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

From: Peter Zijlstra
Date: Wed Aug 01 2018 - 05:36:28 EST


On Tue, Jun 26, 2018 at 11:54:35AM -0600, Jason Gunthorpe wrote:

> What about more like this?
>
> check_shift_overflow(a, s, d) ({

Should that not be: check_shl_overflow() ? Just 'shift' lacks a
direction.

> // Shift is always performed on the machine's largest unsigned
> u64 _a = a;
> typeof(s) _s = s;
> typeof(d) _d = d;
>
> // Make s safe against UB
> unsigned int _to_shift = _s >= 0 && _s < 8*sizeof(*d) : _s ? 0;

Should we not do a gcc-plugin or something that fixes that particular
UB? Shift acting all retarded like that is just annoying. I feel we
should eliminate UBs from the language where possible, like
-fno-strict-overflow mandates 2s complement.

> *_d = (_a << _to_shift);
>
> // s is malformed
> (_to_shift != _s ||

Not strictly an overflow though, just not expected behaviour.

> // d is a signed type and became negative
> *_d < 0 ||

Only a problem if it wasn't negative to start out with.

> // a is a signed type and was negative
> _a < 0 ||

Why would that be a problem? You can shift left negative values just
fine. The only problem is when you run out of sign bits.

> // Not invertable means a was truncated during shifting
> (*_d >> _to_shift) != a))
> })

And I'm not exactly seeing the use case for this macro. What's the point
of a shift-left if you cannot truncate bits. I suppose it's in the name
_overflow, but still.