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

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Date:   Wed, 24 Aug 2022 06:53:07 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...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 12:19:05PM +0300, Andy Shevchenko wrote:
> 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)

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?

Thanks,
Yury