Allowing for ILP in an embedded Java processor

Abstract

Java processors are ideal for embedded and network computing applications such as Internet TV's, set-top boxes, smart phones, and other consumer electronics applications. In this paper, we investigate cost-effective microarchitectural techniques to exploit parallelism in Java bytecode streams. Firstly, we propose the use of a fill unit that stores decoded bytecodes into a decoded bytecode cache. This mechanism improves the fetch and decode bandwidth of Java processors by 2 to 3 times. These additional hardware units can also be used to perform optimizations such as instruction folding. This is particularly significant because experiments with the Verilog model of Sun Microsystems pico Java-II core demonstrates that instruction folding lies in the critical path. Moving folding logic from the critical path of the processor to the fill unit allows to improve the clock frequency by 25%. Out-of-order ILP exploitation is not investigated due to the prohibitive cost, but in-order dual-issue with a 64-entry decoded bytecode cache is seen to result in 10% to 14% improvement in execution cycles. Another contribution of the paper is a stack disambiguation technique that allows elimination of false dependencies between different types of stack accesses. Stack disambiguation further exposes parallelism and a dual in-order issue microengine with a 64-entry bytecode cache yields an additional 10% reduction in cycles, leading to an aggregate reduction of 17% to 24% in execution cycles.