[_yosefk]

rr helps with indeterminism. Memory sanitizers help only a little with that, and, sadly, any thread sanitizer working with POSIX-like threads also helps only a little with that. Reproducing the problem (somewhat differently every time) is one thing, reproducing it in exactly the same way every time and looking at state for as long as you want to is another ballgame entirely. And a 1.2x slowdown is not a ton. 2x is not a ton.

I've developed automated debugging tools, I debugged hundreds and hundreds of rare, irreproducible bugs or just bugs that are hard to figure out, I found things like memory ordering bugs in others' concurrent code just by eyeballing it because the bug was too rare to wait for it to reproduce, I debugged concurrent code on multicore chips without memory coherence, on faulty hardware, etc. You're entitled to your own opinion about what "the hard part" is, but I'm likewise entitled to mine, and I think rr is awesome and I disagree with you on every point.