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

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Date:   Wed, 24 Aug 2022 20:56:02 +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 8:54 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> 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++) {

Of source 0 should be changed to whatever start you have there.

>   unsigned long tmp;
>
>   tmp = (EXPRESSION);
>   if (tmp) {
>     ...
>   }
> }
>
> No?

-- 
With Best Regards,
Andy Shevchenko