Efficient Hardware Looping Units for FPGAs

Looping operations impose a significant bottleneck to achieving better computational efficiency for embedded applications. To confront this problem in embedded computation either in the form of programmable processors or FSMD (Finite-State Machine with Datapath) architectures, the use of customized loop controllers has been suggested. In this paper, a thorough examination of zero-cycle overhead loop controllers applicable to perfect loop nests operating on multi-dimensional data is presented. The design of such loop controllers is formalized by the introduction of a hardware algorithm that fully automates this task for the spectrum of behavioral as well as generated register-transfer level architectures. The presented algorithm would prove beneficial in the field of high-level synthesis of architectures for data-intensive processing. It is also shown that the proposed loop controllers can be efficiently utilized for supporting generalized loop structures such as imperfect loop nests. The performance characteristics (cycle time, chip area) of the proposed architectures have been evaluated for FPGA target implementations. It is shown that maximum clock frequencies of above 230MHz with low logic footprints of about 1.4% of the overall logic resources can be achieved for supporting up to 8 nested loops with 16-bit indices on a modestly-sized Xilinx Virtex-5 device.