JIT Data Structures, Fully Reflective VMs, and Meta-Circular Meta-Tracing

The year leading up to SPLASH has been pretty busy. Beside my own talks on Tracing vs. Partial Evaluation and Optimizing Communicating Event-Loop Languages with Truffle, there are going to be three other presentations on work I was involved in.

Just-In-Time Data Structures

On Thursday, Mattias De Wael is going to present his work on Just-in-Time Data Structures at Onward!. For me this work is exciting, because I hope that this could make programming with standard data structures quite a bit simpler in the long run. Today, ‘scripting’ languages like Ruby and Python provide a small set of very versatile data structures they call lists or arrays that can be used for many many different use cases. But instead of having a one-size-fits-all implementation, a runtime system can ideally identify a specific ‘strategy’ to be used dynamically, and even change it over the course of a program. So, in a nice language, we would merely declare a few fundamental properties such as being ordered, and how to compare items, and the runtime system will figure out whether to use a hash, tree, array, or other kind of implementation. Well, that’s at least my vision. Abstract below:

Today, software engineering practices focus on finding the single right data representation (i.e., data structure) for a program. The right data representation, however, might not exist: relying on a single representation of the data for the lifetime of the program can be suboptimal in terms of performance. We explore the idea of developing data structures for which changing the data representation is an intrinsic property. To this end we introduce Just-in-Time Data Structures, which enable representation changes at runtime, based on declarative input from a performance expert programmer. Just-in-Time Data Structures are an attempt to shift the focus from finding the “right’’ data structure to finding the right sequence of data representations. We present JitDS-Java, an extension to the Java language, to develop Just-in-Time Data Structures. Further, we show two example programs that benefit from changing the representation at runtime.

  • Just-In-Time Data Structures. Mattias De Wael, Stefan Marr, Joeri De Koster, Jennifer B. Sartor, and Wolfgang De Meuter. Proceedings of the 2015 ACM International Symposium on New Ideas, New Paradigms, and Reflections on Programming & Software, ACM, (October 2015)

Towards Fully Reflective Environments

On Friday, Guido Chari is going to present his work Towards Fully Reflective Environments also at Onward!. This is a first step in the direction of bringing all parts of a runtime system to life. While classic systems such as Smalltalk and CLOS go pretty far in reifying language elements and enabling us to interact with them on a meta-level, they typically exclude important aspects of the runtime system such as GC, object representation, interpreter semantics, and JIT compilation. Some of these aspects can be observed via debugging interfaces, but what we have in mind here is to have a complete meta-circular runtime system where each and every aspect is reified and can be adapted at runtime. What Guido is going to present is only a first step, especially since performance is not yet addressed. But, we are working on it.

Modern development environments promote live programming (LP) mechanisms because it enhances the development experience by providing instantaneous feedback and interaction with live objects. LP is typically supported with advanced reflective techniques within dynamic languages. These languages run on top of Virtual Machines (VMs) that are built in a static manner so that most of their components are bound at compile time. As a consequence, VM developers are forced to work using the traditional edit-compile-run cycle, even when they are designing LP-supporting environments. In this paper we explore the idea of bringing LP techniques to the VM domain for improving their observability, evolution and adaptability at run-time. We define the notion of fully reflective execution environments (EEs), systems that provide reflection not only at the application level but also at the level of the VM. We characterize such systems, propose a design, and present Mate v1, a prototypical implementation. Based on our prototype, we analyze the feasibility and applicability of incorporating reflective capabilities into different parts of EEs. Furthermore, the evaluation demonstrates the opportunities such reflective capabilities provide for unanticipated dynamic adaptation scenarios, benefiting thus, a wider range of users.

  • Towards Fully Reflective Environments. Guido Chari, Diego Garbervetsky, Stefan Marr, and Stéphane Ducasse. Proceedings of the 2015 ACM International Symposium on New Ideas, New Paradigms, and Reflections on Programming & Software, ACM, (October 2015)

A Formal Foundation for Trace-Based JIT Compilers

Last but not least, and already on Monday at the WODA workshop, Maarten Vandercammmen is going to present A Formal Foundation for Trace-Based JIT Compilers. The bit that I find most interesting in that work is the meta-circular meta-tracer for a little Scheme, which is hiding in there. The focus of the work changed over time, but initially he was working on identifying the essence of meta-tracing. And he now got a pretty small and manageable framework to experiment with meta-tracing, and even execute multiple levels of interpreters that are all traced through. I’d say that’s a pretty neat foundation for future research, more approachable than a huge practical system such as RPython, but of course, also not without drawbacks. For instance, a proper optimizer is missing so far. But let’s see where this gets us in the future!

Trace-based JIT compilers identify frequently executed program paths at run-time and subsequently record, compile and optimize their execution. In order to improve the performance of the generated machine instructions, JIT compilers heavily rely on dynamic analysis of the code. Existing work treats the components of a JIT compiler as a monolithic whole, tied to particular execution semantics. We propose a formal framework that facilitates the design and implementation of a tracing JIT compiler and its accompanying dynamic analyses by decoupling the tracing, optimization, and interpretation processes. This results in a framework that is more configurable and extensible than existing formal tracing models. We formalize the tracer and interpreter as two abstract state machines that communicate through a minimal, well-defined interface. Developing a tracing JIT compiler becomes possible for arbitrary interpreters that implement this interface. The abstract machines also provide the necessary hooks to plug in custom analyses and optimizations.

  • A Formal Foundation for Trace-Based JIT Compilers Maarten Vandercammen, Jens Nicolay, Stefan Marr, Joeri De Koster, Theo D’Hondt, and Coen De Roover. Proceedings of the 13th International Workshop on Dynamic Analysis. Pittsburgh, PA, USA, ACM, (October 2015)

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