Register minimization beyond sharing among variables

Traditionally, it is assumed that every variable in the input HDL (Hardware Description Language) behavioral description needs to be held in a register; A register can be shared by multiple variables if they have mutually disjoint lifetime intervals. This approach is effective for signal-flow-like computations such as various DSP algorithms. However, it is not the best for the synthesis of control-dominated circuits, which usually have variables/signals of different bit-width as well as very long lifetime. To go beyond register minimization by lifetime-analysis-based sharing, we propose holding some variables in the state registers, some signal nets, or some unclocked sequential networks. We have implemented the proposed method in a software program called VReg. Experimental results have demonstrated that VReg minimizes the number of registers more effectively than the lifetime-analysis-based approach does. Better register minimization also leads to both smaller area and faster designs.