The fastest JavaScript production implementations use just-in-time (JIT) compilation and the vast majority of academic publications about implementations of dynamic languages published during the last two decades focus on JIT compilation. This does not imply that static compilers (AoT) cannot be competitive; as comparatively little effort has been spent creating fast AoT JavaScript compilers, a scientific comparison is lacking. This paper presents the design and implementation of an AoT JavaScript compiler, focusing on a performance analysis. The paper reports on two experiments, one based on standard JavaScript benchmark suites and one based on 31 new benchmarks chosen for their diversity of styles, authors, sizes, provenance, and coverage of the language. The first experiment shows an advantage to JIT compilers, which is expected after the decades of effort that these compilers have paid to these very tests. The second shows more balanced results, as the AoT compiler generates programs that reach competitive speeds and that consume significantly less memory. The paper presents and evaluates techniques that we have either invented or adapted from other systems, to improve AoT JavaScript compilation.

Contextual equivalence is the de facto standard notion of program equivalence. A key theorem is that contextual equivalence is an equational theory. Making contextual equivalence more intensional, for example taking into account the time cost of the computation, seems a natural refinement. Such a change, however, does not induce an equational theory, for an apparently essential reason: cost is not invariant under reduction.

In the paradigmatic case of the untyped λ-calculus, we introduce interaction equivalence. Inspired by game semantics, we observe the number of interaction steps between terms and contexts but—crucially—ignore their own internal steps. We prove that interaction equivalence is an equational theory and we characterize it as B, the well-known theory induced by Böhm tree equality. Ours is the first observational characterization of B obtained without enriching the discriminating power of contexts with extra features such as non-determinism. To prove our results, we develop interaction-based refinements of the Böhm-out technique and of intersection types.

Deterministic two-way transducers capture the class of regular functions. The efficiency of composing two-way transducers has a direct implication in algorithmic problems related to synthesis, where transformation specifications are converted into equivalent transducers. These specifications are presented in a modular way, and composing the resultant machines simulates the full specification. In this talk, I will present how we can achieve efficient composition of two-way transducers, mainly through the use of Reversible Transducers. Reversible machines are machine that are both deterministic and codeterministic. I will show how any two-way transducers can be made reversible, while being easily composable. I will also show how these results can be extended to the setting of infinite words, which is the dedicated setting for model-checking for example.

We revisit finite-state communicating systems with round-based communication under mailbox semantics. Mailboxes correspond to one FIFO buffer per process (instead of one buffer per pair of processes in peer-to-peer systems). Round-based communication corresponds to sequences of rounds in which processes can first send messages, then only receive (and receives must be in the same round as their sends).

A system is called synchronizable if every execution can be re-scheduled into an equivalent execution that is a sequence of rounds. We know from previous results that reachability is PSPACE-complete for synchronizable systems.

In this talk we show that we can check if a mailbox system is synchronizable thanks to a novel automata-based approach. From this we get that checking synchronizability is PSPACE-complete. The same question is undecidable under peer-to-peer semantics. We also show that model-checking synchronizable systems for a natural subclass of regular properties is a PSPACE-complete problem.

Joint work with Anca Muscholl & Grégoire Sutre.

Formal theorem proving is challenging for humans as well as for machines. Thanks to recent advances in LLM capabilities, we believe natural language can serve as a universal interface for reasoning about formal proofs. In this talk, 1) we introduce Pétanque, a new lightweight environment to interact with the Coq theorem prover; 2) we present two interactive proof protocols leveraging natural language as an intermediate representation for designing proof steps; 3) we implement beam search over these interaction protocols, using natural language to rerank proof candidates; and 4) we use Pétanque to benchmark our search algorithms on different datasets. We will then discuss the limitations of our approach and possible future directions.