Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK

[Yury Norov]

On Thu, Jul 26, 2018 at 04:07:51PM +0800, Wei Wang wrote:
> The existing BITMAP_LAST_WORD_MASK macro returns 0xffffffff if nbits is
> 0. This patch changes the macro to return 0 when there is no bit needs to
> be masked.

I think this is intentional behavour. Previous version did return ~0UL
explicitly in this case. See patch 89c1e79eb3023 (linux/bitmap.h: improve
BITMAP_{LAST,FIRST}_WORD_MASK) from Rasmus.

Introducing conditional branch would affect performance. All existing
code checks nbits for 0 before handling last word where needed
explicitly. So I think we'd better change nothing here.

Yury

> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Cc: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/bitmap.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 1ee46f4..12af3d7 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -194,7 +194,10 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
>                                    const unsigned long *maskp, int nmaskbits);
> 
>  #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
> -#define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
> +#define BITMAP_LAST_WORD_MASK(nbits)                           \
> +(                                                              \
> +       nbits ? (~0UL >> (-(nbits) & (BITS_PER_LONG - 1))) : 0  \
> +)
> 
>  #define small_const_nbits(nbits) \
>         (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)
> --
> 2.7.4