linux-kernel - Re: [PATCH v2 3/3] lib/find_bit: optimize find_next

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Date:   Wed, 24 Aug 2022 12:19:05 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Dennis Zhou <dennis@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexey Klimov <aklimov@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Whitcroft <apw@...onical.com>
Subject: Re: [PATCH v2 3/3] lib/find_bit: optimize find_next_bit() functions

On Wed, Aug 24, 2022 at 4:56 AM Yury Norov <yury.norov@...il.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