STACK OPERATIONS FOLDING IN JAVA PROCESSORS

Traditionally, the performance of a stack machine has been limited by the true data dependency. A performance enhancement mechanism, stack o perations folding, was used in Sun Microelectronics' picoJava-I design , and it can fold up to 60% of all stack operations. The authors use t he Java bytecode language as the target machine language, and study Ja va instruction folding on a proposed folding model, the POC model, whi ch is used to illustrate the theoretical folding operations. Various p ractical folding strategies based on the POC model are introduced and evaluated. Statistical data show that the 4-foldable strategy eliminat es 84% of all stack operations, and the 2-, 3-, and 4-foldable strateg ies result in overall program speedups of 1.22, 1.32 and 1.34, respect ively, as compared to a stack machine without folding. Furthermore, th e 4-foldable strategy is the most practical and cost effective of a Ja va stack machine design with a decoder width of 8 bytes. Circuit simul ation results show that a 100MHz 4-foldable folding mechanism can be r ealized with 0.6 mu m CMOS standard cells, or 240MHz with 0.251 mu m C MOS technology.