--- gcc/gcc/cppbuiltin.c
+++ gcc/gcc/cppbuiltin.c
@@ -99,6 +99,8 @@ define_builtin_macros_for_compilation_flags (cpp_reader *pfile)
     cpp_define (pfile, "__OPTIMIZE_SIZE__");
   if (optimize)
     cpp_define (pfile, "__OPTIMIZE__");
+  if (optimize >= 2)
+    cpp_define (pfile, "_FORTIFY_SOURCE=2");
 
   if (fast_math_flags_set_p (&global_options))
     cpp_define (pfile, "__FAST_MATH__");
--- gcc/gcc/doc/invoke.texi
+++ gcc/gcc/doc/invoke.texi
@@ -6658,6 +6658,12 @@ also turns on the following optimization flags:
 Please note the warning under @option{-fgcse} about
 invoking @option{-O2} on programs that use computed gotos.
 
+NOTE: In ALT Linux gcc 4.1.1-alt9 and later versions,
+@option{-D_FORTIFY_SOURCE=2} is set by default, and is activated when
+@option{-O} is set to 2 or higher.  This enables additional compile-time
+and run-time checks for several libc functions.  To disable, specify
+either @option{-U_FORTIFY_SOURCE} or @option{-D_FORTIFY_SOURCE=0}.
+
 @item -O3
 @opindex O3
 Optimize yet more.  @option{-O3} turns on all optimizations specified