[Bug analyzer/98447] incorrect -Wanalyzer-shift-count-overflow warning

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98447

--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
So this can be simplified to

void f (unsigned long *p, int r, int i)
{
  int b = 64, n = r % 64;

  while (i >= 0 && b >= 0)
    {
      if (b <= n)
        p[i--] = 1UL << b;
      b -= n;
    }
}

Here, due to the "n = r % 64", one has n <= 63, so that "1UL << b" can be
executed only when b <= 63, and the shift is necessarily valid (no overflow).

This could be part of a more general bug: I also get the
-Wanalyzer-shift-count-overflow warning if I replace "n = r % 64" by "n = r".
While with the initial code, one is certain that the shift is valid, now this
depends on the value of r. But since the compiler doesn't know the condition
under which this function will be called, it should not assume that r may have
any value.