Object-Relative Addressing: Compressed Pointers in 64-Bit Java Virtual Machines

64-bit address spaces come at the price of pointers requiring twice as much memory as 32-bit address spaces, resulting in increased memory usage. Increased memory usage is especially of concern on machines that are heavily loaded with memory-intensive applications; overall system performance can quickly deteriorate once physical memory is exhausted. This paper reduces the memory usage of 64-bit pointers in the context of Java virtual machines through pointer compression, called Object-Relative Addressing (ORA). The idea is to compress 64-bit raw pointers into 32-bit offsets relative to the referencing object’s virtual address. Unlike previous work on the subject using a constant base address for compressed pointers, ORA allows for applying pointer compression to Java programs that allocate more than 4GB of memory. Our experimental results using Jikes RVM and the SPECjbb and DaCapo benchmarks on an IBM POWER4 machine show that the overhead introduced by ORA is statistically insignificant on average compared to raw 64-bit pointer representation, while reducing the total memory usage by 10% on average and up to 14.5% for some applications.