htable: use "perfect" bit to reduce rehashes.

Steal one bit to indicate an entry is in its perfect position.  This
means we don't have to rehash it when we are rehashing the entire
table to get rid of deleted records, and we can still use it for
comparison during lookup.

Before: $ ./speed 50000000
Initial insert:  462 ns
Details: hash size 134217728, mask bits 5, perfect 81%
Initial lookup (match):  161 ns
Initial lookup (miss):  168 ns
Initial lookup (random):  326 ns
Initial delete all:  164 ns
Details: rehashes 50000000
Initial re-inserting:  167 ns
Deleting first half:  86 ns
Details: rehashes 25000000, delete markers 25000000
Adding (a different) half:  217 ns
Details: delete markers 0, perfect 81%
Lookup after half-change (match):  169 ns
Lookup after half-change (miss):  180 ns
Details: initial churn
Churning second time:  593 ns
Churning third time:  611 ns
Churning fourth time:  619 ns
Churning fifth time:  622 ns
Details: reinserting with spread
Details: delete markers 13800468, perfect 73%
Details: worst run 48 (4 deleted)
Lookup after churn & spread (match):  192 ns
Lookup after churn & spread (miss):  211 ns
Lookup after churn & spread (random):  373 ns
Deleting half after churn & spread:  103 ns
Adding (a different) half after churn & spread:  102 ns
Details: delete markers 29539689, perfect 72%

After:
Initial insert:  467 ns
Details: hash size 134217728, mask bits 5, perfect 81%
Initial lookup (match):  163 ns
Initial lookup (miss):  171 ns
Initial lookup (random):  326 ns
Initial delete all:  170 ns
Details: rehashes 50000000
Initial re-inserting:  169 ns
Deleting first half:  88 ns
Details: rehashes 25000000, delete markers 25000000
Adding (a different) half:  141 ns
Details: delete markers 0, perfect 81%
Lookup after half-change (match):  166 ns
Lookup after half-change (miss):  171 ns
Details: initial churn
Churning second time:  441 ns
Churning third time:  469 ns
Churning fourth time:  466 ns
Churning fifth time:  490 ns
Details: reinserting with spread
Details: delete markers 13800468, perfect 73%
Details: worst run 48 (4 deleted)
Lookup after churn & spread (match):  197 ns
Lookup after churn & spread (miss):  209 ns
Lookup after churn & spread (random):  369 ns
Deleting half after churn & spread:  98 ns
Adding (a different) half after churn & spread:  100 ns
Details: delete markers 29539689, perfect 72%

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)

ccan/htable/tools/speed.c

@@ -73,8 +73,11 @@ static size_t perfect(const struct htable *ht)
 		if (!entry_is_valid(ht->table[i]))
 			continue;
 		if (hash_bucket(ht, ht->rehash(get_raw_ptr(ht, ht->table[i]),
-					       ht->priv)) == i)
+					       ht->priv)) == i) {
+			assert((ht->table[i] & ht->perfect_bit)
+			       == ht->perfect_bit);
 			placed_perfect++;
+		}
 	}
 	return placed_perfect;
 }