-Wstringop-overflow complains about pointers holding explicitly set addresses

[Guy Benyei]

Hello all,
Compiling for RISC-V, I've ran into an error like this:

tmp.c:15:3: error: 'memcpy' writing 4 bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=]
   15 |   memcpy(&str2->c, &str1->c, sizeof(str2->c));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The error can be triggered by a pretty simple function:

void foo(m_struct_t *str1) {
  m_struct_t *str2 = (m_struct_t *)0x123400;
  memcpy(&str2->c, &str1->c, sizeof(str2->c));
}

Debugging the case, I've found the following remark in gcc/pointer-query.cc:

/* Pointer constants other than null are most likely the result
   of erroneous null pointer addition/subtraction.  Unless zero
   is a valid address set size to zero.  For null pointers, set
   size to the maximum for now since those may be the result of
   jump threading.  */

I'd prefer not to disable this warning, as it seems very helpful, but in embedded SW we have just too many cases we have to set an address explicitly. I understand the concern about erroneous null pointer addition/subtraction, but I think these could be detected in other analysis, while stringop overflow would still work for other cases.
I see that the warning can be silenced by zero_address_valid, which is only set for x86 non-generic address space for now. I'm not sure if this enabling zero addresses all over the place is right for RISC-V or other potentially embedded targets.

What do you think?

Thanks
        Guy