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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103194] [12 Regression] ice in
 optimize_atomic_bit_test_and with __sync_fetch_and_and since
 r12-5102-gfb161782545224f5
Date: Wed, 15 Dec 2021 12:47:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103194

--- Comment #22 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #15)
> > Is the behavior well defined for n >=3D 64? I got
> >=20
> > foo.c:11:19: warning: left shift count >=3D width of type
> > [-Wshift-count-overflow]
> >    11 |   long mask =3D 1ll << 65;
> >       |                   ^~
> According to C99
> The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits =
are
> =EF=AC=81lled with zeros. If E1 has an unsigned type, the value of the re=
sult is E1
> =C3=97 2E2, reduced modulo one more than the maximum value representable =
in the
> result type. If E1 has a signed type and nonnegative value, and E1 =C3=97=
 2E2 is
> representable in the result type, then that is the resulting value;
> otherwise, the behavior is unde=EF=AC=81ned.
>=20
> So yes, it's well defined, and the result is zero.

No, that isn't well defined in any C or C++ versions.
E.g. in C++:
https://eel.is/c++draft/expr.shift#1
The behavior is undefined if the right operand is negative, or greater than=
 or
equal to the width of the promoted left operand.
C99 6.5.7:
If the value of the right operand is negative or is
greater than or equal to the width of the promoted left operand, the behavi=
or
is undefined.
What varies is whether shifting negative values left is well defined or not=
, or
whether shifting bits into the sign bit of signed type is well defined or n=
ot.=