strict aliasing can not be turned off

["Liu, Lei" <lei.liu2@xxxxxxxxxxxxx>]

Hi All,

Here is my program.

#include <stdio.h>

struct s1 {
  unsigned int a,b,c;
};
struct s2 {
  unsigned int b,c;
};

int main(void)
{
  struct s1 s;
  s.c =  0xdead;
  struct s2 *p = (struct s2*)&s.b;
  p->c = 0xbeef;
  printf("s.c=%04x\n", s.c);
  return 0;
}

I compile this code with '-O2' flag and get a 's.c=dead'. It's 
reasonable under strict aliasing rule. But when I recompile it with '-O2 
-fno-strict-aliasing', the output is still 's.c=dead'. It seems that 
'-fno-strict-aliasing' does not take effect on the code. The compiler 
generated code are the same:

 8048401:    c7 44 24 08 ad de 00     movl   $0xdead,0x8(%esp)
 8048408:    00
 8048409:    c7 44 24 04 f0 84 04     movl   $0x80484f0,0x4(%esp)
 8048410:    08
 8048411:    c7 04 24 01 00 00 00     movl   $0x1,(%esp)
 8048418:    e8 eb fe ff ff           call   8048308 <__printf_chk@plt>

How can I disable strict aliasing when -O2 is enabled and get the 
expected result 's.c=beef'.

Thanks.
Lei