You're right that it's only putting some extra computation into it, but I don't think there is any way that a prime modulo would make any good hash function worse; the only problem is some wasted effort.
A problem is that I'm not sure of the quality of the hash functions we use internally either. The string hash is a homebrew (afaik), and in several other places pointers are used as hash values. As for the string hash, I made an attempt to search for published alternatives, but I couldn't find any that hashes in O(1) - all of them hashes on the whole string. I think an O(1) hash function is very important in our case. Anyway, it should be possible to sort this out. Pointers are worse. They can contain all sorts of periodicity. An obvious one is that the lowest one or two bits always are zero due to alignment, but there might be more due to the malloc implementation and its allocation patterns, and you'd never know about it (well, in the case of all the block_alloc areas we do know a bit about it, but it's different for each and every one of them). A prime modulo is very effective to put all such concerns to rest. Maybe there are other smart ideas on how to make good hashes out of pointers; I haven't checked yet. One more concern is also how it behaves for the ultimately bad hash function, i.e. one that returns the same value all the time. In that case the hash table is of course O(n) on lookup and store, but it shouldn't try to grow the table all the time due to a reprobe limit. I don't know if he found a clever way to avoid that.
