[Bug tree-optimization/99101] optimization bug with -ffinite-loops

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99101

--- Comment #20 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Michael Matz from comment #19)
> (In reply to Michael Matz from comment #18)
> > But there are other blocks control dependend on bb4, namely bb5 and bb9.
> > To see this observe that bb9 doesn't pdom bb4, but there's a path from bb4 to
> > bb9 (e.g. bb4 -> bb5 -> bb9), where one node in that path that isn't bb4 is
> 
> ... where _each_ node in that path that isn't bb4 ...  Sorry.
> 
> (Essentially this captures the idea that the path-start node that is the
> control
> node is the _latest_ node that determines (non)execution of the path-end,
> i.e. all
> further nodes after the control node unavoidably pass through path-end).

Yes, bb5 and bb9 are control dependent on bb4.

But nothing in bb5 or bb9 ends up necessary (so the IV 'i' is elminated as
well).
We start with obviously necessary stmts

Marking useful stmt: __builtin_puts (&"1"[0]);

Marking useful stmt: xx.0_1 ={v} xx;

Marking useful stmt: __builtin_abort ();

And the only thing we deem necessary is

Marking useful stmt: if (xx.0_1 != 0)

because we mark this due to the self control-dependence of bb6 to itself
when processing the necessary call.  If we didn't do that we'd end up
optimizing
the testcase to just

int main()
{
  __builtin_puts ("1");
  xx;
  abort ();
}

hmm, actually when removing the if (xx) stmt we end up redirecting the
incoming edge to the loop latch and so we'd produce

  <bb 3> :
  __builtin_puts (&"1"[0]);
  xx.0_1 ={v} xx;
  goto <bb 3>; [100.00%]

because we're using yet another idea of post-dominance, that by
inverted_post_order_compute and it's kludge how to handle backwards
unreachable blocks ... (it produces 7, 5, 9, 0, 2, 12, 3, 11, 4, 6, 1)
At one point the code used to walk immediate post-dominators.  This
was changed for PR65337 in g:e4dbb0d449e778bc810d0d627a5aaefd0d7847b1