Front-end policies for improved issue efficiency in SMT processors

The performance and power optimization of dynamic superscalar microprocessors requires striking a careful balance between exploiting parallelism and hardware simplification. Hardware structures which are needlessly complex may exacerbate critical timing paths and dissipate extra power. One such structure requiring careful design is the issue queue. In a simultaneous multi-threading (SMT) processor it is particularly challenging to achieve issue queue simplification due to the increased utilization of the queue afforded by multi-threading. In this paper we propose new front-end policies that reduce the required integer and floating point issue queue sizes in SMT processors. We explore both general policies as well as those directed towards alleviating a particular cause of issue queue inefficiency. For the same level of performance, the most effective policies reduce the issue queue occupancy by 33% for an SMT processor with appropriately sized issue queue resources.