Re: inapropriate 'comparison is always false due to limited range of data type' warning

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Sep 29, 2006 at 10:55:27AM -0400, Rodolfo Hansen wrote:
> v857829@vdsdr-125 ~ $ gcc -O0 test.c -o test
> test.c: In function 'main':
> test.c:5: warning: comparison is always false due to limited range of data 
> type
> 
> 
> does not make sense to me in the following code:
> 
> 
> 
> #include <stdio.h>
> int main (char *argv[], int argc) {
>  unsigned short int number;
> 
>  for (number=0; number < ~(number&(~number)) ; number++) {
>    printf("hello");
>  }
> 
>  return 0;
> }

This question is appropriate for gcc-help mailing list, not here.

> i believe gcc is misinterpreting  ~(number&(~number))
> 
> any coments, help welcome.

No, please read the C promotion rules.  number&(~number) is always 0,
~0 on two's complement arches is -1 (signed int).
number < ~(number&(~number)) comparison is done in promoted mode - int,
so is (int) number < ~0 which is the same as (int) number < -1.  As
no value of unsigned short number promoted to int is negative (unless
sizeof (int) == sizeof (short)), the compiler correctly warns that
the condition is always false.

	Jakub