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 debug to work it seems to me that you need to store the entire machine state including memory for each step. This would make performance incredibly slow, not to mention using a lot of memory. How are these problems solved?
Nathan Fellman wrote:
But does reverse debugging only allow you to roll back next and step commands that you typed, or does it allow you to undo any number of instructions?
You can undo any number of instructions. You're not restricted to, for instance, only stopping at the points where you stopped when you were going forward. You can set a new breakpoint and run backwards to it.
For instance, if I set a breakpoint on an instruction and let it run until then, can I then roll back to the previous instruction, even though I skipped over it?
Yes. So long as you turned on recording mode before you ran to the breakpoint.
Reverse debugging means you can run the program backwards, which is very useful to track down the cause of a problem.
You don't need to store the complete machine state for each step, only the changes. It is probably still quite expensive.