Dynamic Strands: Collapsing Speculative Dependence Chains for Reducing Pipeline Communication

In the modern era of wire-dominated architectures, specific effort must be made to reduce needless communication within out-of-order pipelines while still maintaining binary compatibility. To ease pressure on highly-connected elements such as the issue logic and bypass network, we propose the dynamic detection and speculative execution of instruction strands-linear chains of dependent instructions without intermediate fan-out. The hardware required for detecting these chains is simple and resides off the critical path of the pipeline, and the execution targets are the normal ALUs with a self-bypass mode. By collapsing these strings of dependencies into atomic macro-instructions, the efficiency of the issue queue and reorder buffer can be increased. Our results show that over 25% of all dynamic ALU instructions can be grouped, decreasing both the average reorder buffer occupancy and issue queue occupancy by over a third. Additionally, these strands have several properties which make them amenable to simple performance optimizations. Our experiments show average IPC increases of 17% on a four-wide machine and 20% on an eight-wide machine in Spec2000int and Mediabench applications. Finally, strands ease the IPC penalties of multicycle issue and bypass by reducing dependency pressures, providing opportunity for clock frequency gains as well.