diff --git a/compile.c b/compile.c
index a2efc05438..f2701060d8 100644
--- a/compile.c
+++ b/compile.c
@@ -6198,8 +6198,7 @@ setup_args_core(rb_iseq_t *iseq, LINK_ANCHOR *const args, const NODE *argn,
             argc += 1;
         }
         else {
-            ADD_INSN1(args, argn, splatarray, Qfalse);
-            ADD_INSN(args, argn, concatarray);
+            ADD_INSN(args, argn, concattoarray);
         }
 
         // f(..., *a, ..., k1:1, ...) #=> f(..., *[*a, ...], **{k1:1, ...})
diff --git a/insns.def b/insns.def
index 5f6cb314ff..2b03e3eb47 100644
--- a/insns.def
+++ b/insns.def
@@ -505,7 +505,9 @@ expandarray
     vm_expandarray(GET_CFP(), ary, num, (int)flag);
 }
 
-/* concat two arrays */
+/* concat two arrays, without modifying first array.
+ * attempts to convert both objects to arrays using to_a.
+ */
 DEFINE_INSN
 concatarray
 ()
@@ -516,6 +518,20 @@ concatarray
     ary = vm_concat_array(ary1, ary2);
 }
 
+/* concat second array to first array.
+ * first argument must already be an array.
+ * attempts to convert second object to array using to_a.
+ */
+DEFINE_INSN
+concattoarray
+()
+(VALUE ary1, VALUE ary2)
+(VALUE ary)
+// attr bool leaf = false; /* has rb_check_array_type() */
+{
+    ary = vm_concat_to_array(ary1, ary2);
+}
+
 /* call to_a on array ary to splat */
 DEFINE_INSN
 splatarray
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 97776f7a69..55e79acdd8 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -5204,15 +5204,29 @@ vm_concat_array(VALUE ary1, VALUE ary2st)
     if (NIL_P(tmp1)) {
         tmp1 = rb_ary_new3(1, ary1);
     }
-
-    if (NIL_P(tmp2)) {
-        tmp2 = rb_ary_new3(1, ary2);
-    }
-
     if (tmp1 == ary1) {
         tmp1 = rb_ary_dup(ary1);
     }
-    return rb_ary_concat(tmp1, tmp2);
+
+    if (NIL_P(tmp2)) {
+        return rb_ary_push(tmp1, ary2);
+    } else {
+        return rb_ary_concat(tmp1, tmp2);
+    }
+}
+
+static VALUE
+vm_concat_to_array(VALUE ary1, VALUE ary2st)
+{
+    /* ary1 must be a newly created array */
+    const VALUE ary2 = ary2st;
+    VALUE tmp2 = rb_check_to_array(ary2);
+
+    if (NIL_P(tmp2)) {
+        return rb_ary_push(ary1, ary2);
+    } else {
+        return rb_ary_concat(ary1, tmp2);
+    }
 }
 
 // YJIT implementation is using the C function