We show how to reverse a while language extended with blocks, local variables, procedures and the interleaving parallel composition. Annotation is defined along with a set of operational semantics capable of storing necessary reversal information, and identifiers are introduced to capture the interleaving order of an execution. Inversion is defined with a set of operational semantics that use saved information to undo an execution. We prove that annotation does not alter the behaviour of the original program, and that inversion correctly restores the initial program state.

We propose an approach and a subsequent extension for reversing imperative programs. Firstly, we produce both an augmented version and a corresponding inverted version of the original program. Augmentation saves reversal information into an auxiliary data store, maintaining segregation between this and the program state, while never altering the data store in any other way than that of the original program. Inversion uses this information to revert the final program state to the state as it was before execution. We prove that augmentation and inversion work as intended, and illustrate our approach with several examples. We also suggest a modification to our first approach to support non-communicating parallelism. Execution interleaving introduces a number of challenges, each of which our extended approach considers. We define annotation and redefine inversion to use a sequence of statement identifiers, making the interleaving order deterministic in reverse.Comment: In Proceedings EXPRESS/SOS 2017, arXiv:1709.0004

We present a state-saving approach to reversible execution of imperative programs containing parallel composition. Given an original program, we produce an annotated version of the program that both performs forwards execution and all necessary state-saving of required reversal information. We further produce an inverted version of our program, capable of using this saved information to reverse the effects of each step of the forwards execution. We show that this process implements correct and garbage-free inversion. We give examples of how our implementation of reversible execution can be used for debugging, and demonstrate how a simulation tool we have developed for our approach can be used to examine the program state. Finally, we evaluate the performance and overheads associated with state-saving and inversion.

Reversible computing allows one to run programs not only in the usual forward direction, but also backward. A main application area for reversible computing is debugging, where one can use reversibility to go backward from a visible misbehaviour towards the bug causing it. While reversible debugging of sequential systems is well understood, reversible debugging of concurrent and distributed systems is less settled. We present here two approaches for debugging concurrent programs, one based on backtracking, which undoes actions in reverse order of execution, and one based on causal consistency, which allows one to undo any action provided that its consequences, if any, are undone beforehand. The first approach tackles an imperative language with shared memory, while the second one considers a core of the functional message-passing language Erlang. Both the approaches are based on solid formal foundations.

Software plays a central role in all aspects of reversible computing. We survey the breadth of topics and recent activities on reversible software and systems including behavioural types, recovery, debugging, concurrency, and object-oriented programming. These have the potential to provide linguistic abstractions and tools that will lead to safer and more reliable reversible computing applications. This work has been partially supported by COST Action IC1405 on Reversible Computation-Extending Horizons of Computing.