linux-kernel - Re: [PATCH] x86: generic versions of find_first_(zero

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Message-Id: <1207563950.7880.1246457209@webmail.messagingengine.com>
Date:	Mon, 07 Apr 2008 12:25:50 +0200
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"dean gaudet" <dean@...tic.org>
Cc:	"Alexander van Heukelum" <heukelum@...lshack.com>,
	"Ingo Molnar" <mingo@...e.hu>, "Andi Kleen" <andi@...stfloor.org>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: generic versions of find_first_(zero_)bit, convert    
   i386


On Sun, 6 Apr 2008 13:22:58 -0700 (PDT), "dean gaudet" <dean@...tic.org>
said:
> On Sun, 6 Apr 2008, Alexander van Heukelum wrote:
> 
> > The current generic implementation of ffz is O(lg(n)) already
> 
> it's O(lg(n)) time... the operations all depend on each other.
> 
> the implementation i pointed to is O(lg(n)) code space... and the time 
> depends on how parallel the machine is, they're not dependent on each 
> other.

Indeed. The worst dependencies are in the sum of all the partial
results in this implementation. And addition is associative, so
partial results can be written as ((a+b)+(c+d))+(e+f). Assuming
perfect parallel execution this would lead to O(ln(ln(n))). Good.

Care to implement ffs and __ffs like this?

Greetings,
    Alexander

> -dean
-- 
  Alexander van Heukelum
  heukelum@...tmail.fm

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