[Bug c/59301] New: Please enable -Wstrict-overflow as part of -Wextra

[j at uriah dot heep.sax.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59301

            Bug ID: 59301
           Summary: Please enable -Wstrict-overflow as part of -Wextra
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: j at uriah dot heep.sax.de

The -fstrict-overflow behaviour can lead to surprising results. Consider 
the following code that came up in a forum, complaining about why GCC 
optimizes the first loop into an endless one:

int main (void)
{
    int i = 0;
    while (--i)
        asm("nop");

    for (;;);
}

The (obvious, in that short piece of code) expectation of the programmer 
was that the NOP is executed a finite number of times (basically, just 
"waiting a bit" that way), and the code flow then proceeds to the final 
infinite loop.

Instead, the resulting code is an infinite loop around the NOP statement.

(The original question came out for the AVR target, but the behaviour is 
completely independent of the actual target.)

Specifying the commonly used -Wall -Wextra options doesn't tell the innocent 
programmer the compiler basically already detected some undefined behaviour, 
and might have reordered the code due to that undefined behaviour.  Only 
by specifying -Wstrict-overflow, one gets:

foo.c: In function 'main':
foo.c:4:11: warning: assuming signed overflow does not occur when simplifying
conditional to constant [-Wstrict-overflow]

I think it would be much better to include -Wstrict-overflow into -Wextra, 
so people get aware of the potential problems.