linux-kernel - Re: [PATCH 3/4] string: introduce memchr

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Date:	Mon, 22 Aug 2011 23:56:51 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Akinobu Mita <akinobu.mita@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, Joern Engel <joern@...fs.org>,
	logfs@...fs.org, Marcin Slusarz <marcin.slusarz@...il.com>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 3/4] string: introduce memchr_inv

Le lundi 22 août 2011 à 13:52 -0700, Andrew Morton a écrit :
> On Tue, 23 Aug 2011 01:29:07 +0900
> Akinobu Mita <akinobu.mita@...il.com> wrote:
> 
> > memchr_inv() is mainly used to check whether the whole buffer is filled
> > with just a specified byte.
> > 
> > The function name and prototype are stolen from logfs and the
> > implementation is from SLUB.
> > 
> > ...
> >
> > +/**
> > + * memchr_inv - Find a character in an area of memory.
> > + * @s: The memory area
> > + * @c: The byte to search for
> > + * @n: The size of the area.
> 
> This text seems to be stolen from memchr().  I guess it's close enough.
> 
> > + * returns the address of the first character other than @c, or %NULL
> > + * if the whole buffer contains just @c.
> > + */
> > +void *memchr_inv(const void *start, int c, size_t bytes)
> > +{
> > +	u8 value = c;
> > +	u64 value64;
> > +	unsigned int words, prefix;
> > +
> > +	if (bytes <= 16)
> > +		return check_bytes8(start, value, bytes);
> > +
> > +	value64 = value | value << 8 | value << 16 | value << 24;
> > +	value64 = (value64 & 0xffffffff) | value64 << 32;
> > +	prefix = 8 - ((unsigned long)start) % 8;
> > +

<snip>

> > +	if (prefix) {
> > +		u8 *r = check_bytes8(start, value, prefix);
> > +		if (r)
> > +			return r;
> > +		start += prefix;
> > +		bytes -= prefix;
> > +	}

</snip>

Please note Andrew the previous code just make sure 'start' is aligned
on 8 bytes boundary. (It is suboptimal because if 'start' was already
aligned, we call the slow check_bytes(start, value, 8))

Code should probably do

prefix = (unsigned long)start % 8;
if (prefix) {
	prefix = 8 - prefix;
	r = check_bytes8(start, value, prefix);
	...



> > +
> > +	words = bytes / 8;
> > +
> > +	while (words) {
> > +		if (*(u64 *)start != value64)



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