From 7623d0829a63e7f95ed6f116b7e9f2f4a023e8e7 Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Tue, 2 Apr 2019 11:49:28 +1030 Subject: [PATCH] htable: avoid branch in calculating perfect bit. Final results of tools/speed/10000000 (10 runs) shows a slight slowdown in some tests, but it makes an empty htable smaller. -Initial delete all: 96-98(96.4+/-0.66) ns +Initial delete all: 97-99(98.2+/-0.75) ns -Initial re-inserting: 117-124(121.4+/-1.9) ns +Initial re-inserting: 124-131(126.4+/-2.4) ns -Adding (a different) half: 49-50(49.3+/-0.46) ns +Adding (a different) half: 50-52(51.2+/-0.75) ns Signed-off-by: Rusty Russell --- ccan/htable/htable.c | 23 +++++++++++------------ 1 file changed, 11 insertions(+), 12 deletions(-) diff --git a/ccan/htable/htable.c b/ccan/htable/htable.c index b27d063f..d3c4f9b4 100644 --- a/ccan/htable/htable.c +++ b/ccan/htable/htable.c @@ -11,6 +11,9 @@ /* We use 0x1 as deleted marker. */ #define HTABLE_DELETED (0x1) +/* perfect_bitnum 63 means there's no perfect bitnum */ +#define NO_PERFECT_BIT (sizeof(uintptr_t) * CHAR_BIT - 1) + static void *htable_default_alloc(struct htable *ht, size_t len) { return calloc(len, 1); @@ -58,13 +61,9 @@ static inline bool entry_is_valid(uintptr_t e) return e > HTABLE_DELETED; } -/* We use 0 to mean we don't have a perfect bit, otherwise it's - * bit n - 1 */ static inline uintptr_t ht_perfect_mask(const struct htable *ht) { - if (ht->perfect_bitnum > 0) - return (uintptr_t)1 << (ht->perfect_bitnum - 1); - return 0; + return (uintptr_t)2 << ht->perfect_bitnum; } static inline uintptr_t get_hash_ptr_bits(const struct htable *ht, @@ -232,10 +231,10 @@ static COLD bool double_table(struct htable *ht) ht->bits++; /* If we lost our "perfect bit", get it back now. */ - if (ht->perfect_bitnum == 0 && ht->common_mask) { + if (ht->perfect_bitnum == NO_PERFECT_BIT && ht->common_mask) { for (i = 0; i < sizeof(ht->common_mask) * CHAR_BIT; i++) { - if (ht->common_mask & ((size_t)1 << i)) { - ht->perfect_bitnum = i + 1; + if (ht->common_mask & ((size_t)2 << i)) { + ht->perfect_bitnum = i; break; } } @@ -259,7 +258,7 @@ static COLD bool double_table(struct htable *ht) static COLD void rehash_table(struct htable *ht) { size_t start, i; - uintptr_t e; + uintptr_t e, perfect = ht_perfect_mask(ht); /* Beware wrap cases: we need to start from first empty bucket. */ for (start = 0; ht->table[start]; start++); @@ -271,7 +270,7 @@ static COLD void rehash_table(struct htable *ht) continue; if (e == HTABLE_DELETED) ht->table[h] = 0; - else if (!(e & ht_perfect_mask(ht))) { + else if (!(e & perfect)) { void *p = get_raw_ptr(ht, e); ht->table[h] = 0; ht_add(ht, p, ht->rehash(p, ht->priv)); @@ -299,7 +298,7 @@ static COLD void update_common(struct htable *ht, const void *p) ht->common_mask = ~((uintptr_t)1 << i); ht->common_bits = ((uintptr_t)p & ht->common_mask); - ht->perfect_bitnum = 1; + ht->perfect_bitnum = 0; (void)htable_debug(ht, HTABLE_LOC); return; } @@ -323,7 +322,7 @@ static COLD void update_common(struct htable *ht, const void *p) ht->common_mask &= ~maskdiff; ht->common_bits &= ~maskdiff; if (ht_perfect_mask(ht) & maskdiff) - ht->perfect_bitnum = 0; + ht->perfect_bitnum = NO_PERFECT_BIT; (void)htable_debug(ht, HTABLE_LOC); } -- 2.39.2