Re: Question on GIMPLE shifts

[Richard Biener <richard.guenther@gmail.com>]

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 <exactlywb@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