Reducing instruction fetch cost by packing instructions into register windows

Instruction packing is a combination compiler/architectural approach that allows for decreased code size, reduced power consumption and improved performance. The packing is obtained by placing frequently occurring instructions into an instruction register file (IRF). Multiple IRF entries can then be accessed using special packed instructions. Previous IRF efforts focused on using a single 32-entry register file for the duration of an application. This paper presents software and hardware extensions to the IRF supporting multiple instruction register windows to allow a greater number of relevant instructions to be available for packing in each function. Windows are shared among similar functions to reduce the overall costs involved in such an approach. The results indicate that significant improvements in instruction fetch cost can be obtained by using this simple architectural enhancement. We also show that using an IRF with a loop cache, which is also used to reduce energy consumption, results in much less energy consumption than using either feature in isolation.