linux-ext4 - Re: [PATCH 01/10] string: introduce memweight

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Date:	Wed, 23 May 2012 07:16:00 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Anders Larsen <al@...rsen.net>,
	Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
	linux-fsdevel@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	linux-media@...r.kernel.org, Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>, ocfs2-devel@....oracle.com,
	linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 01/10] string: introduce memweight

On Wed, May 23, 2012 at 09:12:18PM +0900, Akinobu Mita wrote:
> size_t memweight(const void *ptr, size_t bytes)

Why should this return size_t instead of unsigned long?

> {
> 	size_t w = 0;
> 	size_t longs;
> 	const unsigned char *bitmap = ptr;
> 
> 	for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
> 			bytes--, bitmap++)
> 		w += hweight8(*bitmap);
> 
> 	longs = bytes / sizeof(long);
> 	BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
> 	w += bitmap_weight((unsigned long *)bitmap, longs * BITS_PER_LONG);
> 	bytes -= longs * sizeof(long);
> 	bitmap += longs * sizeof(long);
> 
> 	for (; bytes > 0; bytes--, bitmap++)
> 		w += hweight8(*bitmap);
> 
> 	return w;
> }

bitmap_weight copes with a bitmask that isn't a multiple of BITS_PER_LONG
in size already.  So I think this can be done as:

unsigned long memweight(const void *s, size_t n)
{
	const unsigned char *ptr = s;
	unsigned long r = 0;

	while (n > 0 && (unsigned long)ptr % sizeof(long)) {
		r += hweight8(*ptr);
		n--;
		ptr++;
	}

	BUG_ON(n >= INT_MAX / 8)

	return r + bitmap_weight((unsigned long *)ptr, n * 8);
}

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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