[Bug middle-end/92014] New: [10 Regression] bogus warning: writing 8 bytes into a region of size 1 in timezone/zic.c

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92014

            Bug ID: 92014
           Summary: [10 Regression] bogus warning: writing 8 bytes into a
                    region of size 1 in timezone/zic.c
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Last week's r276603 has introduced a bug into the count_nonzero_bytes function
that leads to a false positive -Wstringop-overflow in glibc's timezone/zic.c. 
The test case below reproduces the bogus warning.

$ cat zic.i && gcc -S -O2 -Wall zic.i
struct
{
  char *s1, *s2;
  char c;
} z;


void f (char **a, int i, int j)
{
  char * cp = __builtin_strchr (a[i], '%');

  if (cp && *++cp != 's')
    return;

  z.s1 = __builtin_strdup (a[i]);
  if (!z.s1) __builtin_abort ();

  z.s2 = __builtin_strdup (a[j]);
  if (!z.s2) __builtin_abort ();

  z.c = cp ? *cp : '\0';
}
zic.i: In function ‘f’:
zic.i:21:7: warning: writing 8 bytes into a region of size 1
[-Wstringop-overflow=]
   21 |   z.c = cp ? *cp : '\0';
      |   ~~~~^~~~~~~~~~~~~~~~~
zic.i:4:8: note: destination object declared here
    4 |   char c;
      |        ^