SEED: Scalable, efficient enforcement of dependences

Instruction issue logic is a critical component in modern high-performance out-of-order processors. The ever increasing latencies found in modern processors, mostly associated with memory accesses and longer pipelines, can be attenuated using large issue queues. Conventional designs rely on atomic wakeup-select cycles to ensure compact scheduling. These designs must aggressively utilize broadcasting, compaction, and heavily-ported structures that scale poorly in terms of both power consumption and access time. To provide high scheduling flexibility and large instruction capacity without incurring prohibitive latency and energy overhead, we propose a novel scheme that uses an out-of-order, broadcast-free instruction wakeup block feeding an in-order scheduler. Multi-banked, index-based structures are used throughout this scheme to provide a high degree of scalability while achieving efficient dependence tracking, resulting in good overall performance and energy efficiency. We call this design "Scalable, Efficient Enforcement of Dependences (SEED)". We present a detailed design and analysis of SEED through an extensive evaluation. Compared to a conventional issue queue design, which is assumed favorably to scale in size without any impact on cycle time, the performance degradation of our design is 3% for both INT and FP suites of SPEC CPU2000. For such a small performance cost, SEED enjoys a 19% reduction in total chip power consumption for a 32-entry configuration. We also synthesize SEED and a conventional issue logic with 90nm standard cell logic. Synthesis results show that SEED can cycle twice the speed of a conventional issue logic of equivalent size. Cycling at the same frequency, SEED consumes ten times less dynamic power and five times less static power while achieving substantial area savings.