An execution-backtracking approach to debugging

Spyder, a system for selective checkpointing of computational sequences, is presented. It lets users backtrack from checkpoints without the need to reexecute the program to reach recent prior states. In contrast to more comprehensive (and storage-intensive) checkpointing schemes, backtracking in this approach is constrained to limit storage requirements. The resulting debugger offers a structured view of dynamic events, similar to lexical scope rules' effect on static visibility. The debugger also speeds backtracking to statements before loops and provides what-if capabilities