Placement modes are still specified by the command line keys
ireg_in_iob/oreg_in_iob/ioreg_in_iob, but also introduces more granular
control in the form of attributes at I/O ports:
(* NOIOBFF *) - registers are never placed in this IO,
(* IOBFF *) - registers must be placed in this IO, in case of failure
a warning (not an error) with the reason for nonplacement is issued,
_attribute_absence_ - no diagnostics will be issued: managed to place - good, failed - not bad either.
Signed-off-by: YRabbit <rabbit@yrabbit.cyou>
85210caea4