computeClosure();
  OPT_ValueGraph (OPT_IR ir) {    // TODO!!: compute register lists incrementally    // we need register lists in order to call OPT_Register.getFirstDef()    OPT_DefUse.computeDU(ir);    // add value graph nodes for each symbolic register    addRegisterNodes(ir);    // go through the IR and add nodes and edges to the value graph    // for each instruction, as needed    // each def to the variables it uses    for (Enumeration e = ir.forwardInstrEnumerator(); e.hasMoreElements();) {      OPT_Instruction s = (OPT_Instruction)e.nextElement();      processInstruction(s);    }  }