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

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Date:	Sun, 6 Apr 2008 10:03:43 -0700 (PDT)
From:	dean gaudet <dean@...tic.org>
To:	Alexander van Heukelum <heukelum@...lshack.com>
cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>
Subject: Re: [PATCH] x86: generic versions of find_first_(zero_)bit, convert
 i386

fwiw there's a way to do ffz / ntz which can do lg(n) conditional moves in 
parallel... i'm not sure what (non-x86) architectures this might be best 
on, but it might be a good choice for the generic code... although maybe 
the large number of constants required will be a burden on RISC 
processors.

take a look at figure 5-17 here http://hackersdelight.org/revisions.pdf

int ntz(unsigned x) {
	unsigned y, bz, b4, b3, b2, b1, b0;
	y = x & -x; // Isolate rightmost 1-bit.
	bz = y ? 0 : 1; // 1 if y = 0.
	b4 = (y & 0x0000FFFF) ? 0 : 16;
	b3 = (y & 0x00FF00FF) ? 0 : 8;
	b2 = (y & 0x0F0F0F0F) ? 0 : 4;
	b1 = (y & 0x33333333) ? 0 : 2;
	b0 = (y & 0x55555555) ? 0 : 1;
	return bz + b4 + b3 + b2 + b1 + b0;
}

-dean
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