Proving MCAPI executions are correct using SMT

Asynchronous message passing is an important paradigm in writing applications for embedded heterogeneous multicore systems. The Multicore Association (MCA), an industry consortium promoting multicore technology, is working to standardize message passing into a single API, MCAPI, for bare metal implementation and portability across platforms. Correctness in such an API is difficult to reason about manually, and testing against reference solutions is equally difficult as reference solutions implement an unknown set of allowed behaviors, and programmers have no way to directly control API internals to expose or reproduce errors. This paper provides a way to encode an MCAPI execution as a Satisfiability Modulo Theories (SMT) problem, which if satisfiable, yields a feasible execution schedule on the same trace, such that it resolves non-determinism in the MCAPI runtime in a way that it now fails user provided assertions. The paper proves the problem is NP-complete. The encoding is useful for test, debug, and verification of MCAPI program execution. The novelty in the encoding is the direct use of match pairs (potential send and receive couplings). Match-pair encoding for MCAPI executions, when compared to other encoding strategies, is simpler to reason about, results in significantly fewer terms in the SMT problem, and captures feasible behaviors that are ignored in previously published techniques. Further, to our knowledge, this is the first SMT encoding that is able to run in infinite-buffer semantics, meaning the runtime has unlimited internal buffering as opposed to no internal buffering. Results demonstrate that the SMT encoding, restricted to zero-buffer semantics, uses fewer clauses when compared to another zero-buffer technique, and it runs faster and uses less memory. As a result the encoding scales well for programs with high levels of non-determinism in how sends and receives may potentially match.