Tiny-FPU: Low-Cost Floating-Point Support for Small RISC-V MCU Cores

In the Internet-Of-Things (IoT) domain, microcontrollers (MCUs) are used to collect and process data coming from sensors and transmit them to the cloud. Applications that require the range and precision of floating-point (FP) arithmetic can be implemented using efficient hardware floating-point units (FPUs) or by using software emulation. FPUs optimize performance and code size, whilst software emulation minimizes the hardware cost. We present a new area-optimized, IEEE 754-compliant RISC-V FPU (Tiny-FPU), and we explore the area, code size, performance, power, and energy efficiency of three different implementations of the RISC-V Instruction Set Architecture double and single-precision FP extensions on an MCU-class processor. We show that Tiny-FPU, in its double and single-precision versions, is respectively 54% and 37% smaller than a double and single-precision FPU optimized for performance and energy efficiency. When coupling a RISC-V core with Tiny-FPU, we achieve up to 18.5x and 15.5x speedups with respect to the same core emulating FP operations via software.