Re: [PATCH rdma-next 08/12] overflow.h: Add arithmetic shift helper

[Jason Gunthorpe <jgg@mellanox.com>]

On Mon, Jun 25, 2018 at 11:26:05AM +0200, Rasmus Villemoes wrote:

>    check_shift_overflow(a, s, d) {
>        unsigned _nbits = 8*sizeof(a);
>        typeof(a) _a = (a);
>        typeof(s) _s = (s);
>        typeof(d) _d = (d);
> 
>        *_d = ((u64)(_a) << (_s & (_nbits-1)));
>        _s >= _nbits || (_s > 0 && (_a >> (_nbits - _s -
>    is_signed_type(a))) != 0);
>    }

Those types are not quite right.. What about this?

    check_shift_overflow(a, s, d) ({
        unsigned int _nbits = 8*sizeof(d) - is_signed_type(d);
        typeof(d) _a = a;  // Shift is always performed on type 'd'
        typeof(s) _s = s;
        typeof(d) _d = d;
 
        *_d = (_a << (_s & (_nbits-1)));

	(((*_d) >> (_s & (_nbits-1)) != _a);
    })

And can we use mathamatcial invertability to prove no overlow and
bound _a ? As above.

Jason