[Bug middle-end/117355] [15 regression] firebird miscompiled due to __builtin_dynamic_object_size difference at -O since r15-4396-g72ae35bbc90fea

[siddhesh at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117355
Siddhesh Poyarekar <siddhesh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |siddhesh at gcc dot 
gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #11 from Siddhesh Poyarekar <siddhesh at gcc dot gnu.org> ---
Even smaller reproducer:

typedef unsigned long size_t;

#define STR "string gds__)"

void
gen_blr (void)
{
  char line[256];
  const char *p = STR;
  const char *q = p + sizeof (STR) - 1;

  char *q1 = line;
  for (const char *p1 = p; p1 < q;)
    if ((*q1++ = *p1++) == 'g')
      if (p1 < q && (*q1++ = *p1++) == 'd')
        if (p1 < q && (*q1++ = *p1++) == 's')
          if (p1 < q && (*q1++ = *p1++) == '_')
            if (p1 < q && *p1++ == '_')
              __builtin___strncpy_chk (q1 - 4, "isc", 3,
                                       __builtin_dynamic_object_size
                                       (q1 - 4, 1));
}

cc1plus ends up with a zero size while cc1 gets the conservative whole size,
like Sam observed:

$ gcc/cc1plus -quiet -O -o /dev/null ../firebird.ii
../firebird.ii: In function ‘void gen_blr()’:
../firebird.ii:19:39: warning: ‘char* __builtin___strncpy_chk(char*, const
char*, long unsigned int, long unsigned int)’ writing 3 bytes into a region of
size 0 overflows the destination [-Wstringop-overflow=]
   19 |               __builtin___strncpy_chk (q1 - 4, "isc", 3,
      |               ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
   20 |                                        __builtin_dynamic_object_size
      |                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   21 |                                        (q1 - 4, 1));
      |                                        ~~~~~~~~~~~~
$ gcc/cc1 -quiet -O -o /dev/null ../firebird.ii
$