From 3ef8d833ab6e803cdff714ee454d7a4d47ee1c47 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Fri, 1 Aug 2025 18:17:33 +0200
Subject: [PATCH] rb_gc_impl_mark_and_move: avoid needless writes

Assuming not all objects are moved during compaction, it
is preferable to avoid rewriting references that haven't moved
as to avoid invalidating potentially shared memory pages.
---
 gc/default/default.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/gc/default/default.c b/gc/default/default.c
index 47cfe3fb3b..9038a01e4e 100644
--- a/gc/default/default.c
+++ b/gc/default/default.c
@@ -4420,7 +4420,10 @@ rb_gc_impl_mark_and_move(void *objspace_ptr, VALUE *ptr)
         GC_ASSERT(objspace->flags.during_compacting);
         GC_ASSERT(during_gc);
 
-        *ptr = rb_gc_impl_location(objspace, *ptr);
+        VALUE destination = rb_gc_impl_location(objspace, *ptr);
+        if (destination != *ptr) {
+            *ptr = destination;
+        }
     }
     else {
         gc_mark(objspace, *ptr);