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

['Andy Shevchenko']

On Tue, Nov 23, 2021 at 10:58:44AM +0000, David Laight wrote:
> From: Andy Shevchenko
> > 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.

Have you done any benchmarks? Can we see them?

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

Ditto.


-- 
With Best Regards,
Andy Shevchenko