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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/103483] [12 regression] context-sensitive ranges
 change triggers stringop-overread
Date: Mon, 14 Mar 2022 23:58:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103483

--- Comment #23 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #22)

Your question may have been rhetorical but to be explicit, the real differe=
nce
is hidden in the implementation (which is why these warnings can sometimes =
seem
inconsistent).  GCC doesn't warn for the second test case (copied below)
because it only considers the lower bound of len's range:

int a[2];
void foo (unsigned len)
{
  if (len =3D=3D 1 || len =3D=3D 20)
    __builtin_memset (a, 0, len);
}

But the warning would trigger if GCC decided it was profitable to split the
memset call into two statements:

int a[2];
void foo (unsigned len)
{
  if (len =3D=3D 1)
    a[0] =3D 0;
  else if (len =3D=3D 20)
    __builtin_memset (a, 0, 20);
}

I suspect most users (though not all, otherwise this report would have never
been raised) would consider a warning valid and helpful for the source code=
.=20
But if instead of (len =3D=3D 1 || len =3D=3D 20) the condition were to be =
written in
terms of a relational expression (like len <=3D N) where N were greater tha=
n or
even equal to sizeof (a) + 1, I'd expect complaints about the warning being=
 a
false positive because GCC can't "know" that len =3D=3D N necessarily holds=
.=