Evaluation of Bus Based Interconnect Mechanisms in Clustered VLIW Architectures

With new sophisticated compiler technology, it is possible to schedule distant instructions efficiently. As a consequence, the amount of exploitable instruction level parallelism (ILP) in applications has gone up considerably. However, monolithic register file VLIW architectures present scalability problems due to a centralized register file which is far slower than the functional units (FU). Clustered VLIW architectures, with a subset of FUs connected to any RF provide an attractive solution to address this issue. Recent studies with a wide variety of inter-cluster interconnection mechanisms have reported substantial gains in performance (number of cycles) over the most studied RF-to-RF type interconnections. However, these studies have compared only one or two design points in the RF-to-RF interconnects design space. In this paper, we extend the previous reported work. We consider both multi-cycle and pipelined buses. To obtain realistic bus latencies, we synthesized the various architectures and calculated post-layout clock periods. The results demonstrate that while there is less that 10% variation in interconnect area, the bus based architectures are slower by as much as 400%. Also, neither multi-cycle or pipelined buses nor increasing the number of buses itself is able to achieve performance comparable to point-to-point type interconnects.