netdev - RE: [PATCH] net: pfifo_fast - use ffs(x)-1 instead of array lookup

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B6EA1@saturn3.aculab.com>
Date:	Tue, 13 Mar 2012 16:55:14 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"David Laight" <David.Laight@...LAB.COM>,
	"Maciej Zenczykowski" <zenczykowski@...il.com>,
	"Maciej Zenczykowski" <maze@...gle.com>
Cc:	<netdev@...r.kernel.org>
Subject: RE: [PATCH] net: pfifo_fast - use ffs(x)-1 instead of array lookup

 
> > +	/* For i in 0..7 returns {-1, 0, 1, 0, 2, 0, 1, 0}[i] */
> > +	return ((26468 >> (i+i)) & 3) - 1;
> 
> That expression doesn't seem quite right to me ...
> 	return (int)(0x12131210 >> (i * 4)) & 3) - 1;
> probably does what is wanted.

Hmmm... I'm going blind - I read that as 'i+1' not 'i+i'.
But using 'i * 4' and putting the constant in base 16
make the code rather less obscure.

	David


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