rr has a similar feature set to UndoDB. rr is probably a bit more efficient during recording. However, UndoDB doesn't depend on the performance counters the way

rr has a similar feature set to UndoDB. rr is probably a bit more efficient during recording. However, UndoDB doesn't depend on the performance counters the way rr does, so it works in situations where those counters are unavailable (e.g. some VM guests). Also UndoDB works on ARM, but due to the way ARM implements atomics (see above) rr's approach can't work on ARM. (Until/unless we get ARM to add support for trapping on failed LL/SC.)