From abf70309eb42674982ea51ed41ff5b39b04fe701 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Viktor=20S=C3=B6derqvist?= <viktor.soderqvist@est.tech>
Date: Sat, 26 Nov 2022 02:35:18 +0100
Subject: [PATCH] Shrink dict without rehashing (#11540)

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.
---
 src/dict.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/dict.c b/src/dict.c
index 9975e7872f..9ed461d623 100644
--- 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]--;