OPT_PhysicalRegisterSet phys = ir.regpool.getPhysicalRegisterSet();
    void assignDepthFirstNumbers(OPT_ControlFlowGraph cfg) {      int curDfn = ir.numberInstructions() - 1;      OPT_PhysicalRegisterSet phys = ir.regpool.getPhysicalRegisterSet();      listOfBlocks = null;      for (OPT_BasicBlock bb = reverseTopFirst;            bb != null;           bb = (OPT_BasicBlock)bb.sortedPrev) {        // insert bb at the front of the list        bb.nextSorted = listOfBlocks;        listOfBlocks = bb;        // number the instructions last to first        for (OPT_Instruction inst = bb.lastInstruction();              inst != null;             inst = inst.getPrev()) {          setDFN(inst, curDfn);          curDfn--;        }      }      if (debug) {  printDfns(ir);  }    }