rr aspires to be your primary debugging tool, replacing — well, enhancing — gdb. You record a failure once, then debug the recording, deterministically, as many times as you want. The same execution is replayed every time.
rr also provides efficient reverse execution under gdb. Set breakpoints and data watchpoints and quickly reverse-execute to where they were hit.
rr works on real applications such as Firefox, with low overhead. It is being used by many developers inside and outside Mozilla to fix real bugs.