linux-kernel - Re: [PATCH 2/3] lib: Introduce 2 find bit api: all_is_bit

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Message-ID: <xa1tziy893kg.fsf@mina86.com>
Date:	Fri, 20 Nov 2015 14:57:03 +0100
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Jia He <hejianet@...il.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Yury Norov <yury.norov@...il.com>, Tejun Heo <tj@...nel.org>,
	Martin Kepplinger <martink@...teo.de>,
	George Spelvin <linux@...izon.com>,
	Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Jia He <hejianet@...il.com>
Subject: Re: [PATCH 2/3] lib: Introduce 2 find bit api: all_is_bit_{one,zero}

On Thu, Nov 19 2015, Jia He wrote:
> This patch introduces 2 lightweight bit api.
> all_bit_is_zero return 1 if the bit string is all zero.
> The addr is the start address, the size is the bit size of the bit string.
> all_bit_is_one is the opposite.
>
> Signed-off-by: Jia He <hejianet@...il.com>
> ---
>  lib/find_bit.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
>
> diff --git a/lib/find_bit.c b/lib/find_bit.c
> index 18072ea..1d56d8d 100644
> --- a/lib/find_bit.c
> +++ b/lib/find_bit.c
> @@ -131,6 +131,56 @@ unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
>  EXPORT_SYMBOL(find_last_bit);
>  #endif
>  
> +#ifndef all_bit_is_zero
> +/*
> + * return val: 1 means all bit is zero
> + */
> +unsigned int all_bit_is_zero(const unsigned long *addr, unsigned
> size)

Why does it return unsigned int, not bool?

> +{
> +	unsigned long idx;
> +	unsigned long mask = size;
> +
> +	if (unlikely(size == 0))
> +		return 1;
> +
> +	if (size > BITS_PER_LONG) {
> +		for (idx = 0; idx * BITS_PER_LONG < size; idx++)
> +			if (addr[idx])
> +				return 0;
> +
> +		mask = size - (idx - 1) * BITS_PER_LONG;

Uh?

> +	}
> +
> +	return !(*addr & BITMAP_LAST_WORD_MASK(mask));

BITMAP_LAST_WORD_MASK takes size not mask.

> +}

The whole implementation seems weird, this seems much better to me:

unsigned int all_bit_is_zero(const unsigned long *addr, unsigned size)
{
	for (; size > BITS_PER_LONG; size -= BITS_PER_LONG, ++addr)
		if (*addr)
			return 0;
	return !size || !(*addr & BITMAP_LAST_WORD_MASK(size));
}

But to be honest I’m not entirely sure this is worth the effort.
find_next_bit and find_next_zero_bit may do a little bit more work but
some architectures have specialised optimised versions which will be
lost if all_bit_is_zero and all_bit_is_one are introduced.

> +EXPORT_SYMBOL(all_bit_is_zero);
> +#endif
> +
> +#ifndef all_bit_is_one
> +/*
> + * return val: 1 means all bit is one
> + */
> +unsigned int all_bit_is_one(const unsigned long *addr, unsigned size)
> +{
> +	unsigned long idx;
> +	unsigned long mask = size;
> +
> +	if (unlikely(size == 0))
> +		return 1;
> +
> +	if (size > BITS_PER_LONG) {
> +		for (idx = 0; idx * BITS_PER_LONG < size; idx++)
> +			if (~addr[idx])
> +				return 0;
> +
> +		mask = size - (idx - 1) * BITS_PER_LONG;
> +	}
> +
> +	return !(~(*addr) & BITMAP_LAST_WORD_MASK(mask));
> +}
> +EXPORT_SYMBOL(all_bit_is_one);
> +#endif
> +
>  #ifdef __BIG_ENDIAN
>  
>  /* include/linux/byteorder does not support "unsigned long" type */
> -- 
> 2.5.0
>

-- 
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.o. | Liege of Serenely Enlightened Majesty of         o' \,=./ `o
..o | Computer Science,  ミハウ “mina86” ナザレヴイツ  (o o)
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