static int spread(int h) {
    return (h ^ (h >>> 16)) & HASH_BITS;
  }
Spreads (XORs) higher bits of hash to lower and also forces top bit to 0. Because the table
uses power-of-two masking, sets of hashes that vary only in bits above the current mask will
always collide. (Among known examples are sets of Float keys holding consecutive whole numbers
in small tables.) So we apply a transform that spreads the impact of higher bits downward.
There is a tradeoff between speed, utility, and quality of bit-spreading. Because many common
sets of hashes are already reasonably distributed (so don't benefit from spreading), and
because we use trees to handle large sets of collisions in bins, we just XOR some shifted bits
in the cheapest possible way to reduce systematic lossage, as well as to incorporate impact of
the highest bits that would otherwise never be used in index calculations because of table
bounds.
geode-core/src/main/java/org/apache/geode/internal/concurrent/CompactConcurrentHashSet2.java