diff --git a/ChangeLog b/ChangeLog
index f9dfdee2b1..e6076b3e61 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Jul 26 18:21:58 2013  Koichi Sasada  <ko1@atdot.net>
+
+	* array.c (ary_memcpy): try to enable optimization.
+	  At least on my environments, I don't see any errors
+	  with many trials. Please tell us if you find any GC bugs.
+
 Fri Jul 26 17:49:26 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* win32/file.c (fix_string_encoding): fix target encoding.  the
diff --git a/array.c b/array.c
index 9457851c10..8ab71289a2 100644
--- a/array.c
+++ b/array.c
@@ -69,8 +69,7 @@ ary_memcpy(VALUE ary, long beg, long argc, const VALUE *argv)
 {
 #if 1
     if (OBJ_PROMOTED(ary)) {
-	if (0 /* disalbe now, because it cause mysterious error. */
-	    && argc > 32 /* 32 is magic number */) {
+	if (argc > (128/sizeof(VALUE)) /* is magic number (cache line size) */) {
 	    rb_gc_writebarrier_remember_promoted(ary);
 	    RARRAY_PTR_USE(ary, ptr, {
 		MEMCPY(ptr+beg, argv, VALUE, argc);