpp_define (pfile, "_FORTIFY_SOURCE=(__OPTIMIZE__ > 0 ? 2 : 0)");
 
   /* Misc.  */
   if (flag_gnu89_inline)