On 16/03/2021 02.54, Yury Norov wrote:
> find_bit would also benefit from small_const_nbits() optimizations.
>
> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> ---
> include/asm-generic/bitsperlong.h | 9 +++++++++
> include/linux/bitmap.h | 3 ---
> 2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
> index 3905c1c93dc2..96032f4f908f 100644
> --- a/include/asm-generic/bitsperlong.h
> +++ b/include/asm-generic/bitsperlong.h
> @@ -23,4 +23,13 @@
> #define BITS_PER_LONG_LONG 64
> #endif
>
> +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) < BITS_PER_LONG)
> +
> +/*
> + * The valid number of bits for a bitmap to be small enough, or in other words,
> + * fit into a single machine word is 1 to BITS_PER_LONG inclusively. 0 is not a
> + * valid number for size, and most probably a sing of error.
> + */
> +#define small_const_nbits(n) SMALL_CONST((n) - 1)
> +
I still don't see the point of introducing SMALL_CONST and still don't
like the much-too-generic-name - especially since AFAICT you don't
actually use it anywhere outside the definition of small_const_nbits()?
> #endif /* __ASM_GENERIC_BITS_PER_LONG */
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index adf7bd9f0467..bc13a890ecc1 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
> * so make such users (should any ever turn up) call the out-of-line
> * versions.
> */
You added another comment near its new definition, but the left-behind
comment in bitmap.h is now somewhat confusing, no? I suggest expanding
your new comment a bit so it's clear why we're interested in whether a
bitmap is known at compile-time to consist of exactly one word:
/*
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)
> -
> static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> {
> unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
>
Rasmus
