RE: [PATCH 1/4] bitops: Add single_bit_set()

[David Laight]

From: Andy Shevchenko
> Sent: 23 November 2021 10:48
> 
> On Tue, Nov 23, 2021 at 10:42:45AM +0000, David Laight wrote:
> > From: Vaittinen, Matti
> > > Sent: 22 November 2021 13:19
> > >
> > > On 11/22/21 14:57, Andy Shevchenko wrote:
> > > > On Mon, Nov 22, 2021 at 12:42:21PM +0000, Vaittinen, Matti wrote:
> > > >> On 11/22/21 13:28, Andy Shevchenko wrote:
> > > >>> On Mon, Nov 22, 2021 at 01:03:25PM +0200, Matti Vaittinen wrote:
> > > >
> > > > What do you mean by this?
> > > >
> > > > hweight() will return you the number of the non-zero elements in the set.
> > >
> > > Exactly. The function I added did only check if given set of bits had
> > > only one bit set.
> >
> > Checking for exactly one bit can use the (x & (x - 1)) check on
> > non-zero values - which may even be better on some cpus with a
> > popcnt instruction.
> 
> In the discussed case the value pretty much can be 0, meaning you have
> to add an additional test which I believe diminishes all efforts for
> the is_power_of_2() call.

I wouldn't have thought so.
Code would be:
	if (!scan_for_non_zero())
		return 0;
	if (!is_power_of_2())
		return 0;
	return scan_for_non_zero() ? 0 : 1;

Hand-crafting asm you'd actually check for (x - 1) generating
carry in the initial scan.

The latency of popcnt it worse than arithmetic on a lot of x86 cpu.

	David

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