[Bug c/63445] New: request: make -Wstrict-overflow avoid a class of false positives

[jim at meyering dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63445

            Bug ID: 63445
           Summary: request: make -Wstrict-overflow avoid a class of false
                    positives
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jim at meyering dot net

Thanks for tending and continually improving gcc.

I see some surprising new warnings when using built-from-git (an hour ago) gcc
to compile coreutils.  Here is a reduced test case:

In this example, the code ensures "j - i" will be positive before assigning
that value to an unsigned int "n".  Can gcc be taught to avoid this obvious
false positive?

$ cat f.c

int
f (int i, int j)
{
  unsigned int c = 0;                                                          
  if (i < j)
    {
      unsigned int n = j - i;
      for (unsigned int i = 0; i < n; i++)
        {
          c++;
        }
    }
  return c;
}
$ gcc -O2 -std=c99 -Wstrict-overflow -c f.c
f.c: In function ‘f’:
f.c:9:7: warning: assuming signed overflow does not occur when simplifying
conditional [-Wstrict-overflow]
       for (unsigned int i = 0; i < n; i++)
       ^
$ gcc -v 2>&1|tail -2
gcc version 5.0.0 20141002 (experimental) (GCC)