Traditionally, the performance of a stack machine has been limited by the true data dependency. A performance enhancement mechanism, stack operations folding, was used in Sun Microelectronics' picoJava-I design, and it can fold up to 60% of all stack operations. The authors use the Java bytecode language as the target machine language, and study Java instruction folding on a proposed folding model, the POC model, which is used to illustrate the theoretical folding operations. Various practical folding strategies based on the POC model are introduced and evaluated. Statistical data show that the 4-foldabIe strategy eliminates 84% of all stack operations, and the 2-, 3-, and 4-foldable strategies result in overall program speedups of 1.22, 1.32 and 1.34, respectively, as compared to a stack machine without folding. Furthermore, the 4-foldable strategy is the most practical and cost effective of a Java stack machine design with a decoder width of 8 bytes. Circuit simulation results show that a 100MHz 4-foldable folding mechanism can be realized with 0.6urn CMOS standard cells, or 240MHz with 0.25 urn CMOS technology.
- Java processors
- Stack machines