strict aliasing can not be turned off

[Liu, Lei]

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