[Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this is another case where control dependences do not work as intended.

Marking useful stmt: foo ();

and we have

  <bb 2> [local count: 3508266]:
  x_4 = foo ();
  if (x_4 != 0)
    goto <bb 12>; [33.00%]
  else
    goto <bb 7>; [67.00%]

  <bb 12> [local count: 1157728]:

  <bb 3> [local count: 116930483]:

  <bb 5> [local count: 116930483]:
  foo ();
  goto <bb 3>; [100.00%]


and BB5 is control dependent on BB3 (on the edge 3->5), so we mark the
block necessary but since there's nothing in it we do not make its
control dependences necessary.

I think what triggers this latent bug is the "double" forwarder with
the call in the latch block rather than in the header when one tries
this on

extern int foo();

void blah()
{
  int x = foo();
  if (x)
    while (1) foo ();
}

it's probably latent with a GIMPLE testcase.