[Bug sanitizer/120837] False-positive from -fsanitize=undefined

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120837

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Self-contained testcase:

[[gnu::noipa]] void
bar (void **x, void **y)
{
  x[0] = 0;
  x[1] = 0;
  x[2] = 0;
  y[0] = 0;
  y[1] = 0;
  y[2] = 0;
  y[3] = 0;
  y[4] = 0;
}

[[gnu::noipa]] void *
foo (int x, int y)
{
  void *a[3];
  void *b[5];
  bar (a, b);
  return (x > y ? b : a)[y - 1];
}

int
main ()
{
  if (foo (2, 1) != 0)
    __builtin_abort ();
}

Something during the C GENERIC folding creates invalid code out of it (but C++
doesn't):
*((nNew > nOld ? (void * *) &apNew + 18446744073709551608 : (void * *) &apOld +
18446744073709551608) + (sizetype) ((long unsigned int) nOld * 8))
and later comes ubsan and is really unhappy about the negative offsets to
arrays.