These array lookups are given in the article of 2013 by Simon Gog and Matthias Petri, 2013, "Optimized Succinct Data Structures for Massive Data" in Section 6.2, currently available at http://people.eng.unimelb.edu.au/sgog/optimized.pdf .
A quote from there:
Unfortunately, the development of efficient select operations on 64 -bit integers ( select 64 ) has not been as rapid as for popcnt . There are no direct CPU instructions available to return the position of the i -th set bit.
As was noted at http://vigna.di.unimi.it/Sux/select.php , these two lookups have the overhead of java array accesses, so it would probably be wortwhile to use a native version of this, at least until better processor support becomes available.
For the patch here, I'd expect that some speedup in Java is still possible.