Investigating hardware micro-instruction folding in a Java embedded processor

Bytecode folding is an effective technique for speeding up execution in Java virtual machines. This paper investigates a hardware implementation of the aforementioned technique on BlueJEP, a Java embedded processor. Since BlueJEP is a micro-programmed stack machine, we adopt a micro-instruction oriented approach, folding up to four microinstructions (corresponding to up to four bytecodes, on occasion). A variety of processor versions for different subsets of folding patterns are implemented, simulated and synthesized on a Xilinx FPGA. The measurements and results show that, although the number of execution cycles is reduced, the critical path increase leads to a lower performance. Taking into account the device area, we conclude that for our case, adding a second processor may be preferred over hardware folding. In general, we observe that folding efficiency may only be evaluated properly on a real implementation, rather than using theoretical estimates, due to the increased complexity of the hardware.