Section: New Results

Dynamic Languages: Virtual Machines

Assessing primitives performance on multi-stage execution. Virtual machines, besides the interpreter and just-in-time compiler optimization facilities, also include a set of primitive operations that the client language can use. Some of these are essential and cannot be performed in any other way. Others are optional: they can be expressed in the client language but are often implemented in the virtual machine to improve performance when the just-in-time compiler is unable to do so (start-up performance, speculative optimizations not implemented or not mature enough, etc.). In a hybrid runtime, where code is executed by an interpreter and a just-in-time compiler, the implementor can choose to implement optional primitives in the client language , in the virtual machine implementation language (typically C or C++), or on top of the just-in-time compiler back-end. This raises the question of the maintenance and performance trade-offs of the different alternatives. As a case study, we implemented the String comparison optional primitive in each case. The paper describes the different implementations, discusses the maintenance cost of each of them and evaluates for different string sizes the execution time in Cog, a Smalltalk virtual machine. [18]

Fully Reflective Execution Environments: Virtual Machines for More Flexible Software. VMs are complex pieces of software that implement programming language semantics in an efficient, portable, and secure way. Unfortunately, mainstream VMs provide applications with few mechanisms to alter execution semantics or memory management at run time. We argue that this limits the evolvability and maintainability of running systems for both, the application domain, e.g., to support unforeseen requirements, and the VM domain, e.g., to modify the organization of objects in memory. This work explores the idea of incorporating reflective capabilities into the VM domain and analyzes its impact in the context of software adaptation tasks. We characterize the notion of a fully reflective VM, a kind of VM that provides means for its own observability and modifiability at run time. This enables programming languages to adapt the underlying VM to changing requirements. We propose a reference architecture for such VMs and present TruffleMATE as a prototype for this architecture. We evaluate the mechanisms TruffleMATE provides to deal with unanticipated dynamic adaptation scenarios for security, optimization, and profiling aspects. In contrast to existing alternatives, we observe that TruffleMATE is able to handle all scenarios, using less than 50 lines of code for each, and without interfering with the application's logic. [2]