Re: [PATCH 2/9] lib/bitmap: implement bitmap_{empty,full} with bitmap_weight_eq()

[Yury Norov]

On Sun, Nov 28, 2021 at 10:10 AM Michał Mirosław
<mirq-linux@xxxxxxxxxxxx> wrote:
>
> On Sat, Nov 27, 2021 at 07:56:57PM -0800, Yury Norov wrote:
> > Now as we have bitmap_weight_eq(), switch bitmap_full() and
> > bitmap_empty() to using it.
> >
> > Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> > ---
> >  include/linux/bitmap.h | 26 ++++++++++----------------
> >  1 file changed, 10 insertions(+), 16 deletions(-)
> >
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index 996041f771c8..2d951e4dc814 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -386,22 +386,6 @@ static inline int bitmap_subset(const unsigned long *src1,
> >               return __bitmap_subset(src1, src2, nbits);
> >  }
> >
> > -static inline bool bitmap_empty(const unsigned long *src, unsigned nbits)
> > -{
> > -     if (small_const_nbits(nbits))
> > -             return ! (*src & BITMAP_LAST_WORD_MASK(nbits));
> > -
> > -     return find_first_bit(src, nbits) == nbits;
> > -}
>
> Since this is supposed to be an optimization, I would go all the way and
> replace this with the trivial implementation instead:
>
> bool bitmap_empty(long *bits, size_t nbits)
> {
>         for (; nbits >= BITS_PER_LONG; ++bits, nbits -= BITS_PER_LONG)
>                 if (*bits)
>                         return false;
>
>         if (nbits && *bits & BITMAP_LAST_WORD_MASK(nbits))
>                 return false;
>
>         return true;
> }

This is what current implementations basically do, based on find_first_bit().

I think that for long bitmaps the most time consuming operation is moving
data to L1, and for short bitmaps the difference between approaches is
barely measurable.

But hweght_long on each iteration can't be more effective than the current
version. So, I'll drop this patch for v2 and keep things unchanged.