struct.c: cache member definition in a subclass

Since getting Qnil is already error, it is safe to use rb_attr_get.

* struct.c (struct_ivar_get): cache member definition in a subclass
  Thanks to Sokolov Yura aka funny_falcon <funny.falcon@gmail.com>
  in https://bugs.ruby-lang.org/issues/10585

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50999 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,9 @@
+Tue Jun 23 06:37:10 2015  Eric Wong  <e@80x24.org>
+
+	* struct.c (struct_ivar_get): cache member definition in a subclass
+	  Thanks to Sokolov Yura aka funny_falcon <funny.falcon@gmail.com>
+	  in https://bugs.ruby-lang.org/issues/10585
+
 Tue Jun 23 04:58:06 2015  Eric Wong  <e@80x24.org>
 
 	* benchmark/bm_vm2_struct_big_href_hi.rb: new benchmark

struct.c

@@ -23,12 +23,20 @@ static VALUE struct_alloc(VALUE);
 static inline VALUE
 struct_ivar_get(VALUE c, ID id)
 {
+    VALUE orig = c;
+    VALUE ivar = rb_attr_get(c, id);
+
+    if (!NIL_P(ivar))
+	return ivar;
+
     for (;;) {
-	if (rb_ivar_defined(c, id))
-	    return rb_ivar_get(c, id);
 	c = RCLASS_SUPER(c);
 	if (c == 0 || c == rb_cStruct)
 	    return Qnil;
+	ivar = rb_attr_get(c, id);
+	if (!NIL_P(ivar)) {
+	    return rb_ivar_set(orig, id, ivar);
+	}
     }
 }