The ASYNC_REG is an attribute that
affects many processes in the Vivado tools flow. The
purpose of this attribute is to inform the tool that a register is capable of receiving
asynchronous data in the D input pin relative to the source clock, or that the register
is a synchronizing register within a synchronization chain. The Vivado synthesis, when encountering this attribute treats it as a DONT_TOUCH attribute and pushes the ASYNC_REG property forward in the netlist. This process ensures that the
object with the ASYNC_REG property is not optimized
out, and that tools later in the flow receive the property to handle it correctly.
For information on how other Vivado tools handle this attribute, see Vivado Design Suite Properties Reference Guide (UG912).
You can place this attribute on any register; values are
FALSE (default) and TRUE . This attribute can be
set in the RTL or the XDC.