Make `ary_replace()` to share entry buffers if possible.

src/array.c

@@ -349,12 +349,39 @@ mrb_ary_plus(mrb_state *mrb, mrb_value self)
   return mrb_obj_value(a2);
 }
 
+#define ARY_REPLACE_SHARED_MIN 20
+
 static void
 ary_replace(mrb_state *mrb, struct RArray *a, struct RArray *b)
 {
   mrb_int len = ARY_LEN(b);
 
-  ary_modify(mrb, a);
+  ary_modify_check(mrb, a);
+  if (a == b) return;
+  if (ARY_SHARED_P(a)) {
+    mrb_ary_decref(mrb, a->as.heap.aux.shared);
+    a->as.heap.aux.capa = 0;
+    a->as.heap.len = 0;
+    a->as.heap.ptr = NULL;
+    ARY_UNSET_SHARED_FLAG(a);
+  }
+  if (ARY_SHARED_P(b)) {
+  shared_b:
+    if (ARY_EMBED_P(a)) {
+      ARY_UNSET_EMBED_FLAG(a);
+    }
+    a->as.heap.ptr = b->as.heap.ptr;
+    a->as.heap.len = len;
+    a->as.heap.aux.shared = b->as.heap.aux.shared;
+    a->as.heap.aux.shared->refcnt++;
+    ARY_SET_SHARED_FLAG(a);
+    mrb_write_barrier(mrb, (struct RBasic*)a);
+    return;
+  }
+  if (!MRB_FROZEN_P(b) && len > ARY_REPLACE_SHARED_MIN) {
+    ary_make_shared(mrb, b);
+    goto shared_b;
+  }
   if (ARY_CAPA(a) < len)
     ary_expand_capa(mrb, a, len);
   array_copy(ARY_PTR(a), ARY_PTR(b), len);