1.4.4. Avoid Asynchronous Register Control Signals
Some Intel devices directly support an asynchronous clear function, but not a preset or load function. When the target device does not directly support the signals, the synthesis or placement and routing software must use combinational logic to implement the same functionality. In addition, if you use signals in a priority other than the inherent priority in the device architecture, combinational logic may be required to implement the necessary control signals. Combinational logic is less efficient and can cause glitches and other problems; it is best to avoid these implementations.