Create the helper node only for the specialization needing it

Author: eregon

src/main/java/org/truffleruby/stdlib/ObjSpaceNodes.java

@@ -10,6 +10,7 @@
 package org.truffleruby.stdlib;
 
 import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
+import com.oracle.truffle.api.dsl.Cached;
 import com.oracle.truffle.api.dsl.Specialization;
 import com.oracle.truffle.api.object.DynamicObject;
 import org.truffleruby.Layouts;
@@ -32,8 +33,6 @@ public abstract class ObjSpaceNodes {
     @CoreMethod(names = "memsize_of", isModuleFunction = true, required = 1)
     public abstract static class MemsizeOfNode extends CoreMethodArrayArgumentsNode {
 
-        @Child private ValuesNode matchDataValues = ValuesNode.create();
-
         @Specialization(guards = "isNil(object)")
         public int memsizeOfNil(DynamicObject object) {
             return 0;
@@ -55,7 +54,8 @@ public int memsizeOfString(DynamicObject object) {
         }
 
         @Specialization(guards = "isRubyMatchData(object)")
-        public int memsizeOfMatchData(DynamicObject object) {
+        public int memsizeOfMatchData(DynamicObject object,
+                                      @Cached("create()") ValuesNode matchDataValues) {
             return memsizeOfObject(object) + matchDataValues.execute(object).length;
         }