Impact of intercluster communication mechanisms on ILP in clustered VLIW architectures

VLIW processors have started gaining acceptance in the embedded systems domain. However, monolithic register file VLIW processors with a large number of functional units are not viable. This is because of the need for a large number of ports to support FU requirements, which makes them expensive and extremely slow. A simple solution is to break the register file into a number of smaller register files with a subset of FUs connected to it. These architectures are termed clustered VLIW processors. In this article, we first build a case for clustered VLIW processors with four or more clusters by showing that the achievable ILP in most of the media applications for a 16 ALU and 8 LD/ST VLIW processor is around 20. We then provide a classification of the intercluster interconnection design space, and show that a large part of this design space is currently unexplored. Next, using our performance evaluation methodology, we evaluate a subset of this design space and show that the most commonly used type of interconnection, RF-to-RF, fails to meet achievable performance by a large factor, while certain other types of interconnections can lower this gap considerably. We also establish that this behavior is heavily application dependent, emphasizing the importance of application-specific architecture exploration. We also present results about the statistical behavior of these different architectures by varying the number of clusters in our framework from 4 to 16. These results clearly show the advantages of one specific architecture over others. Finally, based on our results, we propose a new interconnection network, which should lower this performance gap.