compare_by_identity: remove alloc for empty Hash

For non-empty Hashes, this function needs to rehash all the stored values (using the new `compare` and `hash` functions from `identhash`). It does so by writing into a newly allocated `tmp` Hash, and then transferring ownership of its st table into `self`.

For empty Hashes, we can skip allocating this `tmp`, because there's nothing to re-hash. We can just modify our new st table's `type` in-place.

1 files changed, 14 insertions, 7 deletions

diff --git a/hash.c b/hash.c
index cf83675c70..f6525ba4a5 100644
--- a/hash.c
+++ b/hash.c
@@ -4380,15 +4380,22 @@ rb_hash_compare_by_id(VALUE hash)
     ar_force_convert_table(hash, __FILE__, __LINE__);
     HASH_ASSERT(RHASH_ST_TABLE_P(hash));
 
-    tmp = hash_alloc(0);
-    hash_st_table_init(tmp, &identhash, RHASH_SIZE(hash));
-    identtable = RHASH_ST_TABLE(tmp);
+    if (RHASH_TABLE_EMPTY_P(hash)) {
+        // Fast path: There's nothing to rehash, so we don't need a `tmp` table.
+        RHASH_ST_TABLE(hash)->type = &identhash;
+    } else {
+        // Slow path: Need to rehash the members of `self` into a new
+        // `tmp` table using the new `identhash` compare/hash functions.
+        tmp = hash_alloc(0);
+        hash_st_table_init(tmp, &identhash, RHASH_SIZE(hash));
+        identtable = RHASH_ST_TABLE(tmp);
 
-    rb_hash_foreach(hash, rb_hash_rehash_i, (VALUE)tmp);
+        rb_hash_foreach(hash, rb_hash_rehash_i, (VALUE)tmp);
 
-    rb_hash_free(hash);
-    RHASH_ST_TABLE_SET(hash, identtable);
-    RHASH_ST_CLEAR(tmp);
+        rb_hash_free(hash);
+        RHASH_ST_TABLE_SET(hash, identtable);
+        RHASH_ST_CLEAR(tmp);
+    }
 
     return hash;
 }