There was a long discussion about this a couple of months ago. The
summary was that __attribute__ ((mode (word))) was considered harmful.
It's safe enough when you use it within a program, but when you start
to use in a data structure shared by different codes you run into ABI
problems. A typical example is the eh_return_filter_mode target hook,
which was introduced (without documentation!) because Darwin needs to
access the same exception handling information with two different
values of word_mode.