The Impact of Loop Unrolling on Controller Delay in High Level Synthesis

Loop unrolling is a well-known compiler optimization that can lead to significant performance improvements. When used in high level synthesis (HLS) unrolling can affect the controller complexity and delay. We study the effect of the loop unrolling factor on the delay of controllers generated during HLS. We propose a technique to predict controller delay as a function of the loop unrolling factor, and use this prediction with other search space pruning methods to automatically determine the optimal loop unrolling factor that results in a controller whose delay fits into a specified time budget, without an exhaustive exploration. Experimental results indicate delay predictions that are close to measured delays, yet significantly faster than exhaustive synthesis