Re: [PATCH 2/3] find: micro-optimize for_each_{set,clear}_bit()

Marc Zyngier
2021-06-19T16:24:20Z

On Fri, 18 Jun 2021 20:57:34 +0100,
Yury Norov <yury.norov@gmail.com> wrote:
>
> The macros iterate thru all set/clear bits in a bitmap. They search a
> first bit using find_first_bit(), and the rest bits using find_next_bit().
>
> Since find_next_bit() is called shortly after find_first_bit(), we can
> save few lines of I-cache by not using find_first_bit().

Really?

>
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
> include/linux/find.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/find.h b/include/linux/find.h
> index 4500e8ab93e2..ae9ed52b52b8 100644
> --- a/include/linux/find.h
> +++ b/include/linux/find.h
> @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
> #endif
>
> #define for_each_set_bit(bit, addr, size) \
> - for ((bit) = find_first_bit((addr), (size)); \
> + for ((bit) = find_next_bit((addr), (size), 0); \

On which architecture do you observe a gain? Only 32bit ARM and m68k
implement their own version of find_first_bit(), and everyone else
uses the canonical implementation:

#ifndef find_first_bit
#define find_first_bit(addr, size) find_next_bit((addr), (size), 0)
#endif

These architectures explicitly have different implementations for
find_first_bit() and find_next_bit() because they can do better
(whether that is true or not is another debate). I don't think you
should remove this optimisation until it has been measured on these
two architectures.

Thanks,

M.

--
Without deviation from the norm, progress is not possible.