Re: [Ksummit-discuss] [CORE TOPIC] More useful types in the linux kernel

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Fri, Jul 22, 2016 at 08:03:06AM +0100, David Howells wrote:
> Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
> 
> > >  (1) A 'bits' and maybe a 'bits64' type.  Currently you have to use unsigned
> > >      long when you want to deploy a flags field with which you're going to use
> > >      test_bit() and co. - but this typically wastes 32 bits on a 64-bit arch
> > >      because you can't use bits 32-63 as they might not exist.
> > 
> > What is wrong with using DECLARE_BITMAP()? It will allocate exactly as
> > many unsigned longs
> 
> You missed my point.  *unsigned long* is the issue.  The majority of the time
> that wastes 32 bits on a 64-bit machine - especially when we don't need that
> many flags.  On some 64-bit arches we could use unsigned int instead.

I was responding to your statement where you were saying you could not
use bits 32 - 63 as they might not exist. If you have so many bits then
DECLARE_BITMAP is useful. Otherwise we can simply use u{8|16|32} and
BIT() macro.

Thanks.

-- 
Dmitry