An Efficient Implementation of a TAGE Branch Predictor for Soft Processors on FPGA

Abstract

Soft processors are becoming a common component on reconfigurable computing like FPGA. For some accelerators, custom logic functions are implemented as processing elements besides the soft processor. Since the resources in FPGA are fixed and limited, it is desired to implement the soft processor with less logical resources as possible. One of the important parts of the processor is an instruction fetch unit whose performance is dependent on branch prediction. Conventional branch predictors like bimodal or gshare are simple to implement but their prediction accuracy is not good enough. On the other hand, TAGE branch predictor has better prediction accuracy but contains complex logic path for branch prediction, which results in the lower operating frequency. In this paper, we propose a branch predictor called pTAGE, which has almost the same prediction accuracy as TAGE and avoids becoming the critical path of the processor. The branch prediction of pTAGE is pipelined, so prediction result is available on each clock cycle. We implement gshare, TAGE, and pTAGE, respectively in Verilog HDL and evaluate their operating frequency and prediction rate based on FPGA implementation. In this result, pTAGE has almost the same prediction rate as TAGE and 1.41 times higher operating frequency than that of TAGE. Also, we evaluate the performance by varying the latency for updating branch prediction, and the evaluation result shows that pTAGE exhibits higher performance in deep pipelined processors than gshare.