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

From: Peter Zijlstra <[email-masked]>
Date: Wed, 1 Aug 2018 11:36:03 +0200

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.