A low cost split-issue technique to improve performance of SMT clustered VLIW processors

Very Long Instruction Word (VLIW) processors are a popular choice in embedded domain due to their hardware simplicity, low cost and low power consumption. Simultaneous MultiThreading (SMT) is a popular technique for improving processor performance. To maintain execution semantics, a VLIW instruction needs to be issued in entirety, which restricts the opportunities in SMT. Split-issue at operation-level is a technique that allows issuing a VLIW instruction in parts without breaking execution semantics. Issuing an instruction in parts allows non-conflicting part of an instruction to be issued along with other instructions and improves SMT performance. However, implementing split-issue at operation-level requires complex structures and is not practical for an embedded VLIW processor. This paper proposes cluster-level split-issue, which implements split-issue at a cluster-level boundary for clustered VLIW processors. Cluster-level split-issue has a very low hardware overhead in contrast to split-issue at operation-level. Experimental results show that cluster-level split-issue, despite being more restrictive than split-issue at operation-level, achieves similar performance and improves SMT performance significantly.