[Bug tree-optimization/56576] New: wrong code for aliased union at -O3

[dhazeghi at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56576



             Bug #: 56576

           Summary: wrong code for aliased union at -O3

    Classification: Unclassified

           Product: gcc

           Version: 4.8.0

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: tree-optimization

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: dhaze...@yahoo.com





The following code behaves differently at -O3 than at -O2 or below on 4.7 and

mainline 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 as expected on gcc 4.6 at all optimization levels (returns 0).



$ gcc-trunk --version

gcc-trunk (GCC) 4.8.0 20130308 (experimental) [trunk revision 196555]

$ gcc-trunk -O2 trans-reduced.c 

$ ./a.out 

$ echo $?

0

$ gcc-trunk -O3 trans-reduced.c 

$ ./a.out 

$ echo $?

1

$ gcc-4.6 -O3 trans-reduced.c 

$ ./a.out 

$ echo $?

0

$ cat trans-reduced.c 



/* gcc-4.7/gcc-trunk -O3 -m32/-m64 */

union

{

    int f0;

    int f1;

    long f2;

}

a, b;

int c, h;

int *d, *e = &a.f0;

long *f = &b.f2;

int **g = &d;

void fn1 ()

{

    c = 0;

    for (; c <= 3; c++)

    {

        int *i = &b.f1;

        *f = 1;

        *i = 0;

        *g = 0;

        h = *e;

    }

}



int main ()

{

    fn1 ();

    return b.f0;

}