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

[Leon Romanovsky <leon@kernel.org>]

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On Mon, Jun 25, 2018 at 11:11:57AM -0600, Jason Gunthorpe wrote:
> 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.

Rasmus and Jason,

Thanks for the feedback.
The reason why I introduced function, because wanted to reuse
check_mul_overflow macro, but for any reasons which I don't remember
now, I had hard time to fix compilation errors.

Anyway, I'll resubmit.

Thanks


>
> Jason
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