libFuzzer vs. unrolling


After landed, shrink.test stopped passing for us on x86_64h architecture (x86_64 + some haswell extensions).
After looking further into this today, I have noticed that optimizations on haswell are more likely to do unrolling (since it can do more advanced vectorization).
The main loop (inside the callback) in shrink.test gets unrolled thrice, and stays the same on x86_64.

I see how unrolling could mess with coverage, as conceptually the same location in the code becomes e.g. three after unrolling,
and fuzzer should prefer exploring different portions of the program to going around the loop.
Should we then disable unrolling when OPT_FOR_FUZZING is enabled in the spirit of ?
Or just increase the limit on shrink.test?



We still get intermittent failures on libfuzzer/shrink.test every now and then.
I could not get a reproducible example.
Have you seen a similar issue?


Actually now I see a similar issue, where the loop vectorization optimization stops
counters.test from passing on ARM64. I think we should disable some optimizations using OPT_FOR_FUZZING.