IGOR: a system for program debugging via reversible execution

Typical debugging tools are insufficiently powerful to find the most difficult types of program misbehaviors. We have implemented a prototype of a new debugging system, IGOR, which provides a great deal more useful information and offers new abilities that are quite promising. The system runs fast enough to be quite useful while providing many features that are usually available only in an interpreted environment. We describe here some improved facilities (reverse execution, selective searching of execution history, substitution of data and executable parts of the programs) that are needed for serious debugging and are not found in traditional single-thread debugging tools. With a little help from the operating system, we provide these capabilities at reasonable cost without modifying the executable code and running fairly close to full speed. The pro-totype runs under the DUNE distributed operating system. The current system only supports debugging of single-thread programs. The paper describes planned extensions to make use of extra processors to speed the system and for applying the technique to multi-thread and time-dependent executions. 1.