[Bug tree-optimization/59715] [4.7/4.8/4.9 Regression] wrong code at -Os and above on x86_64-linux-gnu

[vries at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59715

--- Comment #5 from vries at gcc dot gnu.org ---
The test passes with -fno-tree-tail-merge. But I don't see anything wrong with
the transformation (merging of empty blocks) done by the pass. 

AFAICT, the problem is introduced by pass_dominator. test.c.126t.slsr looks ok,
but test.c.127t.dom2 look wrong. The test passes with -fno-tree-dominator-opts.

test.c.126t.slsr:
...
  <bb 2>:
  b.0_4 = b;
  if (b.0_4 == 0)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  goto <bb 7>;

  <bb 4>:
  a.1_5 = a;
  if (a.1_5 != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

  <bb 5>:
  b = a.1_5;
  c_7 = 1 % a.1_5;
  if (c_7 != 0)
    goto <bb 6>;
  else
    goto <bb 3>;

  <bb 6>:
  b = 0;

  <bb 7>:
  b.0_9 = b;
  printf ("%d\n", b.0_9);
  return 0;
...
we're taking the path bb2 -> bb4 -> bb5 (b = 2) -> bb6 (b = 0) -> bb7

test.c.127t.dom2:
...
  <bb 2>:
  b.0_4 = b;
  if (b.0_4 == 0)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  goto <bb 6>;

  <bb 4>:
  a.1_5 = a;
  if (a.1_5 != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

  <bb 5>:
  b = a.1_5;
  c_7 = 1 % a.1_5;
  if (c_7 != 0)
    goto <bb 6>;
  else
    goto <bb 3>;

  <bb 6>:
  b.0_9 = b;
  printf ("%d\n", b.0_9);
  return 0;
...
we're taking the path bb2 -> bb4 -> bb5 (b = 2) -> bb6