linux-kernel - Re: Alternative implementation of the generic _

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Date:	Sat, 19 Apr 2008 12:05:23 +0200
From:	Mikael Pettersson <mikpe@...uu.se>
To:	dean gaudet <dean@...tic.org>
Cc:	Joe Perches <joe@...ches.com>,
	Harvey Harrison <harvey.harrison@...il.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Alternative implementation of the generic __ffs

dean gaudet writes:
 > On Fri, 18 Apr 2008, Joe Perches wrote:
 > 
 > > On Fri, 2008-04-18 at 18:11 -0700, dean gaudet wrote: 
 > > > have you benchmarked it?
 > > 
 > > I modified Alexander's benchmark:
 > > http://lkml.org/lkml/2008/4/18/267
 > > to include 32 and 64 bit variants called smallest.
 > > 
 > > On an old ARM:
 > 
 > i'm guessing the 32-bit constants suck :(
 > 
 > the code could be modified to use 16-bit constants only

16-bit immediates don't help, as ARMs express immediate
operands in arithmetic instructions as 8-bit values plus
a 4-bit rotation count (which is multiplied by 2).

Very new ARMs can construct full 16-bit immediates, but
that still takes an additional instruction and an additional
register.
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