The prior unrolling didn't really do much as it left the dependency
chain between iterations. Unrolled the loop for 4 so 4x multiplies
could be pipelined in out-of-order machines.
Results for __nss_hash
Benchmarked on Tigerlake: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
Time as Geometric Mean of N=25 runs
Geometric of all benchmark New / Old: 0.845
type, length, New Time, Old Time, New Time / Old Time
fixed, 0, 4.019, 3.729, 1.078
fixed, 1, 4.95, 5.707, 0.867
fixed, 2, 5.152, 5.657, 0.911
fixed, 3, 4.641, 5.721, 0.811
fixed, 4, 5.551, 5.81, 0.955
fixed, 5, 6.525, 6.552, 0.996
fixed, 6, 6.711, 6.561, 1.023
fixed, 7, 6.715, 6.767, 0.992
fixed, 8, 7.874, 7.915, 0.995
fixed, 9, 8.888, 9.767, 0.91
fixed, 10, 8.959, 9.762, 0.918
fixed, 11, 9.188, 9.987, 0.92
fixed, 12, 9.708, 10.618, 0.914
fixed, 13, 10.393, 11.14, 0.933
fixed, 14, 10.628, 12.097, 0.879
fixed, 15, 10.982, 12.965, 0.847
fixed, 16, 11.851, 14.429, 0.821
fixed, 32, 24.334, 34.414, 0.707
fixed, 64, 55.618, 86.688, 0.642
fixed, 128, 118.261, 224.36, 0.527
fixed, 256, 256.183, 538.629, 0.476
random, 2, 11.194, 11.556, 0.969
random, 4, 17.516, 17.205, 1.018
random, 8, 23.501, 20.985, 1.12
random, 16, 28.131, 29.212, 0.963
random, 32, 35.436, 38.662, 0.917
random, 64, 45.74, 58.868, 0.777
random, 128, 75.394, 121.963, 0.618
random, 256, 139.524, 260.726, 0.535
---
nss/nss_hash.c | 79 +++++++++++++++++++++++++++-----------------------
1 file changed, 42 insertions(+), 37 deletions(-)
@@ -19,58 +19,63 @@
/* This is from libc/db/hash/hash_func.c, hash3 is static there */
/*
- * This is INCREDIBLY ugly, but fast. We break the string up into 8 byte
+ * This is INCREDIBLY ugly, but fast. We break the string up into 4 byte
* units. On the first time through the loop we get the "leftover bytes"
- * (strlen % 8). On every other iteration, we perform 8 HASHC's so we handle
- * all 8 bytes. Essentially, this saves us 7 cmp & branch instructions. If
- * this routine is heavily used enough, it's worth the ugly coding.
+ * (len % 4). On every other iteration, we perform a 4x unrolled version
+ * HASHC. Further unrolling does not appear to help.
*
* OZ's original sdbm hash
*/
uint32_t
__nss_hash (const void *keyarg, size_t len)
{
+ enum
+ {
+ HASH_CONST_P0 = 1, /* (uint32_t)(65599 ^ 0). */
+ HASH_CONST_P1 = 65599, /* (uint32_t)(65599 ^ 1). */
+ HASH_CONST_P2 = 8261505, /* (uint32_t)(65599 ^ 2). */
+ HASH_CONST_P3 = 780587199, /* (uint32_t)(65599 ^ 3). */
+ HASH_CONST_P4 = 1139564289 /* (uint32_t)(65599 ^ 4). */
+ };
+
const unsigned char *key;
- size_t loop;
uint32_t h;
-#define HASHC h = *key++ + 65599 * h
+#define HASHC h = *key++ + HASH_CONST_P1 * h
h = 0;
key = keyarg;
if (len > 0)
{
- loop = (len + 8 - 1) >> 3;
- switch (len & (8 - 1))
- {
- case 0:
- do
- {
- HASHC;
- /* FALLTHROUGH */
- case 7:
- HASHC;
- /* FALLTHROUGH */
- case 6:
- HASHC;
- /* FALLTHROUGH */
- case 5:
- HASHC;
- /* FALLTHROUGH */
- case 4:
- HASHC;
- /* FALLTHROUGH */
- case 3:
- HASHC;
- /* FALLTHROUGH */
- case 2:
- HASHC;
- /* FALLTHROUGH */
- case 1:
- HASHC;
- }
- while (--loop);
- }
+ switch ((len & (4 - 1)))
+ {
+ case 0:
+ /* h starts out as zero so no need to include the multiply. */
+ h = *key++;
+ /* FALLTHROUGH */
+ case 3:
+ HASHC;
+ /* FALLTHROUGH */
+ case 2:
+ HASHC;
+ /* FALLTHROUGH */
+ case 1:
+ HASHC;
+ /* FALLTHROUGH */
+ }
+
+ uint32_t c0, c1, c2, c3;
+ for (--len; len >= 4; len -= 4)
+ {
+ c0 = (unsigned char) *(key + 0);
+ c1 = (unsigned char) *(key + 1);
+ c2 = (unsigned char) *(key + 2);
+ c3 = (unsigned char) *(key + 3);
+ h = HASH_CONST_P4 * h + HASH_CONST_P3 * c0 + HASH_CONST_P2 * c1
+ + HASH_CONST_P1 * c2 + HASH_CONST_P0 * c3;
+
+ key += 4;
+ }
}
return h;
}