Software Thread Level Speculation for the Java Language and Virtual Machine Environment

Thread level speculation (TLS) has shown great promise as a strategy for fine to medium grain automatic parallelisation, and in a hardware context techniques to ensure correct TLS behaviour are now well established. Software and virtual machine TLS designs, however, require adherence to high level language semantics, and this can impose many additional constraints on TLS behaviour, as well as open up new opportunities to exploit language-specific information. We present a detailed design for a Java-specific, software TLS system that operates at the bytecode level, and fully addresses the problems and requirements imposed by the Java language and VM environment. Using SableSpMT, our research TLS framework, we provide experimental data on the corresponding costs and benefits; we find that exceptions, GC, and dynamic class loading have only a small impact, but that concurrency, native methods, and memory model concerns do play an important role, as does an appropriate, language-specific runtime TLS support system. Full consideration of language and execution semantics is critical to correct and efficient execution of high level TLS designs, and our work here provides a baseline for future Java or Java virtual machine implementations.