linux-kernel - Re: [PATCH 1/4] bitops: Add single_bit

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Date:   Tue, 23 Nov 2021 12:47:30 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'Vaittinen, Matti'" <Matti.Vaittinen@...rohmeurope.com>,
        Matti Vaittinen <mazziesaccount@...il.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jiri Kosina <trivial@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yury Norov <yury.norov@...il.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] bitops: Add single_bit_set()

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.

-- 
With Best Regards,
Andy Shevchenko