From 8e96d6f792ccc840b804a4fd443d0b0064b54752 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Sat, 11 Apr 2026 15:42:19 +0200
Subject: rb_gc_obj_optimal_size: don't enlarge small AR table hashes

Ref: https://github.com/ruby/ruby/pull/16653

Frozen hashes backed by AR tables can be smaller than 160B.
---
 gc.c            |  7 ++++++-
 hash.c          | 16 ----------------
 internal/hash.h | 16 ++++++++++++++++
 3 files changed, 22 insertions(+), 17 deletions(-)

diff --git a/gc.c b/gc.c
index 3cf3bdebce..8a0d051e8f 100644
--- a/gc.c
+++ b/gc.c
@@ -3601,7 +3601,12 @@ rb_gc_obj_optimal_size(VALUE obj)
         }
 
       case T_HASH:
-        return sizeof(struct RHash) + (RHASH_ST_TABLE_P(obj) ? sizeof(st_table) : sizeof(ar_table));
+        {
+            if (RB_OBJ_FROZEN(obj) && RHASH_AR_TABLE_P(obj)) {
+                return sizeof(struct RHash) + offsetof(ar_table, pairs) + RHASH_AR_TABLE_BOUND(obj) * sizeof(ar_table_pair);
+            }
+            return sizeof(struct RHash) + (RHASH_ST_TABLE_P(obj) ? sizeof(st_table) : sizeof(ar_table));
+        }
 
       default:
         return 0;
diff --git a/hash.c b/hash.c
index 751d4564d5..4321f479fd 100644
--- a/hash.c
+++ b/hash.c
@@ -471,24 +471,8 @@ ar_set_entry(VALUE hash, unsigned int index, st_data_t key, st_data_t val, st_ha
 #define RHASH_AR_TABLE_SIZE(h) (HASH_ASSERT(RHASH_AR_TABLE_P(h)), \
                                 RHASH_AR_TABLE_SIZE_RAW(h))
 
-#define RHASH_AR_TABLE_BOUND_RAW(h) \
-  ((unsigned int)((RBASIC(h)->flags >> RHASH_AR_TABLE_BOUND_SHIFT) & \
-                  (RHASH_AR_TABLE_BOUND_MASK >> RHASH_AR_TABLE_BOUND_SHIFT)))
-
-#define RHASH_ST_TABLE_SET(h, s)  rb_hash_st_table_set(h, s)
-#define RHASH_TYPE(hash) (RHASH_AR_TABLE_P(hash) ? &objhash : RHASH_ST_TABLE(hash)->type)
-
 #define HASH_ASSERT(expr) RUBY_ASSERT_MESG_WHEN(HASH_DEBUG, expr, #expr)
 
-static inline unsigned int
-RHASH_AR_TABLE_BOUND(VALUE h)
-{
-    HASH_ASSERT(RHASH_AR_TABLE_P(h));
-    const unsigned int bound = RHASH_AR_TABLE_BOUND_RAW(h);
-    HASH_ASSERT(bound <= RHASH_AR_TABLE_MAX_SIZE);
-    return bound;
-}
-
 #if HASH_DEBUG
 #define hash_verify(hash) hash_verify_(hash, __FILE__, __LINE__)
 
diff --git a/internal/hash.h b/internal/hash.h
index 3a5be14ad7..8c5511e82b 100644
--- a/internal/hash.h
+++ b/internal/hash.h
@@ -193,4 +193,20 @@ RHASH_AR_TABLE_SIZE_RAW(VALUE h)
     return (unsigned)ret;
 }
 
+#define RHASH_AR_TABLE_BOUND_RAW(h) \
+  ((unsigned int)((RBASIC(h)->flags >> RHASH_AR_TABLE_BOUND_SHIFT) & \
+                  (RHASH_AR_TABLE_BOUND_MASK >> RHASH_AR_TABLE_BOUND_SHIFT)))
+
+#define RHASH_ST_TABLE_SET(h, s)  rb_hash_st_table_set(h, s)
+#define RHASH_TYPE(hash) (RHASH_AR_TABLE_P(hash) ? &objhash : RHASH_ST_TABLE(hash)->type)
+
+static inline unsigned int
+RHASH_AR_TABLE_BOUND(VALUE h)
+{
+    RUBY_ASSERT(RHASH_AR_TABLE_P(h));
+    const unsigned int bound = RHASH_AR_TABLE_BOUND_RAW(h);
+    RUBY_ASSERT(bound <= RHASH_AR_TABLE_MAX_SIZE);
+    return bound;
+}
+
 #endif /* INTERNAL_HASH_H */
-- 
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