Code size reduction in heterogeneous-connectivity-based DSPs using instruction set extensions

Existing trend of processors shows a progress toward customizable and reconfigurable architectures. In this paper, we study the benefit of combining the architectural design of a VLIW DSP and the concepts of modern customizable processors like ASIPs (application specific instruction set processors) for code size reduction. VLIW DSP architectures exhibit heterogeneous connections between functional units and register files for speeding up special tasks. Such architectural characteristics can be effectively exploited through the use of complex instruction set extensions (ISEs). Although VLIWs are increasingly being used for DSP applications to achieve very high performance, such architectures are known to suffer from increased code size. This paper also addresses how to generate and use ISEs that can result in significant code size reduction in VLIW DSPs without degrading performance. Unfortunately, contemporary techniques for generation of ISEs when applied before resource-binding fail to generate legal ISEs for VLIW architectures with heterogeneous connectivity between the functional units and register files. We propose a heuristic-based approach to generate ISEs for a generalized heterogeneous-connectivity-based VLIW DSP architecture. We achieve an average code size reduction of 25 percent on the MiBench suite with no penalty in performance by applying our ISE generation algorithms on the Tl TMS320C6xx, a representative VLIW DSP. We also show that the overhead of the required architectural assists for our approach is minimal: The TMS320C6xx pipeline meets the required timing with only a limited overhead in area.