How does reverse debugging work?

Question

GDB has a new version out that supports reverse debug (see http://www.gnu.org/software/gdb/news/reversible.html). I got to wondering how that works.

To get reverse

Answer 1

During an EclipseCon session we also asked how they do this with the Chronon Debugger for Java. That one does not allow you to actually step back, but can play back a recorded program execution in such a way that it feels like reverse debugging. (The main difference is that you cannot change the running program in the Chronon debugger, while you can do that in most other Java debuggers.)

If I understood it correctly, it manipulates the byte code of the running program, such that every change of an internal state of the program is recorded. External states don't need to be recorded additionally. If they influence your program in some way, then you must have an internal variable matching that external state (and therefore that internal variable is enough).

During playback time they can then basically recreate every state of the running program from the recorded state changes.

Interestingly the state changes are much smaller than one would expect on first look. So if you have a conditional "if" statement, you would think that you need at least one bit to record whether the program took the then- or the else-statement. In many cases you can avoid even that, like in the case that those different branches contain a return value. Then it is enough to record only the return value (which would be needed anyway) and to recalculate the decision about the executed branch from the return value itself.