[Bug tree-optimization/19431] missed optimization with ifs and deferencing

["rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #15 from rguenth at gcc dot gnu dot org  2007-02-25 22:50 -------
We start with

  D.59982_3 = variable_2(D) + 4B;
  __unary_op = __unary_op.65;
  goto <bb 4> (<L1>);

<L0>:;
  D.61146_10 = __unary_op._M_arg1;
  D.61147_11 = __unary_op._M_arg2;
  D.61093_12 = D.61146_10 + D.61147_11;
  *variable_13 = D.61093_12;
  variable_14 = variable_7 + 4B;
  variable_15 = variable_13 + 4B;

  # variable_7 = PHI <variable_2(D)(2), variable_14(3)>
  # variable_13 = PHI <variable_2(D)(2), variable_15(3)>
<L1>:;
  if (D.59982_3 != variable_7) goto <L0>; else goto <L2>;

and loop header copying and DOM make it optimizable:

  D.59982_3 = variable_2(D) + 4B;
  __unary_op.65._M_arg2 = v_1(D);
  __unary_op.65._M_arg1 = 0;
  __unary_op = __unary_op.65;
  __unary_op$_M_arg2_54 = __unary_op._M_arg2;
  __unary_op$_M_arg1_53 = __unary_op._M_arg1;
  if (variable_2(D) != D.59982_3) goto <L7>; else goto <L2>;

<L2>:;
  return;

  # variable_49 = PHI <variable_2(D)(2)>
  # variable_27 = PHI <variable_2(D)(2)>
<L7>:;
  __unary_op$_M_arg1_25 = __unary_op$_M_arg1_53;
  __unary_op$_M_arg2_17 = __unary_op$_M_arg2_54;
  D.61093_18 = __unary_op$_M_arg1_53 + __unary_op$_M_arg2_54;
  *variable_49 = D.61093_18;
  variable_20 = variable_27 + 4B;
  variable_21 = variable_49 + 4B;
  goto <bb 3> (<L2>);

so a tweaked forwprop fixes it at least before PRE.  But dunno if we want
to do tree combining on conditions here.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19431