Node Collapsing

All combinational nodes by default are collapsible. Nodes to be collapsed (or to be preserved) are flagged through the use of signal attributes in the language. The attributes are:

Istype 'keep' Do not collapse this node.

Istype 'collapse' Collapse this node.

Collapsing provides multi-level optimization for combinational logic. Designs with arithmetic and comparator circuits generally generate large number of product terms which will not fit to any programmable logic device. Node collapsing allows you to describe equations in terms of multi-level combinational nodes, then collapse the nodes into the output until it reaches the product term you specify. The result is an equation that is optimized to fit the device constraints.

Selective Collapsing

In some instances, you may want to keep certain nodes from being collapsed out. For example, some nodes may help in the simulation process. You can specify the nodes you do not want collapsed as Istype 'keep' and the optimizer does not collapse them.