On Wed, Nov 1, 2023 at 5:01 PM Andrew Pinski via Gcc <gcc@gcc.gnu.org> wrote:
>
> On Wed, Nov 1, 2023 at 3:56 AM Daniil Frolov <exactl...@ispras.ru> wrote:
> >
> > Hi!
> >
> > When investigating bit shifts I got an incomprehensible moment with
> > the following example:
> >
> > int f(int x, int k)
> > {
> > int tmp = x >> k;
> > return (tmp & 1) << 10;
> > }
> >
> > If we would like to take a look into GIMPLE then we'll get:
> >
> > int f (int x, int k)
> > {
> > int tmp;
> > int D.2746;
> > int _1;
> > int _5;
> >
> > <bb 2> :
> > tmp_4 = x_2(D) >> k_3(D);
> > _1 = tmp_4 << 10;
> > _5 = _1 & 1024;
> >
> > <bb 3> :
> > <L0>:
> > return _5;
> >
> > }
> >
> > Is the expression '_1 = tmp_4 << 10' considered legal in GIMPLE? Given
> > the
> > semantics of C bit shifts, this statement could modify the sign bit,
> > potentially leading to overflow.
>
> Except it was not undefined in C90.
Also in GIMPLE/GENERIC left-shifts are always logical and the result is
modulo-reduced to the target type. There's no (undefined) arithmetic overflow
involved for any shift operation (but there is for multiply). Only the shift
argument magnitude is constrained.
Richard.
> Thanks,
> Andrew
>
> >
> > ---
> > With best regards,
> > Daniil
