Cheap out-of-order execution using delayed issue

Out-of-order issue mechanisms increase performance by dynamically rescheduling instructions that cannot be statically reordered by the compiler. Such mechanisms are effective but expensive in terms of both complexity and silicon area. It is therefore desirable to find cost-effective alternatives which can provide similar performance gains. In this paper we present delayed issue, a novel technique which allows instructions to be executed out-of-order without the hardware complexity of dynamic out-of-order issue. Instructions are inserted into per-functional unit delay queues using delays specified by the compiler. Instructions within a queue are issued in order; out of order execution results from different instructions being inserted into the queues at various delays. In addition to improving performance, delayed issue reduces code bloat when loops are pipelined.