[Bug middle-end/104067] [12 Regression] wrong code compiling QEMU since r12-4790-g4b3a325f07acebf4

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104067

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
extern void abort ();
struct S { int x; } a[10];
struct S *b;

int
main ()
{
  int i, j = 0;
  struct S *q = a;

  for (i = 100; --i > 0; )
    {
      struct S *p;
      j++;
      if (j >= 10)
        j = 0;
      p = &a[j];

      if (p == q)
        abort ();
      __atomic_thread_fence (__ATOMIC_SEQ_CST);
      q = p;
    }
  return 0;
}

This goes wrong in threadfull2, strlen1 looks correct to me:
  <bb 2> [local count: 10737416]:

  <bb 3> [local count: 1063004409]:
  # j_17 = PHI <j_2(9), 0(2)>
  # q_18 = PHI <prephitmp_16(9), &a(2)>
  # ivtmp_4 = PHI <ivtmp_3(9), 99(2)>
  j_8 = j_17 + 1;
  if (j_8 == 10)
    goto <bb 5>; [34.00%]
  else
    goto <bb 4>; [66.00%]

  <bb 4> [local count: 701582906]:
  _1 = (sizetype) j_8;
  _15 = _1 * 4;
  _14 = &a + _15;

  <bb 5> [local count: 1063004409]:
  # j_2 = PHI <j_8(4), 0(3)>
  # prephitmp_16 = PHI <_14(4), &a(3)>
  if (prephitmp_16 == q_18)
    goto <bb 6>; [0.00%]
  else
    goto <bb 7>; [100.00%]

  <bb 6> [count: 0]:
  abort ();

  <bb 7> [local count: 1063004409]:
  __atomic_thread_fence (5);
  ivtmp_3 = ivtmp_4 - 1;
  if (ivtmp_3 != 0)
    goto <bb 9>; [99.00%]
  else
    goto <bb 8>; [1.00%]

  <bb 9> [local count: 1052374367]:
  goto <bb 3>; [100.00%]

  <bb 8> [local count: 10737416]:
  return 0;

but threadfull2 turns that into something that weirdly starts with:
  <bb 2> [local count: 10737416]:
  j_8 = 1;
  if (j_8 == 10)
    goto <bb 4>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 701582906]:
  # ivtmp_22 = PHI <99(2), ivtmp_12(7)>
  # j_24 = PHI <j_8(2), j_20(7)>
  # q_25 = PHI <&a(2), q_7(7)>
  _1 = (sizetype) j_24;
  _15 = _1 * 4;
  _14 = &a + _15;

  <bb 4> [local count: 705197120]:
  # j_2 = PHI <j_24(3), 0(2)>
  # prephitmp_16 = PHI <_14(3), &a(2)>
  # ivtmp_23 = PHI <ivtmp_22(3), 99(2)>
  # q_26 = PHI <q_25(3), &a(2)>
  if (prephitmp_16 == q_26)
    goto <bb 5>; [0.00%]
  else
    goto <bb 6>; [100.00%]

  <bb 5> [count: 0]:
  abort ();

and for j == 10 turns it into unconditional abort:
  j_20 = j_21 + 1;
  if (j_20 == 10)
    goto <bb 8>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 8> [local count: 357807289]:
  # j_5 = PHI <0(7)>
  # prephitmp_9 = PHI <&a(7)>
  goto <bb 5>; [100.00%]