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Code Issues 1 Releases Wiki Activity

Add some more changes to hashing to improve pointer hashing.

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This commit is contained in:
Calvin Rose 2021-12-07 08:36:08 -06:00
parent 92fdd07ca3
commit 6d9286a202
2 changed files with 31 additions and 5 deletions
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12
src/core/value.c
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@ -325,7 +325,7 @@ int32_t janet_hash(Janet x) {
uint32_t lo = (uint32_t)(as.u & 0xFFFFFFFF); uint32_t lo = (uint32_t)(as.u & 0xFFFFFFFF);
uint32_t hi = (uint32_t)(as.u >> 32); uint32_t hi = (uint32_t)(as.u >> 32);
uint32_t hilo = (hi ^ lo) * 2654435769u; uint32_t hilo = (hi ^ lo) * 2654435769u;
hash = (int32_t)((hilo << 16) | ((hilo >> 16) & 0xFFFF)); hash = (int32_t)((hilo << 16) | (hilo >> 16));
break; break;
} }
case JANET_ABSTRACT: { case JANET_ABSTRACT: {
@ -339,15 +339,17 @@ int32_t janet_hash(Janet x) {
/* fallthrough */ /* fallthrough */
default: default:
if (sizeof(double) == sizeof(void *)) { if (sizeof(double) == sizeof(void *)) {
/* Assuming 8 byte pointer */ /* Assuming 8 byte pointer (8 byte aligned) */
uint64_t i = janet_u64(x); uint64_t i = janet_u64(x);
uint32_t lo = (uint32_t)(i & 0xFFFFFFFF); uint32_t lo = (uint32_t)(i & 0xFFFFFFFF);
uint32_t hi = (uint32_t)(i >> 32); uint32_t hi = (uint32_t)(i >> 32);
hash = (int32_t)(hi ^ (lo >> 3)); uint32_t hilo = (hi ^ lo) * 2654435769u;
hash = (int32_t)((hilo << 16) | (hilo >> 16));
} else { } else {
/* Assuming 4 byte pointer (or smaller) */ /* Assuming 4 byte pointer (or smaller) */
hash = (int32_t)((char *)janet_unwrap_pointer(x) - (char *)0); ptrdiff_t diff = ((char *)janet_unwrap_pointer(x) - (char *)0);
hash >>= 2; uint32_t hilo = (uint32_t) diff * 2654435769u;
hash = (int32_t)((hilo << 16) | (hilo >> 16));
} }
break; break;
} }

24
tools/hashbench/ints1.janet Normal file
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@ -0,0 +1,24 @@
(def f @{})
(var collisions 0)
(loop [x :range [0 300] y :range [0 300]]
(def key (hash (+ (* x 1000) y)))
(if (in f key)
(++ collisions))
(put f key true))
(print "ints 1 collisions: " collisions)
(def f @{})
(var collisions 0)
(loop [x :range [100000 101000] y :range [100000 101000]]
(def key (hash [x y]))
(if (in f key) (++ collisions))
(put f key true))
(print "int pair 1 collisions: " collisions)
(def f @{})
(var collisions 0)
(loop [x :range [10000 11000] y :range [10000 11000]]
(def key (hash [x y]))
(if (in f key) (++ collisions))
(put f key true))
(print "int pair 2 collisions: " collisions)