Transient variable caching in Java's stack-based intermediate representation

摘要

Java's stack-based intermediate representation (IR) is typi- cally coerced to execute on register-based architectures. Un- optimized compiled code dutifully replicates transient vari- able usage designated by the programmer and common op- timization practices tend to introduce further usage (i.e., CSE, Loop-invariant Code Motion, etc.). On register based machines, often transient variables are cached within regis- ters (when available) saving the expense of actually access- ing memory. Unfortunately, in stack-based environments be- cause of the need to push and pop the transient values, further performance improvement is possible. This paper presents Transient Variable Caching (TVC), a technique for eliminat- ing transient variable overhead whenever possible. This opti- mization would find a likely home in optimizers attached to the back of popular Java compilers.` Side effects of the al- gorithm include significant instruction reordering and intro- duction of many stack-manipulation operations. This combi- nation has proven to greatly impede the ability to decompile stack-based IR code sequences. The code that results from the transform is faster, smaller, and greatly impedes decom- pilation.