Messages in this thread | | | Date | Tue, 23 Nov 2021 15:43:19 +0200 | From | 'Andy Shevchenko' <> | Subject | Re: [PATCH 1/4] bitops: Add single_bit_set() |
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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
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