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[Bug tree-optimization/20725] New: [tcb] FRE does not remove a fully redundant load.


Consider:

struct s {
  int *n;
};

int
foo (unsigned int i, struct s *array)
{
  int *p = array[i].n;
  if (p)
    {
      int *q = array[i].n;
      if (p != q)
	return 1;
    }
  return 0;
}

Note that the second load is unnecessary, but FRE does not remove it.

Here is the dump before FRE.

foo (i, array)
{
  int * q;
  int * p;
  int D.1142;
  struct s * D.1141;
  struct s * D.1140;
  unsigned int D.1139;

<bb 0>:
  D.1139_3 = i_2 * 4;
  D.1140_4 = (struct s *) D.1139_3;
  D.1141_6 = D.1140_4 + array_5;
  #   VUSE <TMT.0_15>;
  p_7 = D.1141_6->n;
  if (p_7 != 0B) goto <L0>; else goto <L2>;

<L0>:;
  D.1139_10 = i_2 * 4;
  D.1140_11 = (struct s *) D.1139_10;
  D.1141_12 = array_5 + D.1140_11;
  #   VUSE <TMT.0_15>;
  q_13 = D.1141_12->n;
  if (p_7 != q_13) goto <L1>; else goto <L2>;

<L1>:;
  D.1142_14 = 1;
  goto <bb 4> (<L4>);

<L2>:;
  D.1142_9 = 0;

  # D.1142_1 = PHI <1(2), 0(3)>;
<L4>:;
  return D.1142_1;

}

After FRE:

foo (i, array)
{
  int * q;
  int * p;
  int D.1142;
  struct s * D.1141;
  struct s * D.1140;
  unsigned int D.1139;

<bb 0>:
  D.1139_3 = i_2 * 4;
  D.1140_4 = (struct s *) D.1139_3;
  D.1141_6 = D.1140_4 + array_5;
  #   VUSE <TMT.0_15>;
  p_7 = D.1141_6->n;
  if (p_7 != 0B) goto <L0>; else goto <L2>;

<L0>:;
  D.1139_10 = D.1139_3;
  D.1140_11 = D.1140_4;
  D.1141_12 = D.1141_6;
  #   VUSE <TMT.0_15>;
  q_13 = D.1141_12->n;
  if (p_7 != q_13) goto <L1>; else goto <L2>;

<L1>:;
  D.1142_14 = 1;
  goto <bb 4> (<L4>);

<L2>:;
  D.1142_9 = 0;

  # D.1142_1 = PHI <1(2), 0(3)>;
<L4>:;
  return D.1142_1;

}

-- 
           Summary: [tcb] FRE does not remove a fully redundant load.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20725


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