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: [email protected]
ReportedBy: [email protected]
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;
}
