[Bug tree-optimization/40579] [4.3/4.4/4.5 Regression] gcc -O2 optimization causes infinite loop and wrong output

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2009-06-29 09:10 -------
The following loop invokes undefined behavior:

    long x = -2147483645;
    int i;

    for (i = 0; i < 4; ++i) {
        p = itos(x);
        printf("i=%d p=%s\n",i,p);
        --x;
    }

because at iteration with i == 3 x is decremented and underflows.

This can be avoided as follows, which still shows the bug:

extern void abort (void);
static char * __attribute__((noinline))
itos(int num)
{
  return (char *)0;
}
static void __attribute__((noinline))
foo(int i, const char *x)
{
  if (i >= 4)
    abort ();
}
int main()
{
  int x = -__INT_MAX__ + 3;
  int i;

  for (i = 0; i < 4; ++i)
    {
      char *p;
      --x;
      p = itos(x);
      foo(i, p);
    }

  return 0;
}

Fails when VRP and IVOPTs are enabled.  IVOPTs expresses i in terms of x:

<bb 3>:
  # x_16 = PHI <x_5(4), -2147483644(2)>
  D.1693_15 = (unsigned int) x_16;
  D.1694_14 = 2147483652 - D.1693_15;
  i_13 = (int) D.1694_14;
  i_17 = i_13;
  x_5 = x_16 + -1;
  p_6 = itos (x_5);
  foo (i_17, p_6);
  if (x_5 != -2147483648)
    goto <bb 4>;
  else
    goto <bb 5>;

from which VRP concludes that the test is never true:

Value ranges after VRP:

x_5: [-INF, -2147483645]
...

Folding statement: if (x_5 != -2147483648)
Folding predicate x_5 != -2147483648 to 1

huh.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|c                           |tree-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
      Known to fail|                            |4.3.3 4.4.0 4.5.0
      Known to work|                            |4.2.4
   Last reconfirmed|0000-00-00 00:00:00         |2009-06-29 09:10:49
               date|                            |
            Summary|gcc -O2 optimization causes |[4.3/4.4/4.5 Regression] gcc
                   |infinite loop and wrong     |-O2 optimization causes
                   |output                      |infinite loop and wrong
                   |                            |output
   Target Milestone|---                         |4.3.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40579