[Bug tree-optimization/56577] New: wrong code for aliased union on gcc 4.7 only

["dhazeghi at yahoo dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56577

             Bug #: 56577
           Summary: wrong code for aliased union on gcc 4.7 only
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dhazeghi@yahoo.com


The following code behaves differently at -O3 than at -O2 or below on 4.7 on
x86_64-linux.  At -O3 it returns 1, whereas at -O2 and below it
returns 0, for both -m32 and -m64 targets.

It behaves consistently on gcc 4.6 at all optimization levels (returns 0).  It
may be related to 56576 (derived from a similar testcase) but that bug is also
present on mainline.



$ gcc-4.7 --version
gcc-4.7 (GCC) 4.7.2
$ gcc-4.7 -O2 orig-reduced.c 
$ ./a.out 
$ echo $?
0
$ gcc-4.7 -O3 orig-reduced.c 
$ ./a.out 
$ echo $?
1
$ gcc-4.6 -O3 orig-reduced.c 
$ ./a.out 
$ echo $?
0
$ cat orig-reduced.c 

/* gcc-4.7 -O3 -m32/-m64 (works with trunk or at -O2) */
union
{
    int f0;
    int f1;
    long f2;
}
b, *g, **i = &g;
int a, m;
int *c = (int *)&b.f2, *e, *f = &a;
int d;
int h;
long **j = (long **)&c;
int k;
int **l = &e;
void fn1 ()
{
    d = 0;
    for (; d <= 3; d++)
    {
        k = 0;
        for (; k <= 3; k++)
        {
            int *n = &b.f1;
            *i = 0;
            **j = 1;
            *n = 0;
            *l = 0;
        }
        m = *f;
        a = 1;
        for (; a >= 0; a--)
            h &= 1;
    }
}
int main ()
{
    fn1 ();
    return b.f0;
}