Re: Question on GIMPLE shifts

[Andrew Pinski <pinskia@gmail.com>]

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.

Thanks,
Andrew

>
> ---
> With best regards,
> Daniil