Compute-Capable Block RAMs for Efficient Deep Learning Acceleration on FPGAs

The density of FPGA on-chip memory has been continuously increasing with modern FPGAs having thousands of block RAMs (BRAMs) distributed across their reconfigurable fabric. These distributed BRAMs can provide a tremendous amount of on-chip bandwidth for efficient acceleration of data-intensive applications. In this work, we propose enhancing the ubiquitous FPGA BRAMs with in-memory compute-capabilities. As a result, BRAMs can act as normal storage units or their bitlines can be re-purposed as SIMD lanes executing bit-serial arithmetic operations. Our proposed architectural change results in 1.6× and 2.3× increase in the peak multiply-accumulate throughput of a large Stratix 10 FPGA, at a minimal cost of only 1.8% increase in the FPGA die size and no change to the BRAM's interface to the programmable routing. Then, we present RIMA, a reconfigurable in-memory accelerator architecture for deep learning (DL) inference. RIMA exploits the proposed compute-capable BRAMs and the FPGA's reconfigurability to achieve 1.25× and 3× higher performance compared to the state-of-the-art Brainwave DL soft processor for 8-bit integer and block floating-point precisions, respectively. In addition, RIMA implemented on a Stratix 10 FPGA enhanced with compute-capable BRAMs can achieve an order of magnitude higher performance compared to a same-generation GPU.