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Use a better hash function for libsepol's avtab
This function, based on murmurhash3, has much better distribution than the original. Using the current default of 4096 buckets, there are many fewer collisions: Before: 2893000 entries and 4096/4096 buckets used, longest chain length 1649 After: 2732000 entries and 4096/4096 buckets used, longest chain length 764 The difference becomes much more significant when buckets are increased. A naive attempt to expand the current function to larger outputs doesn't yield any significant improvement; so this function is a prerequisite for increasing the bucket size. Signed-off-by: John Brooks <john.brooks@jolla.com>
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@ -49,10 +49,43 @@
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#include "debug.h"
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#include "private.h"
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static inline int avtab_hash(struct avtab_key *keyp, uint16_t mask)
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/* Based on MurmurHash3, written by Austin Appleby and placed in the
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* public domain.
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*/
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static inline int avtab_hash(struct avtab_key *keyp, uint32_t mask)
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{
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return ((keyp->target_class + (keyp->target_type << 2) +
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(keyp->source_type << 9)) & mask);
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static const uint32_t c1 = 0xcc9e2d51;
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static const uint32_t c2 = 0x1b873593;
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static const uint32_t r1 = 15;
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static const uint32_t r2 = 13;
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static const uint32_t m = 5;
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static const uint32_t n = 0xe6546b64;
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uint32_t hash = 0;
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#define mix(input) { \
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uint32_t v = input; \
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v *= c1; \
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v = (v << r1) | (v >> (32 - r1)); \
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v *= c2; \
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hash ^= v; \
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hash = (hash << r2) | (hash >> (32 - r2)); \
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hash = hash * m + n; \
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}
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mix(keyp->target_class);
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mix(keyp->target_type);
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mix(keyp->source_type);
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#undef mix
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hash ^= hash >> 16;
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hash *= 0x85ebca6b;
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hash ^= hash >> 13;
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hash *= 0xc2b2ae35;
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hash ^= hash >> 16;
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return hash & mask;
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}
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static avtab_ptr_t
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