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

[Leon Romanovsky]

On Mon, Jun 25, 2018 at 11:26:05AM +0200, Rasmus Villemoes wrote:
> On 24 June 2018 at 10:23, Leon Romanovsky <leon@xxxxxxxxxx> wrote:
>
> > From: Leon Romanovsky <leonro@xxxxxxxxxxxx>
> >
> > Add shift_overflow() helper to help driver authors to ensure that
> > shift operand doesn't cause to overflow, which is very common pattern
> > for RDMA drivers.
> >
>
> Not a huge fan. The other _overflow functions have a different behaviour
> (in how they return the result and the overflow status) and are
> type-generic, and I think someone at some point will use such a
> generically-named helper for stuff other than size_t. At least the
> array_size and struct_size helpers have size in their name and are
> specifically about computing the size of something, and are designed to be
> used directly as arguments to allocators, where SIZE_MAX is a suitable
> sentinel. I can't see the other patches in this series, so I don't know how
> you plan on using it, but it should also be usable outside rdma.
>
> Aside: why does b have type size_t?
>
> Does __must_check really make sense for a function without side effects? It
> doesn't tell gcc to warn if the result is not used in a conditional, it
> just warns if the result is not used at all, which wouldn't realistically
> happen for a pure function.
>
> I'd much rather see a type-generic check_shift_overflow (we can agree to
> leave "left" out of the name) with semantics similar to the other
> check_*_overflow functions. Then, if a size_t-eating, SIZE_MAX-returning
> helper is more convenient for rdma, that should be easy to implement on top
> of that. It shouldn't really be that hard to do. Something like
>
> 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);
> }
>
> which should also handle shifts of signed types (though it allows << 0 for
> negative values; that's easy to also disallow). But the exact semantics
> should be documented via a bunch of tests (hint hint) exercising corner
> cases.

I'll respin.

Thanks for the feedback.

>
> Rasmus

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