[Bug tree-optimization/104789] [12 Regression] -Wstringop-overflow false positive at -O3 for an unrolled loop

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104789

--- Comment #9 from Martin Sebor <msebor at gcc dot gnu.org> ---
A much simplified test case that reproduces the same warning (with both GCC 12
and 11) is below.  The underlying problem is that although GCC does have a way
to represent simple disjoint ranges of variable values, it's not used here, and
even if it were, very little code makes uses of this representation beyond a
single contiguous range (or, at most, a converse of it).  Even the disjoint
representation isn't sufficiently flexible to capture arbitrarily complex
ranges (capturing those in their full generality would require a constraint
solver).

$ cat z.c && gcc -O3 -S -Wall z.c
char a[8];

void f (unsigned n)
{
  unsigned i = 0;

  for (unsigned j = 0; i != n; ++j)
    i += 2;

  if (i > 7)
    return;

  while (i % 4)
    a[i++] = 0;
}
z.c: In function ‘f’:
z.c:14:12: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
   14 |     a[i++] = 0;
      |     ~~~~~~~^~~
z.c:1:6: note: at offset 8 into destination object ‘a’ of size 8
    1 | char a[8];
      |      ^

The warning can be avoided (and the emitted object code improved) by changing
the while loop like so:

  while (i % 4)
    {
      if (i > 7) __builtin_unreachable (); 
      a[i++] = 0;
    }

The same suppression works in the test case in comment #7 but GCC then issues
another warning, this one pointing out that an element of the header array is
used uninitialized.  That warning looks valid to me:

/x86_64-pc-linux-gnu/bits/c++allocator.h:33,
                 from
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/allocator.h:46,
                 from
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/vector:61,
                 from pr104789.C:2:
In member function ‘void std::__new_allocator<_Tp>::construct(_Up*, _Args&&
...) [with _Up = unsigned char; _Args = {const unsigned char&}; _Tp = unsigned
char]’,
    inlined from ‘static void std::allocator_traits<std::allocator<_Tp1>
>::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = unsigned char;
_Args = {const unsigned char&}; _Tp = unsigned char]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/alloc_traits.h:516:17,
    inlined from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&)
[with _Tp = unsigned char; _Alloc = std::allocator<unsigned char>]’ at
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/stl_vector.h:1280:30,
    inlined from ‘void commit_temp_packets()’ at pr104789.C:37:17:
/build/gcc-master/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/new_allocator.h:175:11:
warning: ‘header’ may be used uninitialized [-Wmaybe-uninitialized]
  175 |         { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pr104789.C: In function ‘void commit_temp_packets()’:
pr104789.C:10:17: note: ‘header’ declared here
   10 |         uint8_t header[8];
      |                 ^~~~~~