linux-kernel - Re: [PATCH v10 01/10] bitops: Introduce the for_each_set

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Message-ID: <20190322182244.GZ9224@smile.fi.intel.com>
Date:   Fri, 22 Mar 2019 20:22:44 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     linus.walleij@...aro.org, akpm@...ux-foundation.org,
        linux-gpio@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux@...musvillemoes.dk,
        yamada.masahiro@...ionext.com, bgolaszewski@...libre.com,
        linux-arm-kernel@...ts.infradead.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v10 01/10] bitops: Introduce the for_each_set_clump8 macro

On Thu, Mar 14, 2019 at 09:30:02PM +0900, William Breathitt Gray wrote:
> This macro iterates for each 8-bit group of bits (clump) with set bits,
> within a bitmap memory region. For each iteration, "start" is set to the
> bit offset of the found clump, while the respective clump value is
> stored to the location pointed by "clump". Additionally, the
> bitmap_get_value8 and bitmap_set_value8 functions are introduced to
> respectively get and set an 8-bit value in a bitmap memory region.

> +void bitmap_set_value8(unsigned long *const bitmap, const unsigned int size,
> +		       const unsigned long value, const unsigned int start)
> +{
> +	const size_t index = BIT_WORD(start);
> +	const unsigned int offset = start % BITS_PER_LONG;
> +	const unsigned int low_width = (offset + 8 > BITS_PER_LONG) ?
> +				       BITS_PER_LONG - offset : 8;

> +	const unsigned long low_mask = GENMASK(offset + low_width - 1, offset);

I think

	unsigned long low_mask = GENMASK(low_width - 1, 0) << offset;

will generate better code.

> +	const unsigned int high_width = 8 - low_width;
> +	const unsigned long high_mask = GENMASK(high_width - 1, 0);
> +
> +	/* set lower portion */
> +	bitmap[index] &= ~low_mask;
> +	bitmap[index] |= value << offset;
> +
> +	/* set higher portion if space available in bitmap */
> +	if (high_width && start + 8 <= size) {
> +		bitmap[index + 1] &= ~high_mask;
> +		bitmap[index + 1] |= value >> low_width;
> +	}
> +}
> +EXPORT_SYMBOL(bitmap_set_value8);

-- 
With Best Regards,
Andy Shevchenko