https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103194
--- 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 = 1ll << n;
> > > pc2 = __sync_fetch_and_or(&pscc_a_2_3, mask) & mask;
> > > }
> > >
> > > void pscc1(int n)
> > > {
> > > long mask = 1ll << 65;
> > > pc2 = __sync_fetch_and_or(&pscc_a_2_3, mask) & mask;
> > > }
> > >
> > > pscc and pscc1 have different behavior when n >= 64, It seems unsafe to
> > > optimize variable mask?
> >
> > Is the behavior well defined for n >= 64? I got
> >
> > foo.c:11:19: warning: left shift count >= width of type
> > [-Wshift-count-overflow]
> > 11 | long mask = 1ll << 65;
> > | ^~
> According to C99
> The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are
> filled with zeros. If E1 has an unsigned type, the value of the result is E1
> × 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 × 2E2 is
> representable in the result type, then that is the resulting value;
> otherwise, the behavior is undefined.
>
> So yes, it's well defined, and the result is zero.
This is the existing behavior since GCC 7.
