I see this in configure.in:

AC_ARG_ENABLE(nonportable-atomics,
[  --enable-nonportable-atomics  Use optimized atomic code which may produce nonportable binaries],
[if test $enableval = yes; then
   force_generic_atomics=no
 else
   force_generic_atomics=yes
 fi
],
[case $host_cpu in
   i[[456]]86) force_generic_atomics=yes ;;
   *) force_generic_atomics=no ;;
esac
])

I was wondering why the three host_cpus i486, i586, and i686 have special treatment for the default setting as compared to all other cpu architectures?