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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug analyzer/98447] incorrect -Wanalyzer-shift-count-overflow warning
Date: Sat, 26 Dec 2020 18:47:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98447

--- Comment #4 from Vincent Lef=C3=A8vre <vincent-gcc at vinc17 dot net> ---
So this can be simplified to

void f (unsigned long *p, int r, int i)
{
  int b =3D 64, n =3D r % 64;

  while (i >=3D 0 && b >=3D 0)
    {
      if (b <=3D n)
        p[i--] =3D 1UL << b;
      b -=3D n;
    }
}

Here, due to the "n =3D r % 64", one has n <=3D 63, so that "1UL << b" can =
be
executed only when b <=3D 63, and the shift is necessarily valid (no overfl=
ow).

This could be part of a more general bug: I also get the
-Wanalyzer-shift-count-overflow warning if I replace "n =3D r % 64" by "n =
=3D r".
While with the initial code, one is certain that the shift is valid, now th=
is
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 h=
ave
any value.=