linux-kernel - Re: [PATCH 2/9] lib/bitmap: implement bitmap_{empty,full} with bitmap_weight

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Message-ID: <20211128062727.GA276670@lapt>
Date:   Sat, 27 Nov 2021 22:27:27 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
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Subject: Re: [PATCH 2/9] lib/bitmap: implement bitmap_{empty,full} with
 bitmap_weight_eq()

On Sun, Nov 28, 2021 at 05:37:19AM +0100, Michał Mirosław 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.
> [...]
> > -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;
> > -}
> [...]
> > +static __always_inline bool bitmap_empty(const unsigned long *src, unsigned int nbits)
> > +{
> > +	return bitmap_weight_eq(src, nbits, 0);
> > +}
> [..]
> 
> What's the speed difference? Have you benchmarked this?

bitmap_weight_eq() should be faster than find_first_bit(), but the
difference is few cycles, so I didn't bother measuring it.

New version looks just better.