Mutable functional units and their applications on microprocessors

Functional units are the heart of microprocessors as they execute binary instructions of a program. Current microprocessors typically have several types of functional units. In this paper, we propose a new functional unit that combines a floating-point adder and an integer arithmetic and logic unit into a single unit. This functional unit reconfigures itself at run-time to serve different instructions from the program instruction stream. We call such units mutable functional units or MFUs. MFUs can be used in microprocessors to improve functional unit utilization, reduce power consumption, and to improve performance without adding extra functional units. MFUs only require, minor modifications to the existing floating-point adder design. We show that overheads of reconfiguration are small, typically 0 to 1 clock cycle, and at most 2 clock cycles. We demonstrate how integration with a typical current microprocessor can be achieved. This integration allows speedups of non-numerical applications by 8% to 14% while keeping the number of functional units constant. We also show that various enhancements to the base architecture that increase the instruction fetch rate affect the speedups positively.