Re: [suse-programming-e] SuSE-9.0 gcc-3.3.1 optimization turns off important warning

31 Dec 2003

      On Wed, Dec 31, 2003 at 09:22:36AM -0500, Mark Gray wrote:
...
Marcus Meissner  writes:
...
...
But why is there no warning when optimization is turned on?  (When the
warning is actually needed :-)  This is my main point.
It is there with -Wall on SuSE Linux 9.0:
$ cat xx.c 
void f(char *s) {
        while (*s) {
      *s++ = *s | 0x20;
  }
}
$ gcc -O2 -Wall -c xx.c 
xx.c: In function `f':
xx.c:3: Warnung: operation on `s' may be undefined
It is the toupper that turns it off then:
markgray@soyo:/usr/src/packages/BUILD/bug> cat xx.c
#include 
void strupr_OLD (char *s)
{
    while (*s) {
        *s++ = toupper(*s);
    }
}
markgray@soyo:/usr/src/packages/BUILD/bug> gcc -O2 -Wall -c xx.c
markgray@soyo:/usr/src/packages/BUILD/bug>
(This is also 9.0 btw)
Hmm. Probably because *s++ = toupper(*s); is translated by a macro to:

        *s++ = (__extension__ ({ int __res; if (sizeof (*s) > 1) { if (__builtin_constant_p (*s)) { int __c = (*s); __res = __c < -128 || __c > 255 ? __c : (*__ctype_toupper_loc ())[__c]; } else __res = toupper (*s); } else __res = (*__ctype_toupper_loc ())[(int) (*s)]; __res; })); 

Might be to hard for the compiler to spot the problem.

Ciao, Marcus