Compilation as Multi-Language Semantics (2509.19613v1)

Abstract: Modeling interoperability between programs in different languages is a key problem when modeling verified and secure compilation, which has been successfully addressed using multi-language semantics. Unfortunately, existing models of compilation using multi-language semantics define two variants of each compiler pass: a syntactic translation on open terms to model compilation, and a run-time translation of closed terms at multi-language boundaries to model interoperability. In this talk, I discuss work-in-progress approach to uniformly model a compiler entirely as a reduction system on open term in a multi-language semantics, rather than as a syntactic translation. This simultaneously defines the compiler and the interoperability semantics, reducing duplication. It also provides interesting semantic insights. Normalization of the cross-language redexes performs ahead-of-time (AOT) compilation. Evaluation in the multi-LLMs just-in-time (JIT) compilation. Confluence of multi-language reduction implies compiler correctness, and part of the secure compilation proof (full abstraction), enabling focus on the difficult part of the proof. Subject reduction of the multi-language reduction implies type-preservation of the compiler.