Approximate DIV and SQRT instructions for the RISC-V ISA: An efficiency vs. accuracy analysis

In this paper, we present extensions to the floating point unit of the RISC-V ISA with different numeric precision including single precision, half precision and quarter precision. To achieve more energy efficiency transprecision feature is introduced through configurable mantissa precision, which can be used to reduce the number of iterations and thus reduce the energy consumption achieving a programmable trade-off between accuracy and precision. Power estimations based on synthesized results demonstrate that floating-point operations with 8 to 11-bit mantissa can achieve an energy savings of 36% when compared with single precision. We examine the error propagations in benchmark applications with transprecision options, and in a square root-intensive application we report runtime reductions up to 43.65% which translates to 1.77x energy ratio when compared to standard single precision operations.