等价的指令格式定义

class I<bits<6> opcode, dag OOL, dag IOL, string asmstr, list<dag> pattern>
        : Instruction {
  field bits<32> Inst;
  let Inst{5-0}    = opcode;
}

class USE_I<bits<6> opcode, dag OOL, dag IOL, string asmstr, list<dag> pattern> 
       : I <opcode, OOL, IOL, asmstr> {

}

def RET<0x01, (outs), (ins), "nop", []>

class I<dag OOL, dag IOL, string asmstr, list<dag> pattern>
        : Instruction {
  field bits<32> Inst;

  // 使用临时变量Opcode
  bits<6> Opcode = 0;        
  let Inst{5-0}  = Opcode;
}

class USE_I<bits<6> opcode, dag OOL, dag IOL, string asmstr, list<dag> pattern> 
       : I <opcode, OOL, IOL, asmstr> {
  let Opcode = opcode;
}

def RET<0x01, (outs), (ins), "nop", []>

def RET<0x01, (outs), (ins), "nop", []>