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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: Tue, 16 Nov 2021 02:39:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103194

--- Comment #16 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Hongtao.liu from comment #15)
> (In reply to H.J. Lu from comment #13)
> > (In reply to Hongtao.liu from comment #8)
> > > unsigned long pscc_a_2_3;
> > > int pscc_a_1_4;
> > > unsigned long pc2;
> > > void pscc(int n)
> > > {
> > >   long mask =3D 1ll << n;
> > >   pc2 =3D __sync_fetch_and_or(&pscc_a_2_3, mask) & mask;
> > > }
> > >=20
> > > void pscc1(int n)
> > > {
> > >   long mask =3D 1ll << 65;
> > >   pc2 =3D __sync_fetch_and_or(&pscc_a_2_3, mask) & mask;
> > > }
> > >=20
> > > pscc and pscc1 have different behavior when n >=3D 64, It seems unsaf=
e to
> > > optimize variable mask?
> >=20
> > 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.

This is the existing behavior since GCC 7.=