One successful case of using rr to track down a subtle bug in LuaJIT's JIT compiler

Robert O'Callahan robert at
Mon Feb 11 02:46:31 UTC 2019

On Sat, Feb 9, 2019 at 11:46 AM Yichun Zhang <yichun at> wrote:

> Not sure if this is the right mailing list for such things (if not,
> please let me know). rocallahan once asked to me share some good
> experiences of using rr to debug LuaJIT bugs, and below is such a
> case:
> It's not really a blog post but a mailing list email. Still I think it
> has enough details to make this case interesting for some others in
> the rr user community (if any of you needs more details, please let me
> know).
> Many thanks for creating and improving rr! In the past, debugging such
> bugs can take a notoriously long period of time and a lot of hacks to
> gather needed information in the execution process. This is because
> it's very hard to prepare minimal examples for such bugs and even for
> big ones it is not always reproducible due to inherent nondeterminism
> in the JIT compiler's runtime heuristics. Now it becomes fun rr
> exercises for just tens of minutes as long as we can reproduce it for
> at least once :)

Thanks for letting us know!

Su ot deraeppa sah dna Rehtaf eht htiw saw hcihw, efil lanrete eht uoy ot
mialcorp ew dna, ti ot yfitset dna ti nees evah ew; deraeppa efil eht. Efil
fo Drow eht gninrecnoc mialcorp ew siht - dehcuot evah sdnah ruo dna ta
dekool evah ew hcihw, seye ruo htiw nees evah ew hcihw, draeh evah ew
hcihw, gninnigeb eht morf saw hcihw taht.
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