Shrink dict without rehashing (redis#11540)

zuiderkwast · web-flow · commit abf70309eb42 · 2022-11-25T17:35:18.000-08:00
When we're shrinking the hash table, we don't need to hash the keys. 
Since the table sizes are powers of two, we can simply mask the bucket 
index in the larger table to get the bucket index in the smaller table. We 
avoid loading the keys into memory and save CPU time.
diff --git a/src/dict.c b/src/dict.c
@@ -229,7 +229,14 @@ int dictRehash(dict *d, int n) {
 
             nextde = de->next;
             /* Get the index in the new hash table */
-            h = dictHashKey(d, de->key) & DICTHT_SIZE_MASK(d->ht_size_exp[1]);
+            if (d->ht_size_exp[1] > d->ht_size_exp[0]) {
+                h = dictHashKey(d, de->key) & DICTHT_SIZE_MASK(d->ht_size_exp[1]);
+            } else {
+                /* We're shrinking the table. The tables sizes are powers of
+                 * two, so we simply mask the bucket index in the larger table
+                 * to get the bucket index in the smaller table. */
+                h = d->rehashidx & DICTHT_SIZE_MASK(d->ht_size_exp[1]);
+            }
             de->next = d->ht_table[1][h];
             d->ht_table[1][h] = de;
             d->ht_used[0]--;
