Instruction scheduling for clustered VLIW architectures

Clustered VLIW organizations are nowadays a common trend in the design of embedded/DSP processors. In this work we propose a novel modulo scheduling approach for such architectures. The proposed technique performs the cluster assignment and the instruction scheduling in a single pass, which is more effective than doing first the assignment and latter the scheduling. We also show that loop unrolling significantly enhances the performance of the proposed scheduler, especially when the communication channel among clusters is the main performance bottleneck. By selectively unrolling some loops, we can obtain the best performance with the minimum increase in code size. Performance evaluation for the SPECfp95 shows that the clustered architecture achieves about the same IPC (Instructions Per Cycle) as a unified architecture with the same resources. Moreover, when the cycle time is taken into account, a 4-cluster configuration is 3.6 times faster than the unified architecture.