Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH 1/4] bitops: Add single_bit_set() | Date | Tue, 23 Nov 2021 14:36:37 +0000 |
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From: 'Andy Shevchenko' > Sent: 23 November 2021 13:43 > > 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.
Well, on AMD piledriver and bulldozer (etc) 64bit popcnt has a latency of 4. On bobcat the latency is 12. Excavator and Ryzen are better. Intel are ok except for the Atoms (silvermont/goldmont). That isn't going to help.
But run on a cpu without a popcnt instruction and the performance will really be horrid. At best the gain for using popcnt is marginal.
If you want to try a benchmark then code up (and debug): %rsi = buf + length // pointer to end of bitmap %rcx = -length // in bytes 1: jrcxz 8f // jumps if all zeros mov (%rsi, %rcx),%rax mov %rax, %rdx, sub $1, %rax lea 8(%rcx), %rcx jc 1b // jump if zero word and %rdx, %rax jnz 8f // jump if >1 bit set 2: jrcxz 9f cmp (%rsi, %rcx), %rax lea 8(%rcx), %rcx jz 2b 8: xor %eax,%eax ret 9: int %eax ret
I think that is (about) right). The initial loop may be 3 clocks per iteration on a recent Intel cpu.
But I suspect the only real gains are on cpu without popcnt. It isn't as though you'll be doing this as often as (say) the IP checksum function - which I have benchmarked.
David
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