[Bug tree-optimization/71466] wrong code at -O3 on x86_64-linux-gnu

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71466

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
The problem is caused by loop unroller:

144t.ivcanon looks good:

fn1 ()
{
  _Bool c_lsm.12;
  int c_lsm.11;
  _Bool d_lsm.10;
  int d_lsm.9;
  int f;
  int e;
  int _5;
  int c.3_9;
  int _13;
  int c.3_21;

  <bb 2>:
  c = 0;
  goto <bb 4>;

  <bb 3>:
  # e_4 = PHI <e_20(4), 0(5)>
  # c.3_9 = PHI <c.3_21(4), _5(5)>

  <bb 4>:
  # e_35 = PHI <0(2), e_4(3)>
  # c.3_21 = PHI <0(2), c.3_9(3)>
  e_20 = e_35 + 1;
  if (e_20 != 2)
    goto <bb 3>;
  else
    goto <bb 5>;

  <bb 5>:
  _5 = c.3_21 + 1;
  if (_5 <= 1)
    goto <bb 3>;
  else
    goto <bb 6>;

  <bb 6>:
  # _13 = PHI <_5(5)>
  d = 0;
  c = _13;
  return;

}

While 152t.cunroll contains wrongly unrolled code:

fn1 ()
{
  _Bool c_lsm.12;
  int c_lsm.11;
  _Bool d_lsm.10;
  int d_lsm.9;
  int f;
  int e;
  int _1;
  int _5;
  int c.3_9;
  int _10;
  int c.3_12;
  int _13;
  int c.3_16;

  <bb 2>:
  c = 0;
  e_36 = 1;
  if (e_36 != 2)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  _1 = 1;
  if (_1 <= 1)
    goto <bb 4>;
  else
    goto <bb 9>;

  <bb 4>:
  # e_25 = PHI <e_36(2), 0(3)>
  # c.3_16 = PHI <0(2), _1(3)>
  e_8 = e_25 + 1; /* here:   e_8 == 2 */
  if (e_8 != 2)
    goto <bb 6>;
  else
    goto <bb 5>;

  <bb 5>:
  _10 = c.3_16 + 1;
  if (_10 <= 1)
    goto <bb 6>;
  else
    goto <bb 9>;

  <bb 6>:
  # e_17 = PHI <e_8(4), 0(5)>
  # c.3_12 = PHI <c.3_16(4), _10(5)>
  e_20 = e_17 + 1; /* here:   e_20 == 3 */
  if (e_20 != 2)
    goto <bb 7>;
  else
    goto <bb 8>;

  <bb 7>:
  # e_4 = PHI <e_20(6), 0(8)>
  # c.3_9 = PHI <c.3_12(6), _5(8)>
  __builtin_unreachable ();

  <bb 8>:
  _5 = c.3_12 + 1;
  if (_5 <= 1)
    goto <bb 7>;
  else
    goto <bb 9>;

  <bb 9>:
  # _13 = PHI <_5(8), _1(3), _10(5)>
  d = 0;
  c = _13;
  return;
}

I've been bisecting to understand what has changed since gcc 6.