Re: [PATCH 05/12] lib: extend the scope of small_const_nbits() macro

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Thu, Apr 1, 2021 at 3:41 AM Yury Norov <yury.norov@gmail.com> wrote:
>
> find_bit would also benefit from small_const_nbits() optimizations.
> The detailed comment is provided by Rasmus Villemoes.

Thanks, now it looks good!
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>

> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  include/asm-generic/bitsperlong.h | 12 ++++++++++++
>  include/linux/bitmap.h            |  8 --------
>  2 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
> index 3905c1c93dc2..1023e2a4bd37 100644
> --- a/include/asm-generic/bitsperlong.h
> +++ b/include/asm-generic/bitsperlong.h
> @@ -23,4 +23,16 @@
>  #define BITS_PER_LONG_LONG 64
>  #endif
>
> +/*
> + * small_const_nbits(n) is true precisely when it is known at compile-time
> + * that BITMAP_SIZE(n) is 1, i.e. 1 <= n <= BITS_PER_LONG. This allows
> + * various bit/bitmap APIs to provide a fast inline implementation. Bitmaps
> + * of size 0 are very rare, and a compile-time-known-size 0 is most likely
> + * a sign of error. They will be handled correctly by the bit/bitmap APIs,
> + * but using the out-of-line functions, so that the inline implementations
> + * can unconditionally dereference the pointer(s).
> + */
> +#define small_const_nbits(nbits) \
> +       (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> +
>  #endif /* __ASM_GENERIC_BITS_PER_LONG */
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 2cb1d7cfe8f9..a36cfcec4e77 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -230,14 +230,6 @@ int bitmap_print_to_pagebuf(bool list, char *buf,
>  #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
>  #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
>
> -/*
> - * The static inlines below do not handle constant nbits==0 correctly,
> - * so make such users (should any ever turn up) call the out-of-line
> - * versions.
> - */
> -#define small_const_nbits(nbits) \
> -       (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> -
>  static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
>  {
>         unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> --
> 2.25.1
>


-- 
With Best Regards,
Andy Shevchenko