Re: [Qemu-trivial] [Qemu-devel] [PATCH v5 01/46] include: Add IEC binary prefixes in "qemu/units.h"

[Eric Blake]

On 06/29/2018 10:02 AM, Daniel P. Berrangé wrote:
> > Are you suggesting this?
> >
> > #define KiB     (INT32_C(1) << 10)
> > #define MiB     (INT32_C(1) << 20)
> > #define GiB     (INT32_C(1) << 30)
> >
> > #define TiB     (INT64_C(1) << 40)
> > #define PiB     (INT64_C(1) << 50)
> > #define EiB     (INT64_C(1) << 60)

Maybe, but then '8 * GiB' overflows, rather than giving a 64-bit result.

> This feels dangerous to me as now the calculation may or may not be 32-bit
> or 64-bit depending on which constant you happen to pick. I think this
> inconsistency is going to be surprising to both devs and reviewers leading
> to bugs.

Being consistently 64-bit may be annoying, but it's easy enough to 
reason about, compared to worrying whether an overflow will happen.

> Did you consider just adding unsigned versions  ?  eg UKiB, UMiB, UGiB. It
> is a bit ugly but is has the benefit of being obvious whether you're intending
> the calculation  to be signed vs unsigned.

That might also be worthwhile, although I'd tend towards the name KiBU 
instead if UKiB (since I'm already used to U as a suffix).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org