Treffer: Minimizing bank selection instructions for partitioned memory architectures

Bank switching is a technique that increases the code and data memory in microcontrollers without extending the address buses. Given a program in which variables have been assigned to data banks, we present a novel optimization technique that minimizes the overhead of bank switching through cost-effective placement of bank selection instructions. The optimal placement is controlled by a variety of different objectives, such as runtime, low power, small code size or a combination of these parameters. We have formulated the problem as a form of Partitioned Boolean Quadratic Programming (PBQP). We implemented the optimization as part of a PIC Microchip backend and evaluated the approach for several optimization objectives. Our benchmark suite comprises programs from MiBench and DSPStone plus a microcontroller real-time kernel and drivers for microcontroller hardware devices. Our optimization achieved a reduction of program memory space between 2.7% and 18.2%, and an overall improvement with respect to instruction cycles between 5.1% and 28.8%. Our optimization achieved an optimal solution for all benchmark programs.