Baffling USE in x86_64.ad

[Andrew Haley]

In this pattern:

instruct compI_rReg(rFlagsReg cr, rRegI op1, rRegI op2)
%{
  match(Set cr (CmpI op1 op2));
  effect(DEF cr, USE op1, USE op2);

  format %{ "cmpl    $op1, $op2" %}
  opcode(0x3B);  /* Opcode 3B /r */
  ins_encode(REX_reg_reg(op1, op2), OpcP, reg_reg(op1, op2));
  ins_pipe(ialu_cr_reg_reg);
%}

why does the USE appear in the effect?  And the DEF?  The operands
appear in the match expression in the normal way, so I would have
thought the effect expression unnecessary.  It's this pattern: others
don't have the effect:

instruct compL_rReg(rFlagsReg cr, rRegL op1, rRegL op2)
%{
  match(Set cr (CmpL op1 op2));

  format %{ "cmpq    $op1, $op2" %}
  opcode(0x3B);  /* Opcode 3B /r */
  ins_encode(REX_reg_reg_wide(op1, op2), OpcP, reg_reg(op1, op2));
  ins_pipe(ialu_cr_reg_reg);
%}

Thanks,

Andrew.