1. A method for generating a design of a system to be implemented on a target device, comprising: identifying a semantic structure; assigning a signal activity to an output of the semantic structure that describes a toggle rate of the semantic structure according to a characteristic of the semantic structure; performing vectorless estimation on a non-semantic structure to determine a signal activity that describes a toggle rate of the non-semantic structure; and optimizing power for the design of the system according to the signal activity of the semantic structure and the signal activity of the non-semantic structure by modifying one of synthesis, placement, and routing of the system on the target device, wherein at least one of the identifying, assigning, performing, and optimizing procedures is performed by a computer.