htable: use "perfect" bit to reduce rehashes.

[ccan] / ccan / htable / htable.c

diff --git a/ccan/htable/htable.c b/ccan/htable/htable.c
index 48a734071fa93e1cca12a438c129c008ec0e72ee..dec53127470a9f6b06a53555286a9fcbb153dbef 100644 (file)
--- a/ccan/htable/htable.c
+++ b/ccan/htable/htable.c
@@ -18,6 +18,7 @@ struct htable {
        size_t elems, deleted, max, max_with_deleted;
        /* These are the bits which are the same in all pointers. */
        uintptr_t common_mask, common_bits;
+       uintptr_t perfect_bit;
        uintptr_t *table;
 };
 
@@ -50,7 +51,8 @@ static inline uintptr_t get_hash_ptr_bits(const struct htable *ht,
        /* Shuffling the extra bits (as specified in mask) down the
         * end is quite expensive.  But the lower bits are redundant, so
         * we fold the value first. */
-       return (hash ^ (hash >> ht->bits)) & ht->common_mask;
+       return (hash ^ (hash >> ht->bits))
+               & ht->common_mask & ~ht->perfect_bit;
 }
 
 struct htable *htable_new(size_t (*rehash)(const void *elem, void *priv),
@@ -68,6 +70,7 @@ struct htable *htable_new(size_t (*rehash)(const void *elem, void *priv),
                /* This guarantees we enter update_common first add. */
                ht->common_mask = -1;
                ht->common_bits = 0;
+               ht->perfect_bit = 0;
                ht->table = calloc(1 << ht->bits, sizeof(uintptr_t));
                if (!ht->table) {
                        free(ht);
@@ -89,9 +92,9 @@ static size_t hash_bucket(const struct htable *ht, size_t h)
 }
 
 static void *htable_val(const struct htable *ht,
-                       struct htable_iter *i, size_t hash)
+                       struct htable_iter *i, size_t hash, uintptr_t perfect)
 {
-       uintptr_t h2 = get_hash_ptr_bits(ht, hash);
+       uintptr_t h2 = get_hash_ptr_bits(ht, hash) | perfect;
 
        while (ht->table[i->off]) {
                if (ht->table[i->off] != HTABLE_DELETED) {
@@ -99,6 +102,7 @@ static void *htable_val(const struct htable *ht,
                                return get_raw_ptr(ht, ht->table[i->off]);
                }
                i->off = (i->off + 1) & ((1 << ht->bits)-1);
+               h2 &= ~perfect;
        }
        return NULL;
 }
@@ -107,14 +111,14 @@ void *htable_firstval(const struct htable *ht,
                      struct htable_iter *i, size_t hash)
 {
        i->off = hash_bucket(ht, hash);
-       return htable_val(ht, i, hash);
+       return htable_val(ht, i, hash, ht->perfect_bit);
 }
 
 void *htable_nextval(const struct htable *ht,
                     struct htable_iter *i, size_t hash)
 {
        i->off = (i->off + 1) & ((1 << ht->bits)-1);
-       return htable_val(ht, i, hash);
+       return htable_val(ht, i, hash, 0);
 }
 
 void *htable_first(const struct htable *ht, struct htable_iter *i)
@@ -139,13 +143,15 @@ void *htable_next(const struct htable *ht, struct htable_iter *i)
 static void ht_add(struct htable *ht, const void *new, size_t h)
 {
        size_t i;
+       uintptr_t perfect = ht->perfect_bit;
 
        i = hash_bucket(ht, h);
 
-       while (entry_is_valid(ht->table[i]))
+       while (entry_is_valid(ht->table[i])) {
+               perfect = 0;
                i = (i + 1) & ((1 << ht->bits)-1);
-
-       ht->table[i] = make_hval(ht, new, get_hash_ptr_bits(ht, h));
+       }
+       ht->table[i] = make_hval(ht, new, get_hash_ptr_bits(ht, h)|perfect);
 }
 
 static COLD_ATTRIBUTE bool double_table(struct htable *ht)
@@ -164,6 +170,7 @@ static COLD_ATTRIBUTE bool double_table(struct htable *ht)
        ht->max *= 2;
        ht->max_with_deleted *= 2;
 
+       /* FIXME: If we lost our perfect bit, we could reclaim it here! */
        for (i = 0; i < oldnum; i++) {
                if (entry_is_valid(e = oldtable[i])) {
                        void *p = get_raw_ptr(ht, e);
@@ -188,9 +195,11 @@ static COLD_ATTRIBUTE void rehash_table(struct htable *ht)
                e = ht->table[h];
                if (!e)
                        continue;
-               ht->table[h] = 0;
-               if (e != HTABLE_DELETED) {
+               if (e == HTABLE_DELETED)
+                       ht->table[h] = 0;
+               else if (!(e & ht->perfect_bit)) {
                        void *p = get_raw_ptr(ht, e);
+                       ht->table[h] = 0;
                        ht_add(ht, p, ht->rehash(p, ht->priv));
                }
        }
@@ -206,6 +215,7 @@ static COLD_ATTRIBUTE void update_common(struct htable *ht, const void *p)
        if (ht->elems == 0) {
                ht->common_mask = -1;
                ht->common_bits = (uintptr_t)p;
+               ht->perfect_bit = 1;
                return;
        }
 
@@ -224,9 +234,10 @@ static COLD_ATTRIBUTE void update_common(struct htable *ht, const void *p)
                ht->table[i] |= bitsdiff;
        }
 
-       /* Take away those bits from our mask and set. */
+       /* Take away those bits from our mask, bits and perfect bit. */
        ht->common_mask &= ~maskdiff;
        ht->common_bits &= ~maskdiff;
+       ht->perfect_bit &= ~maskdiff;
 }
 
 bool htable_add(struct htable *ht, size_t hash, const void *p)