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

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Date:   Wed, 24 Aug 2022 20:54:46 +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 4:53 PM Yury Norov <yury.norov@...il.com> wrote:
> On Wed, Aug 24, 2022 at 12:19:05PM +0300, Andy Shevchenko wrote:
> > On Wed, Aug 24, 2022 at 4:56 AM Yury Norov <yury.norov@...il.com> wrote:

...

> > > +#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)
>
> Offset in structure, index in array, isn't?
>
> > as I proposed in the first patch? You will drop a lot of
> > divisions / multiplications, no?
>
> Those divisions and multiplications are optimized away, and
> what you suggested blows up the EXPRESSION.
>
> I tried like this:
>    mask = word_op(BITMAP_FIRST_WORD_MASK(__start));
>    idx = __start / BITS_PER_LONG;
>    tmp = (EXPRESSION);
>
>    while (1) {
>         if (tmp) {
>                sz = min(idx * BITS_PER_LONG + __ffs(word_op(tmp)), sz);
>                break;
>         }
>
>         if (++idx > sz)
>                 break;
>
>         tmp = (EXPRESSION);
>    }
>
> And it generated the same code, but looks less expressive to me.
> If you have some elegant approach in mind - can you please share
> it, and how the generated code looks?

for (unsigned long idx = 0; idx < sz; idx++) {
  unsigned long tmp;

  tmp = (EXPRESSION);
  if (tmp) {
    ...
  }
}

No?

-- 
With Best Regards,
Andy Shevchenko