Re: [PATCH v2 3/3] lib/find_bit: optimize find_next_bit() functions

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

On Wed, Aug 24, 2022 at 4:56 AM Yury Norov <yury.norov@gmail.com> wrote:
>
> Over the past couple years, the function _find_next_bit() was extended
> with parameters that modify its behavior to implement and- zero- and le-
> flavors. The parameters are passed at compile time, but current design
> prevents a compiler from optimizing out the conditionals.
>
> As find_next_bit() API grows, I expect that more parameterss will be added.

parameters

> Current designs would require more conditional code in _find_next_bit(),
> which would bloat the helper even more and make it barely readable.
>
> This patch replaces _find_next_bit() with a macro FIND_NEXT_BIT, and adds
> a set of wrappers, so that the compile-time optimization becomes possible.
>
> The common logic is moved to the new macro, and all flavors may be
> generated by providing an EXPRESSION macro parameter, like in this example:
>
>   #define FIND_NEXT_BIT(EXPRESSION, size, start) ...
>
>   find_next_xornot_and_bit(addr1, addr2, addr3, size, start)
>   {
>         return FIND_NEXT_BIT(addr1[idx] ^ ~addr2[idx] & addr3[idx], size, start);
>   }
>
> The EXPRESSION may be of any complexity, as soon as it only refers
> the bitmap(s) and an iterator idx.

...

> +#define FIND_NEXT_BIT(EXPRESSION, size, start)                                 \
> +({                                                                             \
> +       unsigned long mask, idx, tmp, sz = (size), __start = (start);           \
> +                                                                               \
> +       if (unlikely(__start >= sz))                                            \
> +               goto out;                                                       \
> +                                                                               \
> +       mask = word_op(BITMAP_FIRST_WORD_MASK(__start));                        \
> +       idx = __start / BITS_PER_LONG;                                          \
> +                                                                               \
> +       for (tmp = (EXPRESSION) & mask; !tmp; tmp = (EXPRESSION)) {             \

for (unsigned long tmp ...;
But hey, why not loop over idx (which probably should be named as
offset) as I proposed in the first patch? You will drop a lot of
divisions / multiplications, no?

> +               if (idx > sz / BITS_PER_LONG)                                   \
> +                       goto out;                                               \
> +               idx++;                                                          \
> +       }                                                                       \
> +                                                                               \
> +       sz = min(idx * BITS_PER_LONG + __ffs(word_op(tmp)), sz);                \
> +out:                                                                           \
> +       sz;                                                                     \
> +})

-- 
With Best Regards,
Andy Shevchenko