An Extended Shared Logarithmic Unit for Nonlinear Function Kernel Acceleration in a 65-nm CMOS Multicore Cluster

Energy-efficient computing and ultralow-power computing are strong requirements for various application areas, such as internet of things and wearables. While for some applications integer and fixed-point arithmetic suffice, others require a larger dynamic range, typically obtained using floating-point (FP) numbers. Logarithmic number systems (LNSs) have been proposed as energy-efficient alternative, since several complex FP operations translate into simple integer operations. However, additions and subtractions become nonlinear operations, which have to be approximated via interpolation. Even efficient LNS units (LNUs) are still larger than standard FP units (FPUs), rendering them impractical for most general-purpose processors. We show that, when shared among several cores, LNUs become a very attractive solution. A series of compact LNUs is developed, which provide significantly more functionality (such as transcendental functions) than other state-of-the-art designs. This allows, for example, to evaluate the atan2 function with three instructions for only 183.2 pJ/op at 0.8 V. We present the first shared-LNU architecture where these LNUs have been integrated into a multicore system with four 32-b-OpenRISC cores and show measurement results demonstrating that the shared-LNU design can be up to 4.1 × more energy-efficient in common nonlinear processing kernels, compared with a similar area design with four private FPUs.