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

On Sun, Nov 28, 2021 at 10:10 AM Michał Mirosław
[off-list ref] wrote:

On Sat, Nov 27, 2021 at 07:56:57PM -0800, Yury Norov wrote:

quoted

Now as we have bitmap_weight_eq(), switch bitmap_full() and
bitmap_empty() to using it.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 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.