The hash table implementation uses `& (h->size - 1)` to truncate
generated hashes to the number of buckets. This operation is equal to
`% h->size` if and only if the size is a power of two (which seems to be
always the case). One property of the binary and with a power of two
(and probably a small one <=2048) is all higher bits are discarded.
Thus a hash function is needed with a good avalanche effect, which the
current one is not.
Benchmark of building dssp5:
# Current
Time (mean ± σ): 1.347 s ± 0.065 s [User: 1.207 s, System: 0.138 s]
Range (min … max): 1.274 s … 1.436 s 10 runs
# Patch
Time (mean ± σ): 1.336 s ± 0.029 s [User: 1.195 s, System: 0.140 s]
Range (min … max): 1.303 s … 1.376 s 10 runs
Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx>
---
libsepol/cil/src/cil_strpool.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)
diff --git a/libsepol/cil/src/cil_strpool.c b/libsepol/cil/src/cil_strpool.c
index e32ee4e9..beea5c9d 100644
--- a/libsepol/cil/src/cil_strpool.c
+++ b/libsepol/cil/src/cil_strpool.c
@@ -47,16 +47,13 @@ static hashtab_t cil_strpool_tab = NULL;
static unsigned int cil_strpool_hash(hashtab_t h, const_hashtab_key_t key)
{
- const char *p;
- size_t size;
- unsigned int val;
+ unsigned int hash = 5381;
+ unsigned char c;
- val = 0;
- size = strlen(key);
- for (p = key; ((size_t) (p - key)) < size; p++)
- val =
- (val << 4 | (val >> (8 * sizeof(unsigned int) - 4))) ^ (*p);
- return val & (h->size - 1);
+ while ((c = *(unsigned const char *)key++))
+ hash = ((hash << 5) + hash) ^ c;
+
+ return hash & (h->size - 1);
}
static int cil_strpool_compare(hashtab_t h __attribute__ ((unused)), const_hashtab_key_t key1, const_hashtab_key_t key2)
--
2.40.1
