Reflections on a decade of MoarVM, a runtime for the Raku programming language (keynote)

MoarVM was born out of youthful arrogance. I was contributing to Raku (then Perl 6) compiler development, looking at the Parrot virtual machine we were targeting at the time, considering some of our challenges with it (especially regarding performance and threading), and thought: what if I could design and build something better…in my copious free time? Within a few years, helped along by the contributions of tens of other open source volunteers, MoarVM almost entirely replaced Parrot as the language’s runtime of choice. Nearly a decade has passed since work on MoarVM began. Starting out as a simple register-based bytecode interpreter, MoarVM has steadily incorporated many of the tricks of the trade: type specialization, deoptimization, inlining, on stack replacement, JIT compilation, and basic escape analysis. As these fell into place, they drove a further change: hot operations that had been implemented as complex primitives in the VM for speed steadily became bottlenecks for further improvement, as the optimizer had no visibility into them, and thus were gradually eliminated. Most recently, a new generalized dispatch mechanism has arrived, eliminating numerous special-cased mechanisms (for example, method and multiple dispatch caches) with a single, programmable, approach. Being a multi-language VM has never been a goal - yet achieving better performance while managing complexity has led MoarVM to become ever more abstracted from the Raku language. In this session I will review MoarVM’s journey so far, picking out some of the most interesting challenges faced, lessons learned, and trade-offs encountered. I will also discuss the new generalized dispatch mechanism and its concept of resumable dispatch, which is allowing us to take on optimization of some language features that have thus far been stubbornly slow - but which also brings its own share of new challenges.