In this paper we develop an optimized algorithm for performing the Fast Fourier Transform (FFT) on the Vector IRAM (VIRAM) architecture in both the fixed- and floating-point domains. We discuss the impact of various optimizations on the performance of the FFT algorithm on VIRAM, including both an analysis of the usefulness of various VIRAM ISA features as well as a consideration of the performance and accuracy consequences of performing the FFT computations in the fixed-point domain rather than the traditional floating-point domain.

We compare the performance of our most-optimized FFT algorithm on a simulated version of VIRAM to that of eleven high-end fixed- and floating-point Digital Signal Processors (DSPs) and DSP-like architectures, and find that VIRAM outperforms all of the fixed-point DSPs and all but two of the special-purpose floating-point FFT DSPs. On 1024-point FFTs, VIRAM achieves 1.3 GFLOP/s in floating-point mode, and 1.9 GOP/s in fixed-point mode.

Despite its high performance relative to the DSPs, however, we find that the VIRAM architecture is being underutilized by as much as two thirds while running the FFT algorithm. We thus embark on an architectural analysis to determine the underlying cause of this underutilization, and discover that it results from bottlenecks in VIRAM's memory functional units and memory access conflicts in VIRAM's memory system. For larger FFTs, the memory system impact becomes more severe, and we find that the number of memory banks and subbanks plays a crucial role in the scalability of our algorithm's performance to large FFT sizes.




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