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Subject: Re: Alternative implementation of the generic __ffs
From: Joe Perches <joe@perches.com>
To: dean gaudet <dean@arctic.org>
Cc: Harvey Harrison <harvey.harrison@gmail.com>,
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Date: Fri, 18 Apr 2008 17:58:44 -0700
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On Fri, 2008-04-18 at 17:20 -0700, dean gaudet wrote:
> any reasonable compiler should figure out the two are the same... but i 
> really prefer spelling out the lack of dependencies of the computations by 
> breaking it out per-bit.

It seems gcc 4.3 (-Os or -O2) isn't a reasonable compiler.

I think this might be best:

int ffs32(unsigned int value)
{
	int x;

	value &= -value;
	if (!(value & 0x55555555))
		x = 1;
	else
		x = 0;
	if (!(value & 0x33333333))
		x |= 2;
	if (!(value & 0x0f0f0f0f))
		x |= 4;
	if (!(value & 0x00ff00ff))
		x |= 8;
	if (!(value & 0x0000ffff))
		x |= 16;

	return x;
}