From 15b993f1ef6ec436c1c2f411232b27d150a6c168 Mon Sep 17 00:00:00 2001
From: Yanqi Lv <lvyanqi.lyq@alibaba-inc.com>
Date: Fri, 8 Dec 2023 17:48:52 +0800
Subject: [PATCH] Optimize dictExpand of empty dict (#12847)

If a dict is empty before `dictExpand`, we don't need to do rehashing
actually. We can just create a new dict and set it as ht_table[0] to
skip incremental rehashing and free the memory quickly.
---
 src/dict.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/dict.c b/src/dict.c
index e88a843cc..541b2d066 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -273,6 +273,17 @@ int _dictExpand(dict *d, unsigned long size, int* malloc_failed)
     d->ht_table[1] = new_ht_table;
     d->rehashidx = 0;
     if (d->type->rehashingStarted) d->type->rehashingStarted(d);
+    /* If the dict is empty, we finish rehashing ASAP.*/
+    if (d->ht_used[0] == 0) {
+        if (d->type->rehashingCompleted) d->type->rehashingCompleted(d);
+        zfree(d->ht_table[0]);
+        d->ht_size_exp[0] = new_ht_size_exp;
+        d->ht_used[0] = new_ht_used;
+        d->ht_table[0] = new_ht_table;
+        _dictReset(d, 1);
+        d->rehashidx = -1;
+        return DICT_OK;
+    }
     return DICT_OK;
 }