Separate Compilation of Polychronous Specifications

As code generation for synchronous programs requires strong safety properties to be satisfied, compositionality becomes a difficult goal to achieve. Most synchronous languages, such as Esterel, Lustre or Signal require a given module or compilation unit to be insensitive to latency that communication with its environment may incur. In Lustre or Signal, for instance, a compilation unit must satisfy the so-called property of endochrony. To preserve endochrony in an asynchronous environment, an ad-hoc protocol is synthesized to interface the module. However, endochrony is not preserved by composition. Consequently, the protocol has to be rebuilt every time a new module is added in the environment. We propose a methodology and code generation scheme which simplifies this concern. It consists of weakening the global objective of globally preserving endochrony. Instead, we aim at the preservation of a more liberal and compositional objective, weak endochrony [D. Potop-Butucaru and B. Caillaud and A. Benveniste. Concurrency in Synchronous Systems. In Formal Methods in System Design, v. 28(2). Springer, March 2006], which is compositional and much closer from the expected requirement of insensitivity to communication latency. As a result, our code generation scheme supports true separate compilation: a locally compiled synchronous module does not require its synthesized interface with the environment to be rebuilt once composed with another module.