Collective Assertions

We introduce the notion of collective assertions for message-passing-based parallel programs with distributed memory, such as those written using the Message Passing Interface. A single collective assertion comprises a set of locations in each process and an expression on the global state. The semantics are defined as follows: whenever control in a process reaches one of the locations, a "snapshot' of the local state of that process is sent to a coordinator; once a snapshot has been received from each process, the expression is evaluated on the global state formed by uniting the snapshots. We have extended the Toolkit for Accurate Scientific Software (TASS), a verifier based on symbolic execution and explicit state enumeration, to check that collective assertions hold on all possible executions of a C/MPI program. We give several examples of such programs, show that many properties of them are naturally expressed as collective assertions, and use TASS to verify or refute these.