[Bug tree-optimization/83298] [8 Regression] wrong code at -O1, -O2 and -O3 on x86_64-linux-gnu

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83298

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This goes wrong during dom2, before that we have:
  <bb 2> [local count: 161061274]:
  b.1_11 = b;
  if (b.1_11 <= 0)
    goto <bb 7>; [85.00%]
  else
    goto <bb 4>; [15.00%]

  <bb 7> [local count: 136902083]:

  <bb 3> [local count: 912680551]:
  # b.1_12 = PHI <b.1_11(7), b.1_2(8)>
  # RANGE [-2147483647, 1]
  _1 = b.1_12 + 1;
  b = _1;
  b.1_2 = b;
  if (b.1_2 <= 0)
    goto <bb 8>; [85.00%]
  else
    goto <bb 4>; [15.00%]

  <bb 8> [local count: 775778470]:
  goto <bb 3>; [100.00%]

  <bb 4> [local count: 161061274]:
  a.2_3 = a;
  c.3_4 = c;
  _5 = a.2_3 <= 0 ? c.3_4 : 0;
  if (_5 == 0)
    goto <bb 5>; [0.00%]
  else
    goto <bb 6>; [99.96%]

  <bb 5> [count: 0]:
  # USE = nonlocal null
  # CLB = nonlocal null
  __builtin_abort ();

  <bb 6> [local count: 160996849]:
  return 0;

and so all paths from ENTRY go through bb 4.  dom2 decides to thread this:
  <bb 2> [local count: 161061274]:
  b.1_11 = b;
  if (b.1_11 <= 0)
    goto <bb 3>; [85.00%]
  else
    goto <bb 4>; [15.00%]

  <bb 3> [local count: 912680551]:
  # b.1_12 = PHI <b.1_11(2), _1(3)>
  # RANGE [-2147483647, 1]
  _1 = b.1_12 + 1;
  b = _1;
  b.1_2 = _1;
  if (_1 <= 0)
    goto <bb 3>; [85.00%]
  else
    goto <bb 7>; [15.00%]

  <bb 4> [local count: 161061274]:
  a.2_3 = a;
  c.3_4 = c;
  _5 = a.2_3 <= 0 ? c.3_4 : 0;
  if (_5 == 0)
    goto <bb 5>; [0.00%]
  else
    goto <bb 6>; [99.96%]

  <bb 5> [count: 0]:
  # USE = nonlocal null
  # CLB = nonlocal null
  __builtin_abort ();

  <bb 6> [local count: 160996849]:
  return 0;

  <bb 7> [count: 0]:
  a.2_10 = a;
  c.3_15 = c;
  _16 = a.2_10 <= 0 ? c.3_15 : 0;
  goto <bb 5>; [0.00%]

which is wrong, the ranges of b*/_1 are in no way related to the ranges of
a*/c*, so _16 isn't guaranteed to be 0.

Jeff, can you please have a look?