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rr numbers each 'event' it records, and you can pass an event number to the gdb 'run' command to tell it to start from that event. Recent 'rr' now also supports the -e option to replay meaning 'start the debug session pointing at the last recorded event, whatever that was'. Details in the usage page: https://github.com/rr-debugger/rr/wiki/Usage
AIUI you get 'start at an event' basically for free, because 'step backward' is implemented as 'start at the preceding event and then step forward by N', so events are frequent in the trace and the machinery to get to that point without running all the way from the start of the debug session exists anyway. There's some stuff on the website about how this is all implemented, I think.
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If you use https://pernos.co/ then you don't need any of this, but I have a set of only slightly buggy gdbinit scripts that extend the rr debugging experience at <https://github.com/hotsphink/sfink-tools>. The main things it adds are:
1. a `log` command that just records whatever you give it into a plaintext file, together with its "point in time" according to rr. This is useful because when using rr, you tend to move forward and backward in time a lot, and it's hard to keep track of the actual sequence of events and where you are within them. It also creates a checkpoint so you can return to any one of your log points. It also has some niceties like replacing any expression enclosed in curly brackets with the results of executing the gdb expression given, so you can do things like
log starting execution of Init() with v={v}. About to crash. -
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