Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs

[Masahiro Yamada]

Hi Rikard,


On Tue, Aug 6, 2019 at 4:55 AM Rikard Falkeborn
<rikard.falkeborn@xxxxxxxxx> wrote:
>
> On Sun, Aug 04, 2019 at 03:45:16PM +0900, Masahiro Yamada wrote:
> > On Sun, Aug 4, 2019 at 3:36 AM Rikard Falkeborn
> > <rikard.falkeborn@xxxxxxxxx> wrote:
> > >
> > > On Sat, Aug 03, 2019 at 12:12:46PM +0900, Masahiro Yamada wrote:
> > > > On Sat, Aug 3, 2019 at 12:03 PM Masahiro Yamada
> > > > <yamada.masahiro@xxxxxxxxxxxxx> wrote:
> > > >
> > > > >
> > > > > BTW, v2 is already inconsistent.
> > > > > If you wanted GENMASK_INPUT_CHECK() to return 'unsigned long',,
> > > > > you would have to cast (low) > (high) as well:
> > > > >
> > > > >                (unsigned long)((low) > (high)), UL(0))))
> > > > >
> > > > > This is totally redundant, and weird.
> > > >
> > > > I take back this comment.
> > > > You added (unsigned long) to the beginning of this macro.
> > > > So, the type is consistent, but I believe all casts should be removed.
> > >
> > > Maybe you're right. BUILD_BUG_ON_ZERO returns size_t regardless of
> > > inputs. I was worried that on some platform, size_t would be larger than
> > > unsigned long (as far as I could see, the standard does not give any
> > > guarantees), and thus all of a sudden GENMASK would be 8 bytes instead
> > > of 4, but perhaps that is not a problem?
> >
> >
> > How about adding (int) cast to BUILD_BUG_ON_ZERO() ?
>
> I'll have a look.



I found a more important problem in this patch.

You used __is_constexpr(), which is defined in <linux/kernel.h>.

This header does not include <linux/kernel.h>,
so this header is not self-contained anymore.

The following test code fails to build:

#include <linux/bits.h>
unsigned long foo(unsigned long in_bits)
{
        return in_bits & GENMASK(5, 3);
}



However, you cannot include <linux/kernel.h> from <linux/bits.h>.
See the log of 8bd9cb51daac89337295b6f037b0486911e1b408

This header was split out to not pull in <linux/bitops.h>
Including <linux/kernel.h> pulls in <linux/bitops.h> again.


In summary, please use __builtin_constant_p()
instead of __is_constexpr().


You can shorten __builtin_constant_p(high) && __builtin_constant_p(low)
into __builtin_constant_p((low) > (high)).


How about this?


#define GENMASK_INPUT_CHECK(high, low) \
       BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
              __builtin_constant_p((low) > (high)), (low) > (high), 0))


-- 
Best Regards
Masahiro Yamada