dc82685ce0
A mutable hash table only uses the lower few bits of a hash code, because it masks the hash code by [one less than] the power-of-two size of the bucket array. That truncation interacts badly with the hashing function for fixnums, which is the identity function; if the the lower several bits of the fixnum stay the same for many keys and the upper bits change, then there are many hash collisions --- and that's a relatively likely distribution. Fix the mutable hash-table implementations by mixing the hash code to let higher bits influence the lower bits: xor the high half of the bits with the lower half (which doesn't lose information, because xoring again would recover the original number), then do the same for the high one-fourth of bits in the low half, and then (on a 64-bit platform) the high one-eighth of the low one-fourth of the bits. Instead if blaming the way the mutable hash-table implementations only use the lower bits of a hash code, we could blame the hash function on fixnums for not performing this kind of mixing. In this patch, though, we take the view that the hash function's job is to map variation in its domain to variation in the fixnum hash code, and then the hash table's job is to use that fixnum effectively. That separation of responsibilities is now documented with `gen:equal+hash`. There are also improvements here to the hashing function for bignums in CS and to the secondary hashung function for fixnums and bignums in BC. Thanks to Alex Harsanyi for reporting the problem. |
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