[Bug middle-end/104986] New: [12 Regression] bogus writing 1 byte into a region of size 0 with -fwrapv and -O2 -fpeel-loops

["andres at anarazel dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104986

            Bug ID: 104986
           Summary: [12 Regression] bogus writing 1 byte into a region of
                    size 0 with -fwrapv and -O2 -fpeel-loops
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andres at anarazel dot de
  Target Milestone: ---

Hi,

recently started seeing bogus warnings using gcc 12 to build postgres. I
reduced the problem using cvise with some manual cleanups / improvements
afterwards - certainly doesn't quite make sense anymore, but afaics shows a
problem.

Originally I hit this with -O3, but found that -O2 -fpeel-loops is sufficient
to trigger the problem.

repro: https://godbolt.org/z/ejK9h6von

code:
struct inet_struct {
  char family;
  char ipaddr[16];
};

void
inetnot(struct inet_struct *dst1, struct inet_struct *dst2, struct inet_struct
*src) {
  int nb = src->family ? 4 : 6;
  char *psrc = src->ipaddr;
  char *pdst = dst1 ? dst1->ipaddr : dst2->ipaddr;
  while (nb-- > 0)
    pdst[nb] = psrc[nb];
}


gcc-12 -fwrapv -O2 -fpeel-loops -c network2.i

network2.i: In function ‘inetnot’:
network2.i:12:14: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
   12 |     pdst[nb] = psrc[nb];
      |     ~~~~~~~~~^~~~~~~~~~
network2.i:3:8: note: at offset -1 into destination object ‘ipaddr’ of size 16
    3 |   char ipaddr[16];
      |        ^~~~~~
network2.i:3:8: note: at offset -1 into destination object ‘ipaddr’ of size 16


which afaics is bogus, because the loop terminates before reaching offset -1,
the condition is > 0, not >= 0. So the post decrement can't lead to -1 being
reached.

version: gcc version 12.0.1 20220314 (experimental) [master
r12-7638-g823b3b79cd2] (Debian 12-20220313-1) 

Regards,

Andres