Techniques and applications for guest-language safepoints

Safepoints are a virtual machine mechanism that allows one thread to suspend other threads in a known state so that runtime actions can be performed without interruption and with data structures in a consistent state. Many virtual machines use safepoints as a mechanism to provide services such as stop-the-world garbage collection, debugging, and modification to running code such as installing or replacing classes. Languages implemented on these virtual machines may have access to these services, but not directly to the safepoint mechanism itself. We show that safepoints have many useful applications for the implementation of guest languages running on a virtual machine. We describe an API for using safepoints in languages that were implemented under the Truffle language implementation framework on the Java Virtual Machine and show several applications of the API to implement useful guest-language functionality. We present an efficient implementation of this API, when running in combination with the Graal dynamic compiler.We also demonstrate that our safepoints cause zero overhead with respect to peak performance and statistically insignificant overhead with respect to compilation time. We compare this to other techniques that could be used to implement the same functionality and demonstrate the large overhead that they incur.