About strict-aliasing warning

[ccg ijsj <ccg dot ijsj at gmail dot com>]

hi all,

    I wrote a simple testcase with some type prunned pointer and
compiled it with -O2 option.
I assumed that gcc would issue some warnings as told in its documents.
But i am afraid that no warnings at all. I have to specify the
-Wstrict-aliasing or -Wall option manually.
    Is this a document inconsistency or a little bug, or what else I
should pay attention to before doing the test?
    I have tried differenct version of gcc in Ubuntu 8.04/8.10, none
warnings for gcc-3.3/3.4/4.2/4.3 .

    In GCC's documents(http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html)

"-Wstrict-aliasing
    This option is only active when -fstrict-aliasing is active. It
warns about code which might
break the strict aliasing rules that the compiler is using for
optimization. The warning does
not catch all cases, but does attempt to catch the more common
pitfalls. It is included in -Wall.
It is equivalent to -Wstrict-aliasing=3 "

and -O2 would active -fstrict-aliasing by default, which should also
active this options.


BTW: the simple testcase:

  1 #include <stdio.h>
  2
  3 int main()
  4 {
  5         int a = 0x12345678;
  6         short *p = (short *)&a;
  7         short temp;
  8         temp = *p;
  9         *p = *(p+1);
 10         *(p+1) = temp;
 11         printf("%x\n", a);
 12 }

Thanks.


Best,
Jim