RE: [PATCH 1/4] bitops: Add single_bit_set()
From: David Laight
Date: Tue Nov 23 2021 - 05:58:50 EST
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:
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.
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)