Compressing Pointers for the Z Garbage Collector : Runtime compression of pointers in a concurrent setting

Abstract: Pointers in 64-bit architectures are unlikely to exhaust their vast address range, and are as such needlessly big. Reducing the amount of memory a pointer occupies leads to reduced memory demands, better usage of memory, and better locality. Pointer compression is a term that encompasses techniques that aim to make pointers occupy less memory, often to 32-bit for the sake of word alignment. Pointers that are 32-bit embody the opposite problem of having too restricted of an address range, being able to address only 4 GB. Z is a garbage collector in the HotSpot JVM which does not support pointer compression. Partly because the aforementioned address range restriction, and partly because the implementation of compressed pointers which exist in HotSpot would clash with the goals of the garbage collector. This project explores ways of implementing pointer compression for Z that isn't detrimental to the goals of the garbage collector, and aims to find where problems may occur. The outset was to explore compressing speculatively during runtime. The result is a design that relies on a custom bit layout for compressed pointers, inspecting bit layouts of the pointers on each read and write to detect the compression status. This seems to be the most promising in terms of code maintainability and ease of implementation.