Adding limited reconfigurability to superscalar processors

When adding reconfigurability to custom hardware, one must take great care that the reduction in speed due to the reconfigurable logic should not cancel out the gains obtained by reconfiguration. These gains are greatest in very specific and computation-intensive applications, and lessen as the applications become more general and heterogeneous. In the case of superscalar processors, this leads to limiting the amount of reconfigurability to precise changes in existing functional units instead of adding a fully configurable functional unit. We present a detailed study of the modifications necessary in a superscalar processor to allow an FPU to be dynamically reconfigured as several ALUs with a minimal increase in the latency of these functional units. The timing of the FPU's multiplier tree and the decision about reconfiguration are exposed. As there is more than one simple unit involved, this decision is more global than a cycle-by-cycle reconfiguration and must be made for a longer period of time. We discuss possible policies for the dynamic reconfiguration decisions. The results show interesting gains of lip to 56% in the best cases, and average gains of 10%, on typical architectures over a wide range of applications.